Hex Nut & Washer Size Chart — ASME B18.2.2 & B18.22.1 Dimensions
Hex nut and washer dimensions per ASME B18.2.2 and B18.22.1: width across flats, thickness, washer ID and OD, with computed metric columns.
Updated August 22, 2026
The width across flats of a hex nut IS the wrench you grab — a 1/2″ bolt takes a finished hex nut measuring 3/4″ across the flats, so you reach for a 3/4″ wrench (or its 19 mm near-twin, computed in the metric column). ASME B18.2.2 fixes those basics: finished hex nuts are the everyday standard, while heavy hex nuts run wider and thicker at every size and are the default for structural bolting (A563/A194 grades ride on the heavy pattern). The across-corners column is pure geometry — flats ÷ cos 30° — and is what determines whether the nut clears a socket or a counterbore.
Plain washers live in a separate standard, ASME B18.22.1 (folded into B18.21.1 since 2009), and come in two preferred series: Narrow — the “SAE” washer — and Wide — the “USS” washer. At the same bolt size the wide series can have nearly double the outside diameter and a thicker section, which is the entire point: more bearing area over softer material. Washer thicknesses in the standard are nominal with famously loose commercial tolerances (a 0.065 in washer may legally measure 0.051 to 0.080 in), so never stack washers as precision shims.
Finished Hex Nuts — ASME B18.2.2 (Basic Dimensions)
| Bolt size | Width across flats — wrench size (in) | Wrench size (mm, computed) | Across corners (in, computed) | Thickness, basic (in) |
|---|---|---|---|---|
| 1/4″ | 7/16″ (0.4375) | 11.1 | 0.505 | 7/32″ (0.219) |
| 5/16″ | 1/2″ (0.5000) | 12.7 | 0.577 | 17/64″ (0.266) |
| 3/8″ | 9/16″ (0.5625) | 14.3 | 0.650 | 21/64″ (0.328) |
| 7/16″ | 11/16″ (0.6875) | 17.5 | 0.794 | 3/8″ (0.375) |
| 1/2″ | 3/4″ (0.7500) | 19.0 | 0.866 | 7/16″ (0.438) |
| 9/16″ | 7/8″ (0.8750) | 22.2 | 1.010 | 31/64″ (0.484) |
| 5/8″ | 15/16″ (0.9375) | 23.8 | 1.083 | 35/64″ (0.547) |
| 3/4″ | 1-1/8″ (1.1250) | 28.6 | 1.299 | 41/64″ (0.641) |
| 7/8″ | 1-5/16″ (1.3125) | 33.3 | 1.516 | 3/4″ (0.750) |
| 1″ | 1-1/2″ (1.5000) | 38.1 | 1.732 | 55/64″ (0.859) |
Basic (nominal) dimensions; the standard tolerances run a few thousandths under the basics. Across-corners computed as flats ÷ cos 30°; mm computed as 25.4 × in. Jam nuts share these flats at roughly half the thickness.
Heavy Hex Nuts — ASME B18.2.2 (Basic Dimensions)
| Bolt size | Width across flats — wrench size (in) | Wrench size (mm, computed) | Across corners (in, computed) | Thickness, basic (in) |
|---|---|---|---|---|
| 1/4″ | 1/2″ (0.5000) | 12.7 | 0.577 | 15/64″ (0.234) |
| 5/16″ | 9/16″ (0.5625) | 14.3 | 0.650 | 19/64″ (0.297) |
| 3/8″ | 11/16″ (0.6875) | 17.5 | 0.794 | 23/64″ (0.359) |
| 7/16″ | 3/4″ (0.7500) | 19.0 | 0.866 | 27/64″ (0.422) |
| 1/2″ | 7/8″ (0.8750) | 22.2 | 1.010 | 31/64″ (0.484) |
| 9/16″ | 15/16″ (0.9375) | 23.8 | 1.083 | 35/64″ (0.547) |
| 5/8″ | 1-1/16″ (1.0625) | 27.0 | 1.227 | 39/64″ (0.609) |
| 3/4″ | 1-1/4″ (1.2500) | 31.8 | 1.443 | 47/64″ (0.734) |
| 7/8″ | 1-7/16″ (1.4375) | 36.5 | 1.660 | 55/64″ (0.859) |
| 1″ | 1-5/8″ (1.6250) | 41.3 | 1.876 | 63/64″ (0.984) |
Heavy hex basic thickness equals the nominal bolt diameter minus 1/64″ at every size (test-enforced identity). Heavy pattern is the standard for structural (A325/A490 with A563 nuts) and high-temperature (A194) bolting.
