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Wrench & Socket Size Equivalents — SAE & Metric

Inch (SAE) wrench sizes converted to millimeters, with the nearest metric size and how close the fit is. Use a substitute only when the difference is very small.

Ref MRO-0090

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Wrench and socket sizes are the distance across the flats of the fastener head or nut. Inch sizes are in fractions of an inch. Metric sizes are in whole millimeters. A few pairs are almost the same size. Most are not. A loose wrench rounds the corners of the hex and can slip, which damages parts and hurts hands.

How to use it

Find the inch size in the chart. The difference column shows how much bigger (+) or smaller (−) the inch size is than the nearest metric size.
A difference under about 0.1 mm is close enough for light use in a pinch.
If the metric wrench is smaller than the head, it will not fit. If it is larger, it will be loose.
On tight, rusted or high-torque fasteners, always use the exact size, preferably a 6-point socket.

Equivalents chart

Inch size	Inch size (mm)	Nearest metric	Difference (mm)
1/4	6.35	6 mm	+0.35
5/16	7.94	8 mm	−0.06
3/8	9.53	10 mm	−0.47
7/16	11.11	11 mm	+0.11
1/2	12.70	13 mm	−0.30
9/16	14.29	14 mm	+0.29
5/8	15.88	16 mm	−0.12
11/16	17.46	17 mm	+0.46
3/4	19.05	19 mm	+0.05
13/16	20.64	21 mm	−0.36
7/8	22.23	22 mm	+0.23
15/16	23.81	24 mm	−0.19
1	25.40	25 mm	+0.40
1-1/16	26.99	27 mm	−0.01
1-1/4	31.75	32 mm	−0.25
1-5/16	33.34	33 mm	+0.34
1-3/8	34.93	35 mm	−0.07
1-1/2	38.10	38 mm	+0.10

Key points

Closest pairs: 1-1/16 in and 27 mm, 3/4 in and 19 mm, 5/16 in and 8 mm, and 1-3/8 in and 35 mm.
The wrench size is not the bolt size. A 1/2 in hex bolt usually takes a 3/4 in wrench. Metric heads vary by standard; an M10 bolt may take a 16 mm or 17 mm wrench.
Heavy hex nuts and structural bolts use larger wrench sizes than standard hex for the same bolt diameter.
Worn or spread wrenches fit loosely even at the right size. Replace them.

Common mistakes

Using a 13 mm wrench on a 1/2 in head because it "almost fits."
Choosing a wrench by bolt diameter instead of measuring the head.
Using a 12-point socket on a seized fastener.

Governing standards: No single standard sets these equivalents. Inch hex head dimensions are in ASME B18.2.1 and B18.2.2; metric in ASME B18.2.3 series and ISO 4032 and ISO 4014.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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