Washer Size Table
| Nominal | Hole d1 (mm) | OD d2 (mm) | Thickness H (mm) |
|---|---|---|---|
| M2 | 2.2 | 5 | 0.3 |
| M3 | 3.2 | 7 | 0.5 |
| M4 | 4.3 | 9 | 0.8 |
| M5 | 5.3 | 10 | 1 |
| M6 | 6.4 | 12 | 1.6 |
| M8 | 8.4 | 16 | 1.6 |
| M10 | 10.5 | 20 | 2 |
| M12 | 13 | 24 | 2.5 |
| M16 | 17 | 30 | 3 |
| M20 | 21 | 37 | 3 |
| M24 | 25 | 44 | 4 |
| M30 | 31 | 56 | 4 |
| M36 | 37 | 66 | 5 |
A flat washer has three numbers - inside hole, outside diameter, thickness - and the metric standard (ISO 7089, the normal series) ties all three to the nominal screw size in a rigid ladder. The hole is deliberately bigger than the bolt: an M10 washer has a 10.5 mm hole, leaving half a millimeter of clearance so the threaded shank slips through without shaving its threads on the edge. The outside diameter runs about twice the screw size (M10: 20 mm), which is the load-spreading job in one number - double the contact area under the nut or head, so soft materials stop embedding and painted surfaces stop digging in.
Thickness climbs a coarse staircase (0.3 mm at M2 up to 5 mm at M36), with a long flat shelf: M6, M8 share 1.6 mm, M16 through M30 share 3 to 4 mm. That plateau exists because washer stiffness scales with thickness cubed - beyond a point, adding thickness buys little spreading and a lot of cost. This table is the metric normal series; US practice splits the same job into two standards (USS, the wide thick pattern, and SAE, the narrow thin one), so an inch-world part store asks one extra question - which of the two washers - that the metric single series simply eliminates.
How to use
- Match by nominal size, not by eyeball: an M8 washer fits any M8 bolt regardless of head type or thread pitch - washers track the diameter only, which is why the table has no pitch column.
- Check fit with the clearance rule: hole minus bolt should feel like a slack slide fit (0.2 to 1 mm depending on size). A hole that only just passes means you have a reduced-series or fender washer, not the normal series.
- For soft or thin materials, go wider, not thicker: a fender (penny) washer keeps the standard hole but blows up the outside diameter - that is the part that stops a bolt head pulling through sheet metal or plastic.
Frequently asked questions
Why is the washer hole bigger than the bolt?
Clearance by design: the hole (d1) runs 0.2 to 1 mm over the nominal size - M2: 2.2 mm hole, M10: 10.5 mm, M36: 37 mm - and the gap grows with size because thread crests burr and hot-dip galvanizing adds real thickness to a bolt shank. A washer that fit tight would shave plating into the joint and fight the bolt on every assembly; the slack lets the washer seat flat on the surface while the bolt floats in the hole. The same slack is why a washer can rotate freely under the nut head for torque checks - and why a hole that requires a push is your clue you grabbed a reduced or specialty series rather than the ISO normal series in the table.
Does a flat washer stop a bolt from loosening?
No - and this is the most common misuse in the drawer. A plain washer distributes load; it does nothing about rotation. Locking is a separate job done by a nylon-insert nut, a toothed flange, thread adhesive, or a flanged bolt. The classic split (spring) lock washer deserves special distrust: its helical cut is supposed to bite both surfaces, but controlled testing - including NASA static-joint studies - found negligible preload retention versus plain washers, which is why serious engineering standards moved locking to the nut side (all-metal or nylon inserts) and treat split washers as decoration. The table's washers are the plain series: buy them for spreading, add locking somewhere else, and never count on stacking two jobs onto one part.
What is the difference between USS and SAE washers?
Two American series for the same bolt: USS (United States Standard) is the wide, thick pattern for general construction and lag bolts; SAE is narrower and thinner for machine work, closer in feel to the metric normal series. A 1/2 in USS washer might measure 1-3/8 in across with 0.12 in thickness while the SAE twin sits at 1-1/16 in and 0.08 in - same 1/2 in hole. The metric world collapsed this choice: ISO 7089 is one normal series, and where a wider footprint is needed you step to a named special (fender, or the flanged washer built into flange nuts) rather than a parallel standard. Practical upshot for a parts drawer: if the hardware came from metric sources, the table's numbers are complete; if it is inch hardware, identify the series before assuming a thickness.
Why is the outside diameter about twice the bolt size?
Because the washer's job is bearing-area arithmetic: the nut or bolt head already covers some radius around the hole, and the washer extends the loaded ring outward to roughly double the nominal diameter, cutting surface pressure roughly in half or better on the new material it reaches. The ratio actually narrows as sizes grow - about 2.5x at M2, 2x at M10, 1.83x at M36 - because large bolts deliver torque on proportionally stronger substrates, while tiny screws are the ones that dig into wood and plastic. When a catalog lists a reduced outside diameter for weight, or a fender washer triples the diameter for sheet metal, both are re-running this same trade: contact area versus material and space, with the normal series in the table as the default compromise.