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Flat Washer Size Chart: SAE, USS and Metric

Guide · Updated October 28, 2026

A plain flat washer spreads bolt load, protects the part surface and bridges a clearance or slotted hole. Inch washers come in two common patterns that ASME B18.22.1 lists side by side: the narrow (N) series, sold as SAE, with a closer fitting hole and smaller OD, and the wide (W) series, sold as USS, with a larger hole and much larger OD. Metric washers to ISO 7089 are the normal series for hex bolts and screws. The values below are basic dimensions from those standards.

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Step by step

  1. Identify the bolt size: Measure the shank diameter.
  2. Choose narrow or wide: SAE narrow for machinery and closer fit holes; USS wide for wood, soft materials and oversized or slotted holes.
  3. Read ID, OD and thickness: Use the chart for the series you picked.
  4. Check the counterbore or spotface: The washer OD must fit any counterbore or spotface; SAE washers fit far more often.
  5. Check the bearing surface: Use hardened washers (such as F436) under high preload bolts.

Inch flat washers, ASME B18.22.1 (ID / OD / basic thickness)

All values in inches. Thickness varies between a max and min around the basic value; for example 1/4 washers run 0.051 to 0.080 in. The odd 0.734, 1.156 and 1.469 in ODs were chosen so washers are not the size of coins.

Metric flat washers, ISO 7089 (ID / OD / thickness)

ISO 7089 has a chamfer-free design and ISO 7090 is the same washer with a chamfer. ISO 7093 is a large series washer with an OD about three times the bolt, for soft materials.

SAE or USS?

SAE narrow washers have a hole only about 1/32 in larger than the bolt, so they center the bolt and fit inside spotfaces and counterbores. USS wide washers spread load over a larger area and cover sloppy holes, which suits wood, plastics and sheet metal. On a machine part with a normal clearance hole, use SAE. Under the head of a socket head cap screw in a counterbore, a washer is usually left out entirely or a special thin washer is used.

Worked example: bearing area

A 1/2 in SAE washer has bearing area π/4 × (1.062² − 0.531²) = 0.785 × (1.128 − 0.282) = 0.664 in². A 1/2 in USS washer gives 0.785 × (1.891 − 0.316) = 1.236 in², almost twice as much. For a 1/2-13 bolt preloaded to 9,000 lb, bearing stress drops from 13,550 psi to 7,280 psi with the wide washer, which matters on aluminum or wood.

Worked example: fitting a spotface

A 3/8 bolt in a 7/8 in spotface: the SAE washer is 0.812 OD and fits with 0.031 in per side. The USS washer is 1.000 OD and will not fit. For M10, an ISO 7089 washer is 20 mm OD, so the spotface should be about 22 mm.

Common problems

Using USS washers in counterbores, mixing soft and hardened washers on structural bolts, stacking washers to make up length instead of using the right bolt, and using a USS washer under a nut where the larger hole lets it slide off center. Check washer ID with pin gages and thickness with a micrometer.

FAQ

What is the difference between SAE and USS washers?

SAE (narrow) has a smaller ID and OD; USS (wide) has a larger hole and OD for spreading load.

What is the OD of a 1/4 SAE washer?

0.625 in, with a 0.281 in hole.

What is the size of an M8 washer?

ISO 7089: 8.4 mm ID, 16 mm OD, 1.6 mm thick.

How do I check washer thickness?

Check the gage itself before trusting a measurement: gage blocks set mics, calipers and indicators; build any stack free at gageblockcalc.com.

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