What wrench size fits a 1/2 inch nut?

A 1/2 inch bolt uses a hex nut measuring 3/4 inch across the flats, so a 3/4 inch wrench or socket is the correct tool. The nut is named for the bolt thread, not the wrench size.

What size wrench for an M10 nut?

A standard M10 hex nut is 17 mm across the flats. The closest SAE socket is 11/16 inch at 17.46 mm, which is 0.46 mm loose and risks rounding the nut, so use the metric size.

Why is the nut bigger than the bolt size?

The bolt size names the thread diameter. The nut has to surround that thread with enough wall to carry the load, so the across-flats dimension is roughly 1.5 times the bolt diameter.

How thick should a hex nut be?

A standard hex nut is about 0.875 times the bolt diameter thick. A 1/2 inch nut is 0.448 inches. Jam nuts are roughly half that thickness and heavy hex nuts are thicker.

Are SAE and metric nuts interchangeable?

The threads never are. Only the wrench sizes sometimes match closely: 3/8 inch and 10 mm, and 3/4 inch and 19 mm, are close enough to share a socket.

Nut Size Chart

Bolt size to across-flats wrench size and nut thickness, for SAE and metric hex nuts.

Quick answer

A hex nut is named for the bolt thread, not the wrench size. Across the flats is roughly 1.5 × bolt diameter: 1/4" bolt → 7/16" wrench, 3/8" → 9/16", 1/2" → 3/4", 5/8" → 15/16", 3/4" → 1-1/8". In metric: M6 → 10 mm, M8 → 13 mm, M10 → 17 mm, M12 → 19 mm.

SAE hex nuts — ASME B18.2.2

Bolt size	Across flats	Across flats (in)	Nut thickness (in)	Threads per inch (UNC / UNF)
#6	5/16"	0.3125	0.114	32
#8	11/32"	0.3438	0.130	32
#10	3/8"	0.3750	0.130	24
1/4	7/16"	0.4375	0.226	20 / 28
5/16	1/2"	0.5000	0.273	18 / 24
3/8	9/16"	0.5625	0.337	16 / 24
7/16	11/16"	0.6875	0.385	14 / 20
1/2	3/4"	0.7500	0.448	13 / 20
9/16	7/8"	0.8750	0.496	12 / 18
5/8	15/16"	0.9375	0.559	11 / 18
3/4	1-1/8"	1.1250	0.665	10 / 16
7/8	1-5/16"	1.3125	0.776	9 / 14
1	1-1/2"	1.5000	0.887	8 / 12
1-1/8	1-11/16"	1.6875	0.999	7 / 12
1-1/4	1-7/8"	1.8750	1.094	7 / 12

Published standard

Source & method: Values are nominal; commercial tolerance classes vary slightly.

Metric hex nuts — ISO 4032

Bolt size	Across flats (mm)	Nut thickness (mm)	Coarse pitch (mm)	Nearest SAE wrench
M3	5.5	2.4	0.50	7/32"
M4	7	3.2	0.70	9/32"
M5	8	4.7	0.80	5/16"
M6	10	5.2	1.00	3/8"
M8	13	6.8	1.25	1/2"
M10	17	8.4	1.50	11/16"
M12	19	10.8	1.75	3/4"
M14	22	12.8	2.00	7/8"
M16	24	14.8	2.00	15/16"
M20	30	18.0	2.50	1-3/16"
M24	36	21.5	3.00	1-7/16"

Source & method: The nearest SAE column is for wrench selection only. Metric and imperial threads never interchange.

Frequently asked

What wrench size fits a 1/2 inch nut?: A 1/2 inch bolt uses a hex nut measuring 3/4 inch across the flats, so a 3/4 inch wrench or socket is the correct tool. The nut is named for the bolt thread, not the wrench size.
What size wrench for an M10 nut?: A standard M10 hex nut is 17 mm across the flats. The closest SAE socket is 11/16 inch at 17.46 mm, which is 0.46 mm loose and risks rounding the nut, so use the metric size.
Why is the nut bigger than the bolt size?: The bolt size names the thread diameter. The nut has to surround that thread with enough wall to carry the load, so the across-flats dimension is roughly 1.5 times the bolt diameter.
How thick should a hex nut be?: A standard hex nut is about 0.875 times the bolt diameter thick. A 1/2 inch nut is 0.448 inches. Jam nuts are roughly half that thickness and heavy hex nuts are thicker.
Are SAE and metric nuts interchangeable?: The threads never are. Only the wrench sizes sometimes match closely: 3/8 inch and 10 mm, and 3/4 inch and 19 mm, are close enough to share a socket.

Wrenches and nuts

Across-flats sizes from this chart.

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SAE hex nuts — ASME B18.2.2

Metric hex nuts — ISO 4032

Frequently asked

Wrenches and nuts

Wrenches and nuts

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