Washer Size Chart: SAE, USS and Metric Washers

The inside diameter, outside diameter and thickness of SAE, USS and metric flat washers, how SAE and USS washers differ, fender and lock washer sizes, and how to pick and measure a washer.

Updated

Three washers for a 1/4 inch bolt drawn to scale: a small SAE washer, a wider USS washer and a large fender washer, with their outside diameters labeled

Washers are sold by the bolt size they fit, not by their actual hole. A “1/4 inch” washer has a hole a little over 1/4 inch so the bolt slides through, and its outside diameter depends on the type. In the US there are two common series of flat washers, SAE (narrow) and USS (wide); the rest of the world uses metric washers to DIN 125 / ISO 7089.

SAE flat washer sizes

Bolt size Inside diameter (in) Outside diameter (in) Thickness (in) ID × OD (mm)
#6 0.156 0.375 0.049 4.0 × 9.5
#8 0.188 0.438 0.049 4.8 × 11.1
#10 0.219 0.500 0.049 5.6 × 12.7
1/4 0.281 0.625 0.065 7.1 × 15.9
5/16 0.344 0.688 0.065 8.7 × 17.5
3/8 0.406 0.812 0.065 10.3 × 20.6
7/16 0.469 0.922 0.065 11.9 × 23.4
1/2 0.531 1.062 0.095 13.5 × 27.0
9/16 0.594 1.156 0.095 15.1 × 29.4
5/8 0.656 1.312 0.095 16.7 × 33.3
3/4 0.812 1.469 0.134 20.6 × 37.3
7/8 0.938 1.750 0.134 23.8 × 44.5
1 1.062 2.000 0.134 27.0 × 50.8

USS flat washer sizes

Bolt size Inside diameter (in) Outside diameter (in) Thickness (in) ID × OD (mm)
1/4 0.312 0.734 0.065 7.9 × 18.6
5/16 0.375 0.875 0.083 9.5 × 22.2
3/8 0.438 1.000 0.083 11.1 × 25.4
7/16 0.500 1.250 0.083 12.7 × 31.8
1/2 0.562 1.375 0.109 14.3 × 34.9
9/16 0.625 1.469 0.109 15.9 × 37.3
5/8 0.688 1.750 0.134 17.5 × 44.5
3/4 0.812 2.000 0.148 20.6 × 50.8
7/8 0.938 2.250 0.165 23.8 × 57.2
1 1.062 2.500 0.165 27.0 × 63.5

SAE and USS sizes are the nominal values of ASME B18.22.1 (Type A plain washers). Real washers vary by a few thousandths of an inch.

Which to use? SAE washers fit snugly and look neat on machinery, cars and in counterbores. USS washers have a wider face and a looser hole: better for wood, decks and fence work, where they stop the nut from sinking in.

Metric washer sizes

Bolt DIN 125 / ISO 7089 (ID × OD × thick) Large, DIN 9021 (ID × OD × thick)
M3 3.2 × 7 × 0.5 mm 3.2 × 9 × 0.8 mm
M4 4.3 × 9 × 0.8 mm 4.3 × 12 × 1.0 mm
M5 5.3 × 10 × 1.0 mm 5.3 × 15 × 1.2 mm
M6 6.4 × 12 × 1.6 mm 6.4 × 18 × 1.6 mm
M8 8.4 × 16 × 1.6 mm 8.4 × 24 × 2.0 mm
M10 10.5 × 20 × 2.0 mm 10.5 × 30 × 2.5 mm
M12 13 × 24 × 2.5 mm 13 × 37 × 3.0 mm
M16 17 × 30 × 3.0 mm 17 × 50 × 3.0 mm
M20 21 × 37 × 3.0 mm 22 × 60 × 4.0 mm
M24 25 × 44 × 4.0 mm 26 × 72 × 5.0 mm

The DIN 125 washer is the standard one in any metric hardware box. DIN 9021 is the metric equivalent of a fender washer: about three times the bolt diameter across.

Fender washers

Fender washers have a normal hole and a very wide face. They’re sold by bolt size and outside diameter:

Size Outside diameter Common uses
#10 × 1 in 1 in (25 mm) Sheet metal, plastic panels
1/4 × 1 in, 1/4 × 1 1/4 in 1 to 1 1/4 in (25–32 mm) Car body panels, plywood, tarps
5/16 × 1 1/2 in 1 1/2 in (38 mm) Trailers, soft wood
3/8 × 1 1/2 in, 3/8 × 2 in 1 1/2 to 2 in (38–51 mm) Decks, docks, heavy panels

Lock washers

A split lock washer is a ring cut through and slightly twisted, so it bites into the nut and the surface. They’re sized by bolt just like flat washers: a 1/4 inch split lock washer fits a 1/4 inch bolt and is about 0.487 in (12.4 mm) across. Internal and external tooth lock washers grip with small teeth instead, and are common on electrical connections. Many engineers now prefer a properly torqued bolt, a nyloc nut or thread locker over a lock washer; see the bolt torque chart.

How to measure a washer

  1. Measure the hole (inside diameter) with calipers or against a ruler at actual size. Subtract a little: a 0.281 in hole is a 1/4 inch washer.
  2. Measure across (outside diameter) to tell SAE from USS: 0.625 in is SAE 1/4, 0.734 in is USS 1/4.
  3. Metric or inch? If the hole is a round number plus a few tenths of a millimeter (6.4, 8.4, 10.5 mm), it’s metric. Use mm to inches to compare.

For bolt sizes and the matching wrench, see the screw and bolt size chart and the wrench size chart.

More DIY charts: bolt torque · tap drill sizes · number and letter drill sizes · nail sizes · thread pitch.

Frequently asked questions

What is the difference between SAE and USS washers?

For the same bolt, a USS washer is wider, thicker and has a slightly bigger hole. A 1/4 inch SAE washer is 0.625 in across with a 0.281 in hole; a 1/4 inch USS washer is 0.734 in across with a 0.312 in hole. SAE washers suit machinery and close-fitting holes; USS washers spread the load better on wood.

What size is a 1/4 inch washer?

A 1/4 inch washer fits a 1/4 inch bolt, so its hole is a little bigger than 1/4. An SAE 1/4 washer has a 0.281 in (7.1 mm) hole and 0.625 in (15.9 mm) outside diameter; a USS 1/4 washer has a 0.312 in (7.9 mm) hole and 0.734 in (18.6 mm) outside diameter.

What size washer for an M8 bolt?

A standard M8 washer (DIN 125 / ISO 7089) has an 8.4 mm hole, 16 mm outside diameter and is 1.6 mm thick. For soft materials, a large M8 washer (DIN 9021) is 24 mm across.

Are washer sizes measured by the hole or the outside?

By the bolt they fit. A washer sold as 3/8 inch or M10 fits a 3/8 inch or M10 bolt; its actual hole is slightly larger, and the outside diameter depends on the type. Fender washers add the outside size too, like 1/4 × 1 1/4 inch.

What is a fender washer used for?

Spreading the load over a wide area so the bolt head doesn't pull through soft or thin material: sheet metal, plastic, plywood, drywall anchors and car body panels. A typical 1/4 inch fender washer is 1 to 1 1/4 inches across.

More measuring guides