Drill bit sizes: mm to fractional inches, and what is close enough

The exact relation

One inch is exactly 25.4 millimeters, defined in 1959 and listed in NIST SP 811 and by the BIPM. So the conversion has no rounding in it: divide millimeters by 25.4 for inches, multiply inches by 25.4 for millimeters. Drill bits are marked in whole millimeters, in fractions of an inch (usually sixteenths up to 1/2 inch), or in number and letter sizes, which are a third system not covered here.

For a quick conversion of any value, use the mm to inches converter; the fractions of an inch guide has the full 1/64 to 1 inch chart.

Fractional bits in millimeters

These are the common sixteenth-inch sizes. Each value is the fraction multiplied by 25.4, rounded to two decimals.

Metric bits in fractional inches

Here is the other direction: the nearest 1/64 inch to each metric size, and by how much the two differ. A positive difference means the fractional bit is larger than the metric one.

Which pairs are practically the same

Across 2 to 13 mm, the closest match is 2 mm and 5/64 inch, only 0.02 mm apart. The worst is 3 mm against 1/8 inch, 0.17 mm apart. The pattern is useful: 8 mm (7.94 mm for 5/16) and 10 mm (9.92 mm for 25/64) are within a few hundredths of a millimeter, while mid-range sizes can be off by a quarter of a millimeter.

Because the table rounds to the nearest 64th, a finer fraction would be closer, but bits in 1/64 steps are uncommon outside full engineering sets.

When swapping is fine and when it is not

• Clearance holes for screws and bolts: a swap of 0.1 to 0.2 mm is usually fine, because the hole is meant to be a little larger than the bolt anyway. Pick the larger bit if in doubt.
• Tap drill holes for threads: stay as close as you can to the size in the tap chart. A hole that is 0.2 mm too large can leave too little thread to hold.
• Wall plugs: use the diameter printed on the plug. A hole that is too large makes the plug spin; one that is too small makes it hard to seat. Brick and masonry bits are sold in both metric and fractional sizes, so match the plug, not your guess.
• Wood dowels and pins: for a snug fit, test on scrap. Wood compresses, so a hole 0.1 mm under the dowel is common practice, but a hole over its size is not.
• Reamed or press-fit holes: never swap. Those tolerances are in hundredths of a millimeter.

Number and letter drill sizes

Besides metric and fractional bits there are number sizes (#1 to #80) and letter sizes (A to Z), common in US machine shops. They sit between the fractions: letter bits run from A (0.234 inch) to Z (0.413 inch), which covers 5.94 to 10.49 mm. If a drawing calls for a letter or number bit, convert its decimal inch value with 25.4 and look at the nearest metric bit in the table above instead of rounding to a fraction.

A simple way to choose a bit

• Convert the size you need to millimeters or inches with the right unit: 3/8 inch is 3 ÷ 8 × 25.4 = 9.53 mm.
• Look for a bit marked in the same unit as your hardware first. Metric screws go with metric bits, imperial screws with fractional ones.
• If you must cross over, check the difference column above and choose the bit that is equal or slightly larger for clearance, and equal or slightly smaller for pilot holes.
• Check the measured bit with calipers if the marking is worn. Both inches to cm and the inches to cm chart help with the sum when a spec is given in centimeters.

Where this fits in the series

This is the first post in the tools series. The next one covers socket and wrench sizes in mm and inches, where the same close-but-not-equal pairs cause rounded bolt heads. To read the sixteenth ticks that these sizes come from, see how to read a tape measure in inches and mm. The method for reading any mm value also appears in phone and tablet sizes in cm, where 1 inch = 2.54 cm is applied to screens.