Overview
A drill chart is a reference table that helps you match a drill bit size to a specific hole purpose, such as tapping threads, allowing a fastener to pass through, or preparing a hole for reaming. The term “drill chart” can refer to a general drill size reference, a tap drill chart, a clearance drill chart, or a PCB manufacturing chart—each serves a distinct purpose but uses the same basic principle: look up your desired outcome, then read across the row to find the correct drill diameter. This article focuses on explaining how to read and apply drill charts correctly, with decision guidance that most online lookup tables do not provide.
What a drill chart is used for
Drill charts exist because few holes are drilled to their “final” size in a single step. If you are tapping a thread, you must first drill an undersized hole to leave material for the tap to cut threads into. If a bolt or screw needs to slide through, you drill a larger clearance hole. If you need a precision final diameter, you drill undersize and then ream to the exact size. Each of these tasks requires a different hole size, and a drill chart tells you which drill bit diameter gets you to the right starting point.
Without a chart, you would have to calculate thread geometry, convert between metric and inch measurements, or memorize dozens of drill sizes. A good drill chart compresses all that into a lookup: find your fastener size or target diameter, identify your hole purpose, and read the recommended bit size.
The quick rule: start with the hole purpose
The most common mistake when using a drill chart is picking the wrong column. Drill charts organize data by purpose first, then by fastener size. Before you look up a number, ask yourself: Do I need to tap threads into this hole, allow a fastener to pass through, start a pilot hole, or prepare for reaming? That question determines which chart column you use. A clearance hole is deliberately oversized; a tap drill is deliberately undersized. Using one when you need the other produces a hole that does not work.
Common drill chart types
Drill charts come in several flavors, each suited to a different task. Knowing which type applies to your job saves time and prevents mistakes.
Drill size charts
A general drill size chart lists the diameters of drill bits in common sizing systems. These charts typically include fractional sizes (like 1/16”, 1/8”, 3/16”), metric sizes (1.0 mm, 1.5 mm, 2.0 mm), and numbered or lettered sizes (like #1, #2, or A, B, C), along with decimal-inch equivalents to help compare sizes across systems. Most drill size charts do not recommend which size to use for a specific job; they simply show what diameters exist and how to convert between systems.
Tap drill charts
A tap drill chart connects a thread callout—such as #4-40 (a size 4 screw with 40 threads per inch) or M5 × 0.8 (a metric thread)—to the correct hole size to drill before tapping. In most applications, tap drill sizes are selected to provide approximately 70–75% thread engagement. This percentage represents how many threads the tap will cut into the material; higher engagement creates stronger threads but also increases tapping torque and the risk of breaking the tap. A tap drill chart typically includes the thread size, pitch or threads per inch, tap drill size in both fractional and decimal form, and sometimes multiple recommendations based on whether you prioritize strength or ease of tapping.
Clearance drill charts
When a bolt or screw must slide through a hole rather than cut threads, you use a clearance hole. A clearance drill chart specifies three fit classes: close fit (minimal gap), normal fit (typical assembly), and free fit (looser, for easier insertion or tolerance accommodation). The difference between these fits can be as little as the width of a few drill sizes, so consulting a chart prevents under- or over-sizing.
PCB and NC drill charts
In electronics manufacturing, a drill chart takes the form of an NC (numerical control) drill chart or tool table. A NC Drill Chart file contains drilling information including positions, tool numbers, size, quantity and so on. Unlike machining drill charts, PCB drill charts include hole count by template, hole tolerance, and terminating layers. This information tells the fabrication house which drill tools to load, how many holes each tool must make, the size tolerance for each hole, and whether holes are plated (electrically conductive) or non-plated (mechanical only). For example, Class 2 boards use +/- 3 mils for plated through-holes and +/- 2 mils for non-plated holes smaller than 250 mils.
