Wrench Size Chart: SAE vs Metric, Sockets and Allen Keys

Every common wrench size in inches and millimeters, which SAE and metric sizes you can swap and which will round off a nut, the wrench for each bolt, and Allen key sizes.

Updated October 13, 2026

Open-end wrenches drawn to scale from 5/16 inch to 3/4 inch, each labeled with its size in millimeters and the closest metric wrench, with matching pairs like 3/4 inch and 19 mm highlighted

Wrench sizes are measured across the flats of the nut or bolt head, the distance between two opposite flat sides. US tools are SAE (fractions of an inch); almost everything else, including most cars made since the 1980s, is metric. A few sizes are close enough to swap, but most aren’t: a wrench even 0.3 mm too big will slip and round the corners off a tight nut.

SAE to metric wrench chart

SAE wrench	mm	Closest metric	Difference	Swap?
1/4	6.35	6 mm	0.35 mm	No
5/16	7.94	8 mm	0.06 mm	Yes
11/32	8.73	9 mm	0.27 mm	No
3/8	9.53	10 mm	0.48 mm	No
7/16	11.11	11 mm	0.11 mm	Yes
1/2	12.70	13 mm	0.30 mm	Loose
9/16	14.29	14 mm	0.29 mm	No
5/8	15.88	16 mm	0.12 mm	Yes
11/16	17.46	17 mm	0.46 mm	No
3/4	19.05	19 mm	0.05 mm	Yes
13/16	20.64	21 mm	0.36 mm	No
7/8	22.23	22 mm	0.23 mm	Loose
15/16	23.81	24 mm	0.19 mm	Loose
1	25.40	25 mm	0.40 mm	No
1 1/16	26.99	27 mm	0.01 mm	Yes
1 1/8	28.58	29 mm	0.43 mm	No
1 1/4	31.75	32 mm	0.25 mm	Loose

Yes means the sizes are within about 0.15 mm, close enough to use either way. Loose works for a nut that isn’t tight, as long as the bigger wrench goes on the smaller nut; on a stuck nut, use the right size. No means one wrench won’t fit, or will round the nut.

Metric to SAE wrench chart

Metric	Inches	Closest SAE	Difference
6 mm	0.236	1/4 in	0.35 mm
7 mm	0.276	9/32 in	0.14 mm
8 mm	0.315	5/16 in	0.06 mm
10 mm	0.394	13/32 in	0.32 mm
11 mm	0.433	7/16 in	0.11 mm
12 mm	0.472	15/32 in	0.09 mm
13 mm	0.512	1/2 in	0.30 mm
14 mm	0.551	9/16 in	0.29 mm
15 mm	0.591	19/32 in	0.08 mm
16 mm	0.630	5/8 in	0.12 mm
17 mm	0.669	11/16 in	0.46 mm
18 mm	0.709	11/16 in	0.54 mm
19 mm	0.748	3/4 in	0.05 mm
21 mm	0.827	13/16 in	0.36 mm
22 mm	0.866	7/8 in	0.22 mm
24 mm	0.945	15/16 in	0.19 mm
27 mm	1.063	1 1/16 in	0.01 mm
30 mm	1.181	1 3/16 in	0.16 mm
32 mm	1.260	1 1/4 in	0.25 mm

The 10 mm, 13 mm, 15 mm, 17 mm and 18 mm sizes have no good SAE match, which is why they’re the ones people go looking for. Keep a metric set if you work on cars, bikes or furniture.

Which wrench fits which bolt

US bolt	Wrench	Metric bolt	Wrench (ISO)	Wrench (older DIN)
1/4 in	7/16 in	M5	8 mm	8 mm
5/16 in	1/2 in	M6	10 mm	10 mm
3/8 in	9/16 in	M8	13 mm	13 mm
7/16 in	5/8 in	M10	16 mm	17 mm
1/2 in	3/4 in	M12	18 mm	19 mm
9/16 in	13/16 in	M14	21 mm	22 mm
5/8 in	15/16 in	M16	24 mm	24 mm
3/4 in	1 1/8 in	M20	30 mm	30 mm

These are for standard hex bolts and nuts; heavy hex nuts take a bigger wrench. For the bolts themselves (diameter and threads), see the screw and bolt size chart.

Socket drive sizes

The drive size is the square hole on the back of the socket that fits the ratchet. It has nothing to do with the nut size, and it’s always in inches, even on metric sockets.

Drive	Typical socket sizes	Used for
1/4 in	5/32–9/16 in, 4–14 mm	Small fasteners, electronics, interior trim
3/8 in	1/4–7/8 in, 6–22 mm	Most car and home repairs
1/2 in	3/8–1 1/4 in, 10–32 mm	Lug nuts, suspension, high torque
3/4 in	3/4–2 in, 19–50 mm	Trucks, farm and heavy equipment

6-point or 12-point? A 6-point socket grips the flats of the nut and is less likely to round it, so use it on tight or rusty fasteners. A 12-point fits on at twice as many angles, which helps in tight spaces.

Allen key (hex key) sizes

Allen keys are also measured across the flats. Socket head cap screws, the round-headed screws in furniture and machines, each take one standard size:

US screw	Allen key	Metric screw	Allen key
#4	3/32 in	M3	2.5 mm
#6	7/64 in	M4	3 mm
#8	9/64 in	M5	4 mm
#10	5/32 in	M6	5 mm
1/4 in	3/16 in	M8	6 mm
5/16 in	1/4 in	M10	8 mm
3/8 in	5/16 in	M12	10 mm
1/2 in	3/8 in	M16	14 mm

Button head and flat head screws often take a size smaller. Flat-pack furniture almost always uses 4, 5 or 6 mm keys.

How to find a nut’s size

Measure across two opposite flats (not corner to corner) with calipers, or hold a ruler against the nut and read it carefully. Then find the nearest size in the charts above: about 12.7 mm is 1/2 in, about 13 mm is metric. If you have a ruler but no calipers, use the mm ruler on your screen after calibrating it, or convert with inches to mm.

More DIY charts: bolt torque · drill bit sizes · thread pitch · nail sizes · lumber sizes.

Frequently asked questions

Is 10 mm the same as 3/8 inch?

No. A 3/8 in wrench is 9.53 mm, almost half a millimeter smaller than 10 mm. It won't fit a 10 mm nut, and a 10 mm wrench on a 3/8 in nut is loose enough to round it. The closest SAE size to 10 mm is 13/32 in, which most sets don't include.

What metric wrench is the same as 3/4 inch?

19 mm. A 3/4 in wrench is 19.05 mm, only 0.05 mm bigger, so the two are interchangeable. Other close pairs: 5/16 in and 8 mm, 7/16 in and 11 mm, 5/8 in and 16 mm, and 1 1/16 in and 27 mm.

What size wrench for a 3/8 inch bolt?

9/16 in for a standard 3/8 in hex bolt. A 1/4 in bolt takes 7/16 in, 5/16 in takes 1/2 in, 1/2 in takes 3/4 in, and 5/8 in takes 15/16 in.

What size Allen key for an M6 socket head screw?

5 mm. Metric socket head cap screws use an Allen key about 0.8 times the screw size: M3 takes 2.5 mm, M4 takes 3 mm, M5 takes 4 mm, M8 takes 6 mm and M10 takes 8 mm.

Is 13 mm the same as 1/2 inch?

Close, but not the same: 1/2 in is 12.70 mm. A 13 mm wrench fits a 1/2 in nut loosely and can round it if the nut is tight. A 1/2 in wrench won't go onto a 13 mm nut at all.