Wrench & Socket Size Chart: SAE vs Metric Conversion

Few things are more frustrating than a socket that almost fits. Metric (mm) and SAE (inch) wrenches overlap in size, and a few pairs are close enough to swap — but most are not, and using the wrong one rounds off bolt heads fast. This guide shows every common size in both systems, the exact difference, and when it is safe to substitute.

SAE Wrench Sizes in Millimeters

SAE sizes are fractions of an inch. The table converts each to exact millimeters and shows the nearest metric wrench and the gap between them.

SAE size	Exact mm	Nearest metric	Difference	Swap?
1/4″	6.35 mm	6 mm	0.35 mm	No
5/16″	7.94 mm	8 mm	0.06 mm	Usually OK
3/8″	9.53 mm	10 mm	0.48 mm	No
7/16″	11.11 mm	11 mm	0.11 mm	Loose — avoid on tight fasteners
1/2″	12.70 mm	13 mm	0.30 mm	No
9/16″	14.29 mm	14 mm	0.29 mm	No
5/8″	15.88 mm	16 mm	0.13 mm	Loose — avoid on tight fasteners
11/16″	17.46 mm	17 mm	0.46 mm	No
3/4″	19.05 mm	19 mm	0.05 mm	Usually OK
13/16″	20.64 mm	21 mm	0.36 mm	No
7/8″	22.23 mm	22 mm	0.22 mm	No
15/16″	23.81 mm	24 mm	0.19 mm	Loose — avoid on tight fasteners
1″	25.40 mm	25 mm	0.40 mm	No

Metric Wrench Sizes in Inches

Metric size	Inches	Nearest SAE (1/32″)	Difference	Swap?
6 mm	0.236″	1/4″	0.35 mm	No
7 mm	0.276″	9/32″	0.14 mm	Loose — avoid on tight fasteners
8 mm	0.315″	5/16″	0.06 mm	Usually OK
9 mm	0.354″	11/32″	0.27 mm	No
10 mm	0.394″	13/32″	0.32 mm	No
11 mm	0.433″	7/16″	0.11 mm	Loose — avoid on tight fasteners
12 mm	0.472″	15/32″	0.09 mm	Usually OK
13 mm	0.512″	1/2″	0.30 mm	No
14 mm	0.551″	9/16″	0.29 mm	No
15 mm	0.591″	19/32″	0.08 mm	Usually OK
16 mm	0.630″	5/8″	0.13 mm	Loose — avoid on tight fasteners
17 mm	0.669″	21/32″	0.33 mm	No
18 mm	0.709″	23/32″	0.26 mm	No
19 mm	0.748″	3/4″	0.05 mm	Usually OK
20 mm	0.787″	25/32″	0.16 mm	Loose — avoid on tight fasteners
21 mm	0.827″	13/16″	0.36 mm	No
22 mm	0.866″	7/8″	0.22 mm	No
24 mm	0.945″	15/16″	0.19 mm	Loose — avoid on tight fasteners

Which Sizes Are Truly Interchangeable?

Only a handful of pairs are within about 0.1 mm, which is inside the normal manufacturing clearance of a good wrench:

3/4″ and 19 mm — 0.05 mm apart. The best-known swap.
5/16″ and 8 mm — 0.06 mm apart.
7/8″ and 22 mm are 0.22 mm apart and 1/2″ and 13 mm are 0.30 mm apart — both will feel loose and can slip under load.

Anything marked “No” in the tables will either not fit or will sit loose on the corners of the nut. On a tight or rusted fastener that is how heads get rounded.

How to Tell If a Bolt Is Metric or SAE

Where it came from: most cars, bikes and appliances built since the 1980s are metric. Older US machinery and some US trucks and tools still use SAE.
Head markings: metric bolts often show a strength number such as 8.8 or 10.9; SAE bolts use radial lines (3 lines = grade 5, 6 lines = grade 8).
Measure it: measure the head across the flats with calipers. A reading near a whole millimeter (for example 13.0 mm) means metric; near a fraction of an inch (12.7 mm = 1/2″) means SAE.

6-Point vs 12-Point Sockets

A 6-point socket grips the flat sides of the nut and is less likely to round it — use it for tight, rusted or high-torque fasteners. A 12-point socket is easier to fit in tight spaces because it engages at more angles, but it grips closer to the corners. If you have to use a near-size socket, a 6-point is the safer choice.

Frequently Asked Questions

Is a 19 mm socket the same as 3/4 inch?

Almost. 3/4″ is 19.05 mm, only 0.05 mm larger, so they are usually interchangeable.

What SAE size is closest to 10 mm?

10 mm is 0.394″. The closest SAE sizes are 3/8″ (9.53 mm, too small) and 13/32″ (10.32 mm, rarely sold). Use a 10 mm wrench.

Can I use a 1/2 inch socket on a 13 mm bolt?

No. 1/2″ is 12.70 mm, which is 0.30 mm too small for a 13 mm head. The next size up, 9/16″ (14.29 mm), is far too loose.

Wrench and Socket Size Chart: SAE vs Metric