Tap drill, clearance drill, pilot hole, and reamer-prep hole
The most important decision when reading a drill chart is identifying which hole type matches your task. This section explains each type and includes a quick reference matrix to help you choose correctly.
| Hole Purpose | Why You Drill This Way | Typical Scenario | Key Tradeoff |
|---|---|---|---|
| Tap drill | Pre-drill before cutting threads; sized to leave material for tap to engage | Threaded insert; fastened assembly | Higher engagement = stronger but harder to tap |
| Clearance drill | Final hole allows fastener to pass through; no threads cut | Bolt through bracket; screw into assembly | Tight fit risks jamming; loose fit adds tolerance stack |
| Pilot hole | Small starter hole guides larger bit or reduces splitting risk | Long fastener in wood; initial centering | Reduces accuracy if too large or small |
| Reamer-prep hole | Deliberately undersized hole left for precision reaming | Precision bushing fit; tight tolerance | Reaming time adds cost; skipping it may miss tolerance |
Tap drill
A tap drill is the hole you make before using a tap to cut threads. The tap itself is a threading tool; it cuts or forms helical grooves into the walls of the drilled hole. The pre-drilled hole must be small enough to leave material for the tap to cut, but large enough that the tap does not jam or break. A standard tap drill chart for inch threads lists tap sizes from #0 to 1-1/2”, with corresponding pre-tap hole sizes. Similarly, a metric tap drill chart lists tap sizes from M1.6 to M39.
The challenge is that the “correct” tap drill size depends on material, tap type (cutting vs. forming), thread class, and how deep the hole is. A simplified chart assumes common conditions and cannot cover every case. Material matters: aluminum requires a different tap drill than steel because aluminum is softer and does not hold threads as securely. Forming taps (which displace material rather than cut it) require a different starting hole than cutting taps because they create the thread profile without removing chips.
Clearance drill
A clearance drill is a hole sized so that a fastener can slide through freely without binding. Unlike a tap drill, a clearance hole is drilled to its final size; no further threading or reaming occurs. The size depends on the fastener diameter and the tightness of fit you need.
Close fit means minimal gap; the fastener slides through with slight friction, useful when you need precise alignment. Normal fit is the most common; the fastener passes through easily but still sits close to the hole edge. Free fit is loose; useful when tolerance stack-up is large or when you want the fastener to thread into a hole in a different layer or part. A clearance drill chart shows all three options for each fastener size so you can choose based on assembly requirements.
Pilot hole
A pilot hole is a small starter hole used to guide a larger drill or to reduce the risk of wood splitting or metal tearing when drilling. Pilot holes are not threaded and are typically much smaller than the final hole diameter. They serve a centering and stress-relief function rather than a sizing function.
In woodworking, a pilot hole roughly 60–70% of the screw’s shank diameter reduces splitting. In metal, a pilot hole can guide a larger drill more accurately by preventing wander or chatter. Pilot holes are less commonly found in formal drill charts; instead, they are specified as a recommendation by the tool or fastener manufacturer.
Reamer-prep hole
Reaming is a finishing operation that enlarges and smooths a hole to a precise diameter and finish. When the final hole diameter or surface finish must be tight, you first drill a hole slightly undersize, then pass a reamer (a precision tool with multiple flutes) through to bring the hole to final size. The reamer-prep hole is intentionally left undersize; the reamer then cuts the last fraction of a millimeter or thousandth of an inch.
Reaming is used when tolerance is tight, when hole runout or straightness matters, or when a smooth, burnished surface is required (such as for a precision fit or a bushing). The challenge is deciding how much material to leave for reaming; too much and you waste time and tool life; too little and the reamer cannot cut and may chatter or break.
How to read fractional, number, letter, wire gauge, and metric drill sizes
Drill bits are available in multiple sizing systems, and a single chart may include all of them. Understanding each system helps you interpret the numbers and find equivalents.
Fractional drill sizes
Fractional drill sizes are expressed as inches: 1/16”, 1/8”, 3/32”, 1/4”, and so on. They are the most common sizes in inch-based shops. The advantage is intuitive: 1/4” is twice 1/8”. The disadvantage is that fractional arithmetic is unfamiliar to many modern users, and fine distinctions are hard to spot (is 7/64” close to 3/32”?).
To compare fractional sizes, convert to decimal inches. 1/8” = 0.125”, 7/64” = 0.109”, and 3/32” = 0.094”. With decimals, you can see that 7/64” is closer to 1/8” than to 3/32”, even though the fractions look unrelated. Decimal equivalents also make metric-to-inch conversion easier: if you need a hole close to 3 mm (0.118”), you can see that 7/64” (0.109”) or 1/8” (0.125”) are nearby options.