Type A Plain Washers — ASME B18.22.1 Preferred Series (Basic Dimensions)
| Bolt size | Series | ID (in) | OD (in) | OD (mm, computed) | Nominal thickness (in) |
|---|---|---|---|---|---|
| 1/4″ | Narrow | 0.281 | 0.625 | 15.9 | 0.065 |
| 1/4″ | Wide | 0.312 | 0.734 | 18.6 | 0.065 |
| 5/16″ | Narrow | 0.344 | 0.688 | 17.5 | 0.065 |
| 5/16″ | Wide | 0.375 | 0.875 | 22.2 | 0.083 |
| 3/8″ | Narrow | 0.406 | 0.812 | 20.6 | 0.065 |
| 3/8″ | Wide | 0.438 | 1.000 | 25.4 | 0.083 |
| 7/16″ | Narrow | 0.469 | 0.922 | 23.4 | 0.065 |
| 7/16″ | Wide | 0.500 | 1.250 | 31.8 | 0.083 |
| 1/2″ | Narrow | 0.531 | 1.062 | 27.0 | 0.095 |
| 1/2″ | Wide | 0.562 | 1.375 | 34.9 | 0.109 |
| 5/8″ | Narrow | 0.656 | 1.312 | 33.3 | 0.095 |
| 5/8″ | Wide | 0.688 | 1.750 | 44.4 | 0.134 |
| 3/4″ | Narrow | 0.812 | 1.469 | 37.3 | 0.134 |
| 3/4″ | Wide | 0.812 | 2.000 | 50.8 | 0.148 |
| 7/8″ | Narrow | 0.938 | 1.750 | 44.4 | 0.134 |
| 7/8″ | Wide | 0.938 | 2.250 | 57.1 | 0.165 |
| 1″ | Narrow | 1.062 | 2.000 | 50.8 | 0.134 |
| 1″ | Wide | 1.062 | 2.500 | 63.5 | 0.165 |
Narrow = “SAE” washers, Wide = “USS” washers (trade names; the standard calls them Type A N and W). Thickness is nominal with wide commercial tolerance — 0.065 nominal legally spans 0.051–0.080 in. mm computed as 25.4 × in.
Sources & Further Reading
- ASME B18.2.2 (Nuts for General Applications) as reproduced in Birmingham Fastener and Eugene Fastener finished-hex-nut charts and AFT Fasteners’ heavy-hex-nut table; boltingspecialist.com and bigboltnut.com B18.2.2 min–max tables as independent brackets of every basic value
- ASME B18.22.1-1965 Type A plain washers (incorporated into ASME B18.21.1-2009) as reproduced in the Amesweb preferred-sizes chart, the Brighton-Best International SAE flat washer product data sheet (narrow series), and the Southern Fasteners USS Type A Wide sheet — every cell verified in at least two
- Across-corners = flats ÷ cos 30°; mm = 25.4 × in (computed, never transcribed)
Frequently Asked Questions
What wrench fits which bolt?
Read the width-across-flats column: a finished hex nut takes a wrench 1.5× the bolt diameter at 1/2″ and above (3/4″ wrench for 1/2″ bolt, 1-1/8″ for 3/4″), while the smallest sizes run slightly larger than 1.5× (7/16″ wrench for a 1/4″ bolt). Heavy hex nuts take the next wrench up — 7/8″ for a 1/2″ structural bolt. The computed mm column shows why a 19 mm socket “works” on a 3/4″ nut: 19.05 mm is the exact conversion.
What is the difference between a heavy hex nut and a finished hex nut?
Width and thickness, not thread: at every size the heavy pattern is wider across the flats (7/8″ vs 3/4″ at 1/2″ bolt size) and thicker (its basic thickness is the bolt diameter minus 1/64″). The extra section exists for higher-strength and structural service — A563 grades for A325/A490 structural bolts and A194 nuts for high-temperature studs are all heavy hex. If a nut came off industrial equipment, odds are it is heavy pattern.
SAE vs USS washers — which one is the standard washer?
Both are; they are the Narrow and Wide preferred series of ASME B18.22.1 Type A. “SAE” (narrow) washers have smaller holes and ODs — better fit on machinery with precisely sized bolts. “USS” (wide) washers have looser holes and much larger ODs — better load spreading on wood and oversized holes. At 3/8″ the difference is 0.812 vs 1.000 in OD; by 1″ it is 2.0 vs 2.5 in.
Why is my “flat washer” not flat and not the thickness the chart says?
Because B18.22.1 thicknesses are nominal with intentionally wide commercial tolerances — a nominal 0.065 in washer is in spec anywhere from 0.051 to 0.080 in, and stamping leaves burrs and dish. That is fine for load spreading, which is the washer’s job. When thickness actually matters (shimming, preload stack-ups), specify machined or hardened washers (ASTM F436 for structural) instead of commercial Type A.
Try the Calculators
Hex Nut & Washer Size Chart — ASME B18.2.2 & B18.22.1 Dimensions — reuven.tools/reference/hex-nut-washer-dimensions — verified against: ASME B18.2.2 (Nuts for General Applications) as reproduced in Birmingham Fastener and Eugene Fastener finished-hex-nut charts and AFT Fasteners’ heavy-hex-nut table; boltingspecialist.com and bigboltnut.com B18.2.2 min–max tables as independent brackets of every basic value; ASME B18.22.1-1965 Type A plain washers (incorporated into ASME B18.21.1-2009) as reproduced in the Amesweb preferred-sizes chart, the Brighton-Best International SAE flat washer product data sheet (narrow series), and the Southern Fasteners USS Type A Wide sheet — every cell verified in at least two; Across-corners = flats ÷ cos 30°; mm = 25.4 × in (computed, never transcribed)