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Washer Size Chart
Quick answer: SAE washers are narrower with a tighter hole for machine work; USS washers are wider and thicker for structural bolting. A 1/2″ SAE is 1-1/16″ across, the USS is 1-3/8″.
SAE and USS washer dimensions side by side for every common bolt size, what a washer is really doing in the joint, and an honest assessment of which lock washers work.
Flat washer size chart
Two standards exist side by side and are not interchangeable. SAE washers are narrower with a smaller hole, sized for machine screws and precision work. USS washers are wider and thicker with a looser hole, meant for structural bolting where the extra bearing area matters more than the fit.
| Bolt size | SAE outside dia | USS outside dia | Thickness (in) |
|---|---|---|---|
| #6 | 5/16 | - | 0.036 |
| #8 | 3/8 | - | 0.036 |
| #10 | 7/16 | - | 0.049 |
| 1/4 | 5/8 | 3/4 | 0.065 |
| 5/16 | 11/16 | 7/8 | 0.065 |
| 3/8 | 13/16 | 1 | 0.065 |
| 7/16 | 59/64 | 1-1/4 | 0.065 |
| 1/2 | 1-1/16 | 1-3/8 | 0.095 |
| 5/8 | 1-5/16 | 1-3/4 | 0.095 |
| 3/4 | 1-15/32 | 2 | 0.134 |
| 1 | 2 | 2-1/2 | 0.134 |
A dash means that size is not made in the USS pattern. Inside diameters run slightly over the nominal bolt size in both standards so the bolt passes freely.
What a washer is actually doing
- Spreading load. A bolt head bearing directly on wood or aluminium crushes it. The washer distributes that force across a wider footprint.
- Protecting the surface. A rotating nut galls and scars whatever it turns against. The washer takes that wear instead.
- Bridging an oversized hole. Slotted or oversized holes need a washer large enough to bear on solid material either side.
- Providing a consistent friction surface. Torque specs assume a predictable bearing surface; a washer gives one.
Lock washers and why they mostly do not work
| Type | Mechanism | Verdict |
|---|---|---|
| Split (helical) lock | Spring tension plus a biting edge | Largely ineffective on hardened fasteners — flattens at full torque and adds nothing |
| External tooth | Teeth bite both surfaces | Works on softer materials; damages coatings |
| Internal tooth | Teeth bite, hidden under head | Same, but tidier appearance |
| Wedge-lock pairs | Cam faces resist rotation | Genuinely effective; the current best practice |
| Nylon-insert nut | Nylon grips threads | Effective; single-use, and not for heat |
| Thread-locking fluid | Adhesive fills threads | Effective and cheap; choose the right strength grade |
The split lock washer is the most-used and least-effective option on this list. Under full torque on a hardened bolt it flattens completely and behaves as a slightly springy flat washer. Where vibration is the real problem, wedge-lock pairs or thread-locking fluid do the job it is credited with.
Fender washers
A fender washer has a very large outside diameter relative to its hole — a 1/4″ fender washer might be 1-1/4″ across. They exist to spread load over thin sheet metal or to cover an oversized or damaged hole. The trade-off is that a large thin washer dishes under high torque, so they belong in low-torque, load-spreading applications rather than structural ones.
Frequently asked questions
What is the difference between SAE and USS washers?
SAE washers are narrower with a tighter hole, intended for machine screws and precision assemblies. USS washers are wider and thicker with a looser hole, intended for structural bolting where spreading the load matters more than a close fit. A 1/2 in SAE washer is 1-1/16 in across; the USS is 1-3/8 in.
Do split lock washers actually work?
Largely not, on hardened fasteners. At full torque a split washer flattens completely and behaves as a slightly springy flat washer. Where vibration loosening is a genuine concern, wedge-lock washer pairs, nylon-insert nuts or thread-locking fluid are far more effective.
Which side of a washer faces the nut?
For a plain flat washer it does not matter — both faces are the same. For stamped washers with a slightly rounded and a slightly sharp edge, the rounded (die-broken) side should face the bolt head or nut, and the sharp side toward the work.
Do I need a washer under both the head and the nut?
Under the nut always, since that is the part that rotates. Under the head only if the head also turns during tightening, or if the material underneath is soft enough to need the load spread.