Number and wire gauge drill sizes
Numbered drill sizes (like #1, #2, … #80) are used for small holes, typically ranging from around 0.228” (#1) down to 0.0135” (#80). Wire gauge sizes (often noted alongside numbered sizes because they occupy overlapping diameter ranges) run from 0 (largest) down to 36 (smallest) in reverse—higher numbers are smaller, which is counter-intuitive if you are used to fractional or metric numbering.
The numbering system exists for historical reasons and does not map to any obvious logic; there is no simple formula to predict the diameter of a #43 drill from the diameter of a #42 drill. For this reason, numbered and wire gauge drills must always be looked up in a chart. The advantage is that a large range of small sizes is available in a compact notation; the disadvantage is that there is no way to guess the size without a reference.
Letter drill sizes
Letter drill sizes (A through Z) are another inch-based system for medium and small holes. Like numbered sizes, they fill gaps between fractional sizes but follow no obvious pattern. Letter sizes typically range from A (the largest, roughly 0.234”) to Z (the smallest, roughly 0.404”), though not all letters are used for all applications.
Letter sizes are less common than fractional or numbered sizes in general machining, but they appear in some tap drill and clearance drill charts, especially for older standards or specialized applications. Like numbered sizes, they require a chart lookup.
Metric drill sizes
Metric drill sizes are specified in millimeters and follow a systematic progression: 1.0, 1.2, 1.5, 2.0, 2.5, 3.0, and so on. The advantage is that metric sizing is logical and easy to interpolate; a 2.4 mm drill is between 2.0 and 2.5 mm. The disadvantage for users working in inch-based shops is that metric and inch sizes do not line up neatly, so you often have to choose the nearest inch equivalent.
When you have a metric hole diameter and must choose an inch drill, convert the millimeter dimension to decimal inches (divide by 25.4), then find the nearest fractional, numbered, or lettered drill size. For example, 5 mm ÷ 25.4 = 0.197 inches, which is close to 13/64” (0.203”) or the #20 drill (0.161”). The choice depends on whether your hole can be slightly oversized or undersized and whether you have the nearest bit in stock.
How to choose the nearest drill size when conversions do not match
In practice, the drill size you calculate or convert from a chart may not exist in your toolbox or inventory. Learning when to round up, round down, or substitute requires understanding the hole’s purpose and tolerance.
Compare decimal equivalents first
Always convert all sizes to decimal inches or millimeters before comparing. Decimal format removes ambiguity; it is easy to see that 0.1285” (a common decimal for around 13/64”) is very close to 3.26 mm but slightly different from 3.25 mm (which is 0.128”). Once you have decimals, you can rank available drills by closeness to your target and then apply the next decision rule.
Decide whether oversize or undersize is safer
For a tap drill, undersizing is nearly always safer because you cannot add material back if you drill too large. For a clearance hole, oversizing is usually acceptable and sometimes preferred because a loose fit does not jam. For a reaming operation, undersizing by the reaming allowance is mandatory; oversizing means the reamer cannot cut enough and produces an oversize hole.
If you have two equally close options (one slightly over, one slightly under your target), the hole purpose determines which to choose. Tapping benefits from undersize because it ensures good thread engagement. Clearance tolerates oversize and often prefers it. Reaming requires the right amount of undersize to give the reamer material to remove without forcing or chatter.
Check tolerance before choosing a rounded value
When you convert between metric and inch or round a calculated value, verify that the result stays within the intended tolerance. For example, if your drawing specifies 5.0 ± 0.2 mm and you convert to inch (0.197 ± 0.008”), the nearest common drill might be 3/16” (0.1875”), which is 0.0095” under the minimum and exceeds tolerance. In this case, you might choose 13/64” (0.203”) instead, even though it is slightly over, because it stays within tolerance.
This step is easy to overlook when reading a simplified chart, but it can be the difference between a part that assembles correctly and one that fails acceptance inspection.
Worked examples for using a drill chart
The following examples walk through real scenarios to show how to apply chart-reading principles.
Choosing a tap drill from a thread callout
Suppose you need to tap a #6-32 thread into aluminum. A tap drill chart shows that the recommended tap drill for #6-32 is a #36 drill (0.1065”). However, you know aluminum is soft and prone to stripping. You look for a note in the chart about material or decide to ask whether the shop prefers a slightly larger tap drill for aluminum to reduce tapping torque and risk of tap breakage. Some shops use a #33 drill (0.113”) instead for aluminum, accepting slightly lower thread engagement but reducing tool breakage risk.
