Drill Bit Size Chart: Inch, Metric and Tap Drill Sizes

Drill bits in the US come in fractions of an inch, usually in steps of 1/64 inch, while the rest of the world uses millimeters. The chart below converts between them, and the sections after it answer the two questions people most often get wrong: which bit to drill before tapping a thread, and how big a pilot hole a wood screw needs.

Have an odd size, like 0.309 in or 7.8 mm? The drill size calculator finds the nearest fractional, number, letter and metric bit for any size.

Drill bit size chart: fraction, decimal and mm

Fraction	Decimal (in)	mm	Closest metric bit
1/16	0.0625	1.59	1.6 mm
5/64	0.0781	1.98	2.0 mm
3/32	0.0938	2.38	2.4 mm
7/64	0.1094	2.78	2.8 mm
1/8	0.1250	3.18	3.2 mm
9/64	0.1406	3.57	3.6 mm
5/32	0.1563	3.97	4.0 mm
11/64	0.1719	4.37	4.4 mm
3/16	0.1875	4.76	4.8 mm
13/64	0.2031	5.16	5.2 mm
7/32	0.2188	5.56	5.6 mm
15/64	0.2344	5.95	6.0 mm
1/4	0.2500	6.35	6.4 mm
17/64	0.2656	6.75	6.8 mm
9/32	0.2813	7.14	7.1 mm
5/16	0.3125	7.94	8.0 mm
11/32	0.3438	8.73	8.7 mm
3/8	0.3750	9.53	9.5 mm
25/64	0.3906	9.92	10.0 mm
13/32	0.4063	10.32	10.3 mm
7/16	0.4375	11.11	11.1 mm
15/32	0.4688	11.91	12.0 mm
1/2	0.5000	12.70	12.7 mm

To convert any other size: inches × 25.4 = millimeters. The inches to mm converter handles fractions like 7/64 directly.

Tap drill sizes

Before cutting threads with a tap, you drill a hole slightly smaller than the bolt, so the tap has material to cut the threads into. These are the standard sizes for about 75% thread depth (the full list, including fine threads and pipe taps, is in the tap drill chart):

Thread	Tap drill	Decimal (in)	mm
#4-40	#43	0.089	2.26
#6-32	#36	0.1065	2.71
#8-32	#29	0.136	3.45
#10-24	#25	0.1495	3.80
#10-32	#21	0.159	4.04
1/4-20	#7	0.201	5.11
1/4-28	#3	0.213	5.41
5/16-18	F	0.257	6.53
3/8-16	5/16	0.3125	7.94
1/2-13	27/64	0.4219	10.72

Metric tap drills are easier: for coarse threads, subtract the pitch from the diameter.

Thread	Tap drill
M3 × 0.5	2.5 mm
M4 × 0.7	3.3 mm
M5 × 0.8	4.2 mm
M6 × 1.0	5.0 mm
M8 × 1.25	6.8 mm
M10 × 1.5	8.5 mm
M12 × 1.75	10.2 mm

Pilot and clearance holes for screws

A pilot hole goes in the piece the screw threads into. It should be about the size of the screw’s core, so the threads still bite, and it stops hardwood from splitting. A clearance hole goes in the piece being fastened down, so the screw passes through freely and pulls the two pieces together.

Screw	Pilot, softwood	Pilot, hardwood	Clearance hole
#6	5/64 in	3/32 in	9/64 in
#8	3/32 in	7/64 in	11/64 in
#10	7/64 in	1/8 in	13/64 in
#12	1/8 in	9/64 in	7/32 in

These are typical starting points: screw makers’ charts vary a little. When in doubt, test on a scrap piece first.

Checking a bit’s size

The size is usually stamped on the shank. If it’s worn off, measure across the shank (the smooth part, not the flutes) with calipers, or hold it against a mm ruler and compare with the chart. A drill gauge, a metal plate with labeled holes, is the fastest way to sort a box of mixed bits.

For the screws and bolts themselves, see the screw and bolt size chart.

More DIY charts: tap drill chart · number and letter drill sizes · thread pitch · wrench sizes · sandpaper grit · pipe sizes.