Drill Bit Size Chart: Fractional, Number, Letter and Metric Table (Free PDF)

The Fraction column lists your marked size; that’s what you’ll find first. From there, just read across to find the exact equivalent in decimals and metrics. Sizes increases in 1/64 in steps from 1/64 in to 1 in.

Each fraction is also shown in its reduced form (e.g., 2/64 is listed as 1/32, 16/64 is listed as 1/4). So if you’re looking for 12/64 but don’t see it on a chart, check under 3/16.

Decimal: This is what the fraction looks like when rounded to four decimal places, showing its precise value. Why are the decimals odd-looking? Because they’re one 64th, or .015625 inches.

Millimeters is the same diameter expressed at an exact 25.4 mm/inch and rounded to three places.

There is a rounding trap. Some charts will show sizes like 17/32 as …125 at the fifth decimal point. So 0.5313 on some and 0.5312 on others. This table uses the round-half-even convention for ties that ANSI-style decimal equivalent charts follow, so it matches the wall chart in most US shops.

The decimal column is not predictive for hole size. The fraction on the shank is nominal. When a hole is freehand drilled it will run measurably over size. A good twist drill cuts at or just above nominal.

Be aware also that at the small end of the scale a 1/64 step is very coarse. Between 1/64 and 1/32, the diameter doubles. This is exactly why we have the number and letter gauge system. The number, letter, and metric tables below fills those gaps.

For example, to find out what size an unmarked drill is, first look at the decimal column. Get some calipers and measure across the drill's margin nearest the shank end. (Because the point end wears down, the shank end is most accurate.)

Compare the reading on your caliper to the chart. In case your caliper reads halfway between two fractional entries, check both the number table and the letter table before making your decision. It will most likely be one of the in-between gauge sizes, and less likely to be a worn fractional size.

The above table represents just the sizing system. This doesn't tell you which drill to select when tapping a thread, or which one to choose for starting a bolt or screw.

Other charts perform those tasks, such as tap drill charts for threading and clearance or pilot hole size charts for screw starts and fastener through holes. Those charts selects from this list of drills for a particular application.

In the Number size column, find your gauge; then, on the right-hand side under Millimeters (mm) and Decimal (in) you’ll see diameter.

Before we go any further though, it’s important to remember one key fact about this table: The numbers work backwards. For example, #1 is the largest drill (at 0.2280 in), while #80 is the smallest (at 0.0135 in). In other words: A higher number = a smaller hole.

ANSI/ASME B94.11M describes a wire gauge numbering system known as the number series. Although it's derived from an earlier British number system called Stubs Steel Wire Gauge (from the 19th century), it is not same.

Decimal sizes are standard tabulated sizes; i.e., they don't come from a formula or anything. Furthermore, the increments are not constant, they jump by 0.0035 in between #57 and #56, while they only jump by 0.0020 in between #8 and #7. There is no way to estimate number drill size, and that's why you have to lookup the size.

And that’s it for the series.  Above 0.2280 in the letter series take over at A (0.2340 in).

Below #80 the series continue as normal to #97 for micro drilling of circuit boards. If you go beyond #80, you’ll be buying a specialty item.

Confusing the number drill and the number screw gauge is a frequent mistake. For example, using a #8 drill would result in a hole 0.1990 in wide, not something you want to try on an #8 wood screw. Instead, look in the clearance hole or pilot hole charts for screw-to-drill combination.

Fractional numbers are spaced coarsely in 1/64 in increments; there’s a need for number drills to plug those holes. Those number drills also alternate with the fractional set: #52 to #48 all fall between 5/64 and 1/16, while #30 (0.1285 in) is right between 9/64 and 1/8.

Those are the sizes most frequent used as close-fit clearance drills or tap drills for machine screws. The tap drill chart tells you what number goes with what thread.

If it's a loose, unmarked small drill, get out your drill gauge plate, take a measurement on your calipers, and compare to the decimal column.

A drill that runs a thou or two unders has just been around the block too much, sizes this small go fast. Think of a bigger shortfall as a different gauge.

In the Letter size column find the letter stamped onto the shank then match it to the decimal (in) or millimeters (mm) columns for the corresponding diameter. The letters is listed in an easy order unlike the numbers. The smallest letter is A which is 0.2340 in while Z is the largest at 0.4130 in. The letter series resume right where the number series leaves off. Letter A is only 0.0060 in larger than number 1.

The nearest 1/64 fraction (in) column shows how each letter relate to the fractional progression. This tell us what 1/64 size it’s closest to and by how much that letter drill vary from that size.

There is one exact equivalency that is worth committing to memory. Letter E = exactly 1/4 in. An E drill and a 1/4 in drill are identical tools marked differently. Letter K is just .0002 in shy of 9/32. Close enough that it can be used interchangeable for all practical purposes.

