Drill Bit Size Chart

Number drills run from #80, the smallest, at 0.0135 in (0.343 mm), up to #1, the largest, at 0.228 in (5.791 mm). The bigger the number, the smaller the drill. A #7 drill is 0.201 in (5.105 mm) and a #30 is 0.1285 in (3.264 mm).

Letter drills carry on from the number drills. A is 0.234 in (5.944 mm), just above #1, and they climb to Z at 0.413 in (10.49 mm). Letter E is 0.25 in (6.35 mm), exactly the same size as a 1/4 inch drill.

A letter drill, every time. The biggest number drill, #1, is 0.228 in and the smallest letter drill, A, is 0.234 in. So the full order of size runs from #80 up to #1, then A up to Z, then on through the fractions.

28 mm is 1.1024 in. The nearest fractional drills are 1 7/64 in (28.178 mm, 0.18 mm over) and 1 3/32 in (27.781 mm, 0.22 mm under). Neither is exact. For a clearance hole take the 1 7/64 in, which is over. For anything that has to fit, use a 28 mm drill.

Turn the fraction into a decimal, then multiply by 25.4. 1 5/16 in is 1.3125 in, which is 33.3375 mm, shown on this page as 33.338 mm. The nearest metric drills are 33.5 mm (0.16 mm over) and 33.0 mm (0.34 mm under).

1/4 inch is exactly 6.35 mm. The nearest metric drills are 6.4 mm (0.05 mm over) and 6.3 mm (0.05 mm under). Letter E is the same size as 1/4 inch, so if you only have letter drills, E does the same job.

Close a caliper across the margins as near the cutting end as you can, because the body tapers slightly towards the shank. Drills are made to size or a hair under, never over, and wear makes them smaller, so the reading sits at or just below the true size. Take the nearest standard size at or just above it, then check it in the size finder.

Twist drills are made to ISO tolerance class h8, which allows the diameter to sit at the nominal size or a little under it, never over. The hole is a different story. Runout and a point that wanders often make it slightly bigger than the drill, so measure the hole when the fit matters.

For a clearance hole, yes: go to the next size up. For a tapped hole, a reamed hole or a press fit, look at the gap first. The metric table shows how far each fractional drill is over or under, for example 1 7/64 in against 28 mm is 0.18 mm over. For taps, use the tap drill chart rather than swapping by eye.

A clearance hole is bigger than the bolt so the bolt passes through, and a tap drill is smaller than the bolt so the thread has metal to cut into. Use the tap drill chart for tap drills and the bolt and screw size chart for clearance holes, then pick the drill from this chart.