Drill Bit Size Chart: Decimal, Number, Letter and Metric Sizes in Inches and Millimeters

1. Four Sizing Systems, One Hole

The first thing to know about drill sizes is that there is no single system, and never was. Four naming schemes are in daily use, each one inherited from a different trade and a different era, and each one covering only part of the range. A fractional drill is named by its diameter as an ordinary fraction of an inch. A number drill is named by an old wire-gauge number, where a larger number means a smaller drill. A letter drill uses the alphabet the same way, but running the other way, so a later letter is a larger drill. A metric drill is named by its diameter in millimeters and needs no conversion at all.

The systems interleave rather than nest, which is the part that trips people up. Number drills fill the small end of the range and run continuously into the letter series at the top; letter drills then stop at Z, which is still below one half inch; fractional drills cover the small sizes in coarse steps and take over properly above that. A hole of a given size can therefore be reached by a fractional drill, a number drill or a letter drill, and the three designations will not resemble each other.

That is why the working method is always the same: reduce the designation to its decimal diameter, and compare decimals. Everything else on this page follows from that one move.

The ranges the four series cover

Note the direction of each scale, because three of the four run in different ways. A larger number drill is smaller. A later letter is larger. A larger fraction is larger. A larger metric size is larger. Only the number series runs backwards, and it is the one people misread.

2. Fractional Sizes, 1/64 inch to 1 inch

Fractional drills are the sizes you find in a hardware store index and the ones a drawing usually calls for outside precision work. They step by a constant 1/64 of an inch, so the series is evenly spaced in name even though it is coarse by any modern standard: there are only two sizes between 3/16 inch and 1/4 inch.

The fractional series is also the only one of the four whose true value is exact. Three thirty-seconds of an inch is exactly 0.09375 inch, not an approximation of it, because it is a ratio of two whole numbers. Every other series on this page was defined by measuring or by convention, and carries a decimal that stops where the standard chose to stop. That distinction is what produces the disagreement discussed in the sixth section: exact values survive arithmetic, while rounded values do not.

Read the millimeter column as the exact value multiplied by 25.4, rounded to three decimals. Because the underlying value is exact, the millimeter figures can be checked by hand at any row: nineteen sixty-fourths is 0.296875 inch, and that times 25.4 is 7.541 mm. A chart that shows a different figure for that row has either rounded earlier in the chain or made an arithmetic slip, and the section on disagreement tells you how to tell those two apart.

3. Number Sizes, #1 to #80

Number drills descend from the old wire gauge, which counted the number of drawing passes a rod took to reach a diameter: more passes, thinner wire, larger number. The drill series kept the convention, so #1 is the largest at 0.2280 inch and #80 is the smallest at 0.0135 inch, and the inversion holds throughout the series: #60 is 0.0400 inch while #40, a smaller number, is 0.0980 inch. Reading a number drill index without noticing that the scale runs backwards is a classic and expensive error.

The purpose of the series is resolution. Where fractions step in gaps of 0.015625 inch, number drills close up to steps of a thousandth or two at the small end, and that is the resolution thread work needs. Number drills are therefore the natural home of the tap drill, and the series is dense exactly where threads are small.

The decimal inch values above are the standard numbers, taken to four decimals. They are not derived from anything and are not approximations of a fraction: the standard simply states them, which is why this series has no rounding ambiguity of its own. The millimeter column is the standard value multiplied by 25.4, so it can differ from a chart that lists a metric drill of nominally the same size by the usual small amount.

4. Letter Sizes, A to Z

Letter drills continue the same job as number drills where the number series runs out about a quarter inch, and they fill the gap up to one half inch. Unlike the number series, the letter series runs the intuitive way: A is the smallest at 0.2340 inch and Z is the largest at 0.4130 inch, so a later letter is a bigger drill.

Twenty-six letters cover 0.1790 inch of range, which works out at an average step near 0.0072 inch, but the steps are not even. The series exists to provide tap drills for inch threads in the middle of the range, and it is spaced to suit those threads rather than to be uniform. Several letters are better known by the thread they drill than by their own diameter, which is the subject of the tap drill section below.

One entry in this table deserves a note, because it is the only place in the whole chart where a published copy disagrees with the standard. Letter K is 0.2810 inch in the standard tables and in most published copies, but at least one widely copied online table prints 0.2811 inch. The difference is a tenth of a thousandth of an inch, well under any drill tolerance, and it is almost certainly a transcription slip rather than a different standard. If you find 0.2811 in a chart you are using, that chart has a typo, not a different convention.

5. Metric Sizes, 1.0 mm to 25 mm

Metric drills are the simple case, because the designation is the diameter. There is nothing to invert and nothing to round: an 8 mm drill is 8 mm, and the millimeter figure on the drill shank is the measurement rather than a label for it. The series below follows the nominal sizes of ISO 235, stepping in tenths of a millimeter up to 10 mm and in half millimeters from 10.5 mm to 25 mm.

Because the metric size is exact by definition, a metric drill has no rounding ambiguity at all, and the only figure on this page derived from it is the decimal inch equivalent in the second column. That is the reverse of the situation in the fractional series, where the inch value is exact and the millimeter value is derived. It is a useful check on any conversion: the series whose designation is a whole quantity is the one whose other column carries the rounding.