This choice is not in the chart because simplified charts assume standard conditions. By understanding thread percentage and material, you can deviate safely from the published value when the situation calls for it.
Choosing a clearance drill for a bolt
You need to bolt two aluminum brackets together with a 1/4”-20 bolt (a quarter-inch diameter with 20 threads per inch). The bolt shaft is 0.25” diameter. A clearance drill chart shows three options: 0.266” for close fit, 0.281” for normal fit, and 0.313” for free fit.
You check your tolerance stack-up and assembly procedure. If you expect some position tolerance in the hole locations and want the bolt to slide in easily without forcing, you choose the normal fit (0.281”), which maps to a 9/32” drill. If your brackets are thin and you are worried about cracking around the hole, you might choose free fit (0.313”, close to 5/16”) to avoid jamming the bolt. Conversely, if alignment is critical and the bolt shank must not slip, you choose close fit (0.266”, close to 17/64”).
Finding a nearby inch drill for a metric hole
Your drawing calls for a 4.5 mm hole (0.177 inch). You check your available drills and do not have a 4.5 mm bit. The nearest options are 4.4 mm (0.173”) and 4.8 mm (0.189”), or you can use inch sizes: 9/64” (0.141”), 5/32” (0.156”), or 11/64” (0.172”).
The hole is for a clearance fit bolt, so slight oversizing is acceptable. The 11/64” drill (0.172”) is closest to 4.5 mm and is 0.005” under, which is within typical clearance tolerance. Alternatively, 4.8 mm is 0.189”, which is about 0.012” over, still acceptable for a clearance hole. You choose 4.8 mm if you want to favor the metric side, or 11/64” if you are working in an inch-based shop.
Why the drilled hole may not match the chart exactly
A chart value is a nominal (average) starting point, not a guarantee of final hole size. Many factors cause the actual finished hole to differ from what the chart predicts.
Runout, wear, and drill wander
A drill bit spinning in a chuck is not perfectly centered. Runout (the circular motion of the bit around its nominal axis) causes the hole to be slightly oversized and possibly non-round. A dull or worn bit produces more runout and enlarges the hole further. Drill wander—deviation from the intended path as the bit travels through the material—is especially noticeable in deep holes or hard materials. None of these effects appear on a chart because they depend on equipment condition, setup, and operator technique.
Material behavior and chip evacuation
Different materials respond differently to the same drill size. Soft aluminum chips readily and clogs the flutes of the drill; the hole may end up oversized if the bit is clogged and slides rather than cutting. Hard stainless steel or titanium may produce tight, long chips that jam in the flutes and force the bit to wander or vibrate. Thin material or blind holes (holes that do not go all the way through) create chip evacuation challenges; if chips cannot escape, they jam and heat builds up, sometimes causing the hole to enlarge or the tap to seize.
Plating, reaming, and finishing operations
In PCB manufacturing, electroplating adds material to the hole wall after drilling. A drilled hole nominally 10 mils (0.01”) wide can grow to 12 mils or more after plating, depending on the thickness. Conversely, if the hole is a press fit for a terminal, the plating growth may push it out of tolerance. Mechanical reaming reduces the finished hole to a tighter tolerance by removing a controlled amount of material, but it assumes the pre-ream hole was drilled correctly and does not account for plating or other prior steps. Each finishing operation shifts the final hole size relative to the charted pre-drill value.
Common mistakes when using a drill chart
Drill charts are powerful tools, but misuse creates frustrating failures. Here are the most frequent mistakes and how to avoid them.
Using a clearance size when you need a tap drill
This is the most common column-selection error. A clearance hole is much larger than a tap drill for the same fastener size. If you drill a clearance hole and then try to tap threads, the tap will have almost no material to engage and will fail to cut usable threads. The hole will be stripped (threads will not hold) or the fastener will rattle. Always verify that you are reading from the correct chart section before drilling.
Treating rounded conversions as exact
When a chart shows both metric and inch values, the inch value is often a rounded conversion. For example, 5 mm converts to 0.1969”, but a chart might round it to 0.197” or recommend the nearest standard size, 13/64” (0.2031”). The difference (0.0062”) is small but can exceed tolerance for a precision assembly. Verify rounding does not push your hole outside acceptable limits before committing to a rounded value.