Worn shanks can create reading traps. Letters can be confused with numbers, such as I looking like 1 or O being mistaken for a zero. Neither a 0 drill nor a #1 drill is anywhere near the size of those letter drills, so be careful not to mistake an I for a 1 or an O for a zero. When you see a questionable mark, verify with your calipers.

You’ll also see the letter K. On most charts, this is 0.2810 in., but some old charts has it listed as 0.2811 in. And then there’s the letter series. Z marks the end of the line. The only option after that in the inch system are the fractional sizes above 0.4130 in.

Why do they exist? Letter drills exist because 1/4 in and larger fasteners needs finer steps than 1/64 in provides for their tapping and clearance range. You see those call outs all the time in machine work like F, I, Q and U. Those are letter drills.

Tap drill chart explains which letter goes with what thread. If you’re using a fractional set and get a call out for a letter drill, then you can find its size in the compromise column that's the last one.

In a clearance hole it won't hurt as much (a few ten-thousandths). For tapping though, it alter how far into the thread you engage. So look at the tap drill chart first when subbing. Actualy, it should of been checked earlier.

To use it: Find your size in the Metric drill (mm) column, then look at a Decimal equivalent (in) column to see the exact size in inches.

As the sizes get bigger the step pattern shift. From .5-2.5 mm they increase in 0.05 mm increments. Up to 10 mm they is in 0.1 mm increments. Up to 13.8 mm there is selected popular sizes like 10.2, 10.5, 10.8 mm etc. Then they is in 0.25 mm increments from 14-17.5 mm. Lastly from 18-25 mm they are in 0.5 mm increments.

This is converted precisely using 25.4 mm/inch for the decimal equivalent (in). Rounding to four places allow digit-for-digit comparison with the other three tables above (fractional, number, letter).

The Nearest 1/64 fraction (in) column name the closest fractional drill and the deviation. When you have a metric size called out but only an imperial index on the bench, this is the deviation you want.

Deviations of less than about 0.002 in get drowned out by normal drilling oversize. Larger ones is worth another glance at the equivalents table below.

Now this list is the series you see on those decimal-equivalent wall charts people keep around. This is their favorite series, but it isn’t all of metric. BS 328 defines 230 sizes between .2 and 25 mm.

Guhring makes them, and Dormer makes them, down to 0.1 mm intervals through 13 mm and beyond. And they carry half-tenths special sizes such as 3.25 or 4.15 mm to tap with.

So yes, there’s a 12.4 mm drill and you can get one. It’s just not a popular stock size in most drill manufacturers’ sets.

Examine the seams where systems meet. 1/8 + 0.0010 in = 3.2 mm. 6.5 mm is 0.0011 in under letter F. 1/2 in exactly = 12.7 mm. It’s left out on purpose. 12.5 and 13 mm bracket it.

And 13 mm just happens to be capacity of a typical 1/2 in chuck; another practical detail. Above 13 mm, metric tends to become reduced shank drills. Big holes tend to be step-drilled or bored rather than drilled in one go. This is not a recommendation; it is just how the size system work.

For metric tap drills like M10 x 1.5, you can check the tap drill chart. For metric clearance holes, see the clearance hole chart.

If you’re doing an imperial drawing but have only metric tools (or vice-versa) use this table to determine what the equivalent is:

In the Metric drill (mm) column, find the call out; then in the Decimal (in) column verify its exact value. The Closest inch-system drill column will identify the one closest of the entire set of three inch system sizes (fractional, letter, and #/number).

Its own diameter will be shown in the Closest drill decimal (in) column, while Difference (in) will display how much bigger the substitute drill is different than the metric. Therefore if it’s a positive number, the substitute will drill a slightly larger hole.

Actualy, number and letter gauges get closer than that; in fact, below 10 mm they land nearer the metric size than any fractional step, so you see rows like 2.0 mm to #47, or 6.5 mm to F.

There are several where the two drill sizes can be used interchangeably for most purposes, because the typical excess of a drilled hole is far more than a thousandth of an inch here: 2.0 mm and #47, 9.5 mm and 3/8, 1.5 mm and #53, etc.

The question becomes how much bigger or smaller is OK? In a clearance hole, you don't mind if the hole is a little too large, but if it is too small, it might bind the fastener. You should of go with the positive difference.

For a tap drill, do the opposite: use only what is listed on the tap drill chart (even 0.002 in shifts the thread engagement noticeably). Don't substitute for a dowel, bearing, or press-fit hole at all; just ream to size.

For example, the table ends at 13 mm. This is where the inch series steps up (i.e. 1/64 in is 0.4 mm), and the increments then becomes too large for the substitution to remain tight.

It’s only metric-to-inch anyway. If you’re going the reverse direction, find the nearest match by scanning the decimal column in the metric table using the decimal number from the tables above for the corresponding inch drill.