Screw and Bolt Size Chart: US and Metric

A screw or bolt size gives two things: the diameter and the thread. In the US that’s written like #8-32 or 1/4-20 (diameter, then threads per inch); in metric, like M6 × 1.0 (diameter in millimeters, then the distance between threads). Add the length and you have the whole description: 1/4-20 × 1 in, or M6 × 1.0 × 25.

Have a screw in your hand? Lay it on the screw size checker: it draws each size at real size on your screen, threads included, so you can match yours without calipers. Measured it already? The thread identifier tells you the thread from its diameter and thread count.

US screw and bolt size chart

Sizes smaller than 1/4 inch use numbers; from 1/4 inch up, sizes are fractions of an inch.

Size	Diameter (in)	Diameter (mm)	Coarse thread (UNC)	Fine thread (UNF)	Closest metric
#0	0.060	1.52	—	80	M1.6
#2	0.086	2.18	56	64	M2
#4	0.112	2.84	40	48	M3
#6	0.138	3.51	32	40	M3.5
#8	0.164	4.17	32	36	M4
#10	0.190	4.83	24	32	M5
#12	0.216	5.49	24	28	M5 / M6
1/4	0.250	6.35	20	28	M6
5/16	0.3125	7.94	18	24	M8
3/8	0.375	9.53	16	24	M10
7/16	0.4375	11.11	14	20	M11 (rare)
1/2	0.500	12.70	13	20	M12
5/8	0.625	15.88	11	18	M16
3/4	0.750	19.05	10	16	M20

Number sizes follow a simple formula: diameter = 0.060 + 0.013 × number (in inches). Wood screws use the same numbers: a #8 wood screw has about the same shank as a #8 machine screw.

“Closest metric” means close in diameter only. US and metric threads never match, so a metric nut won’t go onto a US bolt even when they look the same size.

Metric bolt size chart

Size	Diameter	Coarse pitch	Fine pitch	Wrench (ISO)
M3	3 mm (0.118 in)	0.5 mm	—	5.5 mm
M4	4 mm (0.157 in)	0.7 mm	—	7 mm
M5	5 mm (0.197 in)	0.8 mm	—	8 mm
M6	6 mm (0.236 in)	1.0 mm	—	10 mm
M8	8 mm (0.315 in)	1.25 mm	1.0 mm	13 mm
M10	10 mm (0.394 in)	1.5 mm	1.25 mm	16 mm (DIN: 17)
M12	12 mm (0.472 in)	1.75 mm	1.25 / 1.5 mm	18 mm (DIN: 19)
M16	16 mm (0.630 in)	2.0 mm	1.5 mm	24 mm
M20	20 mm (0.787 in)	2.5 mm	1.5 mm	30 mm

If a metric size is written without a pitch, such as “M8 × 30”, it has the coarse thread. Pitch is the distance from one thread to the next, so a smaller number means finer threads.

Wrench sizes for US hex bolts

Bolt	Wrench or socket
1/4 in	7/16 in
5/16 in	1/2 in
3/8 in	9/16 in
7/16 in	5/8 in
1/2 in	3/4 in
5/8 in	15/16 in
3/4 in	1 1/8 in

These are for standard hex bolts. Heavy hex bolts and some nuts take the next size up.

How to measure a screw or bolt

1. Diameter: measure across the outside of the threads with calipers, lay the bolt on a mm ruler, or match it on the screw size checker and read to the nearest half millimeter. Then find the nearest size in the charts above. A bolt that measures about 6.3 mm is a 1/4 in; one that measures 5.9 to 6 mm is an M6.
2. Threads: for US bolts, count the thread peaks along 1 inch: that’s threads per inch. For metric bolts, measure across 10 threads and divide by 10 to get the pitch. A thread gauge does this instantly.
3. Length: measure from under the head to the tip. For flat-head screws that sit flush, measure the whole screw, head included. To choose a length for a new job, the bolt length calculator adds up the parts, washers and nut for you.

If your measurement falls between two sizes, try a nut you know the size of. It’s the only foolproof way to tell an M8 × 1.25 from a 5/16-18, which differ by only 0.06 mm in diameter.

To convert a measurement, use mm to inches or inches to mm. For the drill sizes that go with these screws, see the drill bit size chart.

More DIY charts: bolt torque · washer sizes · wrench sizes · thread pitch · nail sizes · wire gauge.