Ignoring material, tap type, or thread engagement
A general chart assumes mild steel, cutting taps, and standard thread engagement (often 70–75%). If your material is aluminum, titanium, or a high-strength alloy, if you are using a forming tap instead of a cutting tap, or if you need deeper or shallower engagement, the chart value may not be optimal. Consult material-specific guidance or test in scrap material before committing to production. Do not assume a published chart covers every case.
How to structure a drill chart as a shareable reference
If you maintain or publish a drill chart—whether as a shop reference, an internal standard, or a dataset shared with suppliers—clear field structure ensures the chart is useful and accurate.
Recommended fields for a machining drill chart
- Drill size label (e.g., “1/16”“, “#43”, “M2.0”): The size in its primary system
- Sizing system (Fractional, Numbered, Letter, Metric, Wire Gauge): Helps readers identify which sizing rules apply
- Diameter (inches): Decimal-inch equivalent for cross-system comparison
- Diameter (millimeters): Millimeter equivalent; helpful for users with metric tools or international drawings
- Use case (Tap drill, Clearance, Pilot, General): Clarifies purpose and reduces confusion
- Thread callout (e.g., “#4-40”, “M3 × 0.5”): The fastener or thread this size targets
- Fit type (Close fit, Normal fit, Free fit): Relevant for clearance drills
- Notes: Material caveats, thread percentage assumptions, or special conditions
A well-structured chart is easy to filter, sort, and reference. If you publish it as a dataset—for example, by uploading a CSV or XLSX file to a tool that generates an interactive reference page with a filterable table and charts—readers can quickly locate the row they need without scrolling or searching manually. Tools like TablePage allow you to upload CSV, TSV, XLSX, or XLS files and instantly generate a shareable page with a filterable table and charts, making it easy to distribute a drill chart across your organization or to customers without requiring them to open a spreadsheet or PDF.
Recommended fields for a PCB drill chart
- Tool number (e.g., T01, T02): References the drill tool loaded in the NC machine
- Finished hole size: Diameter in millimeters or mils
- Hole count: Total number of holes using this tool
- Tolerance (e.g., ±0.003”, ±0.08 mm): Tightness of the hole-size specification
- Plated / Non-plated: Whether the hole conducts electrical current or is mechanical only
- Terminating layers (e.g., “Surface to Layer 3”): Indicates through-hole depth if the board is multilayer
- Quantity by hit count: Optional; number of hits (holes) per tool for verification
PCB drill charts include hole count by template, hole tolerance, and terminating layers. This structure ensures the fabricator understands what to drill and where, and allows you to track tool wear or quality issues by reviewing hit counts and hole sizes after drilling.
Drill chart FAQ
What is the difference between a drill chart and a tap drill chart?
A general drill chart lists bit diameters and sizing systems; it tells you what drill sizes exist and how they convert between inches, millimeters, and other scales. A tap drill chart connects a specific thread callout (like #6-32 or M5 × 0.8) to the recommended hole size to drill before tapping that thread. A drill chart answers “what sizes are available?” A tap drill chart answers “what size do I drill for this thread?”
Do drill chart values change by material?
The nominal drill diameter stays the same across materials—a 1/8” drill is still 0.125” whether you are drilling steel, aluminum, or plastic. However, the effectiveness of the drill diameter changes with material. For tapping, the material determines how aggressively the tap will engage threads and how much torque and heat build up. Tap drill sizes are selected to provide approximately 70–75% thread engagement in standard conditions, but this percentage is a starting point; soft materials may benefit from a slightly larger tap drill to reduce tapping torque, while hard or brittle materials may require extra caution or a different approach entirely.
Material also affects cutting speed, feed rate, and tool life; a simplified chart does not account for these factors. Always consult material-specific guidance when working with alloys or composites outside the chart’s assumed scope.
When should a hole be reamed instead of drilled to final size?
Use reaming when the final hole diameter must be precise (within a few thousandths of an inch or micrometers) or when the surface finish matters (smooth, burnished walls rather than torn or rough). Drilling alone is fast but produces a hole with some runout and a rough interior. Reaming removes a thin layer—typically 0.010” to 0.050” depending on the tool and material—and produces a hole that is rounder, straighter, and smoother.
The trade-off is time and tool cost: reaming is slower than drilling and adds a second operation. Use it only when the tighter tolerance or finish justifies the cost. For general-purpose holes (clearance, pilots, non-critical sizing), drilling alone is sufficient.