In practice the metric series and the inch series do not meet cleanly, and a workshop holding only an inch drill index cannot cut a true 8 mm or 10 mm hole. The next section gives the nearest inch bit for the common metric sizes, with the diameter error stated so you can judge whether the substitution is acceptable for the job in front of you.

6. Why Two Charts Disagree in the Third Decimal

Put two published drill charts side by side and some of the millimeter figures will not match, even though both charts are using the same standard and neither has made an arithmetic mistake. The cause is a single decision about rounding order, and it only affects the fractional series, because that is the only series whose true value has more digits than the value printed on the chart.

The exact value of 3/32 inch is 0.09375 inch. Multiply that by 25.4 and you get 2.381 mm. But a chart that prints the inch figure first must round it to four decimals, which gives 0.0938 inch, and converting that rounded figure gives 2.383 mm. Both readings are defensible; they simply round at different points in the chain.

This page converts the exact value, never the printed value. The rule matters because the printed inch figure is a lossy representation, and converting a rounded number compounds one rounding with another. There are 32 fractional sizes in the table above where the two orders give different millimeter figures. The table below shows the common ones.

The differences are all 0.001 or 0.002 mm, which is a fortieth of a typical drill tolerance and has no practical consequence. The reason to understand them is diagnostic: if a chart you are checking differs from this one by one or two thousandths of a millimeter on a fractional size, it has rounded in the other order and is fine. If it differs by more than that, or if it differs on a number or letter drill, it has an actual error, because those series have no rounding to argue about. For the wider method behind these reductions, see How to Convert Units, and for exact factors across every category see the unit conversion chart.

7. The Nearest Inch Bit to a Metric Hole

A drawing in millimeters and a drill index in inches is an ordinary mismatch, and the honest answer is that no inch drill cuts a true metric size above a few millimeters. What you can do is find the nearest inch bit and know how far off it is. The table gives the closest match in any of the three inch series for five common metric holes, along with the diameter error, so the decision is made on a number rather than on a guess.

Whether a substitution is acceptable depends entirely on the fit the hole is for. The errors above run from a few hundredths to a few tenths of a millimeter. On a clearance hole for a fastener that is nothing; on a locating hole for a dowel or a press fit for a bearing it is the whole tolerance. The habit that prevents the problem is to order the metric drill when the drawing is metric, and to keep the two indexes separate rather than to bridge them at the drill press. To move between the units on the drawing itself, the site has an millimeters to inches converter and the reverse inches to millimeters converter.

8. Why So Many Tap Drills Are a Number or a Letter

A tap drill has to leave the right amount of material for the thread: enough that the thread is strong, not so much that the tap has to cut the whole depth of the thread in one pass. The conventional choice leaves about 75 per cent of a full thread, which for a given pitch lands on a diameter that almost never coincides with a convenient fraction. That is precisely why the number and letter series exist, and why so many tap drills are listed as a number or a letter rather than as a fraction.

The table gives the standard 75 per cent tap drill for the coarse inch threads most often cut in a shop, drawn from the number and letter tables above so that the diameters agree with the rest of this page. A 1/4-20 thread takes a #7 drill, a 5/16-18 takes an F, and a 1/2-13 takes 27/64 inch: three different series for three threads of a similar size, which is the whole point.

Two practical notes. The 75 per cent figure is the one for aluminum, brass and plastics; steel, stainless and cast iron are often started with the 50 per cent drill, which is a size or two larger, because a coarser thread is easier to cut in a hard material and the loss of thread strength is acceptable. And a tap drill is a starting size, not a guarantee: the finished thread depth depends on the tap itself and on how far it is driven, so a hole that meets its drill diameter can still fail a thread gauge.

The same reasoning about material and clearance applies to sheet and plate, where the sizing chart problem is a different one: there the number is a legacy gauge rather than a wire number, and the mapping to thickness depends on the metal. The sheet metal gauge chart covers that case, including why one gauge number means a different thickness in every metal.

9. Sizing a Hole on a Drawing

The most useful habit with drill sizes is the same one that applies to every legacy unit: specify the hole by its diameter and its tolerance, not by the drill that happens to cut it. A decimal inch or a millimeter figure has one meaning and is the figure an inspector will measure; a drill designation is a convenient name for a cutter, and several different cutters can answer to nominally the same callout once tolerances are in play.

Three rules of thumb carry most of the benefit:

• Give the hole diameter and its tolerance, and add the drill size in brackets only where the shop floor actually needs it. The bracket is a convenience, and the diameter is the requirement.
• Do not bridge unit systems on the shop floor. If the drawing is in millimeters, order the metric drill; if it is in inches, use the inch index. A substituted drill is a silent error that no inspection step is looking for.
• Remember that drill tolerance is a range, not a point. A nominal drill cuts slightly oversize in most materials, so a hole specified to a tight tolerance is reamed or bored after drilling rather than drilled to size.

For the general method behind converting any of these figures, and for the families where a factor does not exist, see How to Convert Units. For the failure modes that come from getting a conversion wrong, the case studies in the eight most expensive unit conversion mistakes are worth an hour. And because a drill size is finally a length like any other, the length conversion guide covers the unit that results from it.

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