How to Identify a Washer Size from Its Inner and Outer Diameter
Measure the bore first. A metric washer's bore is the bolt size plus a small clearance: 6.4 mm fits M6, 8.4 mm fits M8, 10.5 mm fits M10, 13 mm fits M12, 17 mm fits M16, and 25 mm fits M24. Then measure the outside diameter to find the series. On an M8 washer, 15 mm OD is DIN 433 / ISO 7092 (small), 16 mm is DIN 125 / ISO 7089 (standard) and 24 mm is DIN 9021 / ISO 7093 (large, "fender"). Thickness confirms it.
If none of the metric rows fit, check the inch tables: an SAE 1/2" washer has a 13.5 mm bore, which is easy to mistake for an M12. The tables below cover DIN/ISO M2–M36 and ASME SAE/USS #6–1". They do not cover spring, tooth or square washers, or washers made to a customer drawing.
This page is for the common situation where you have a washer, or a drawing with only a bore and an outside diameter, and need to name the standard before you can reorder or quote it. The dimensions are the nominal values we stamp to every day; the same tables sit behind our washer size finder.
1. Measure it properly
- Bore (inner diameter, d1): use the inside jaws of a caliper and take two readings at 90° to each other. Washer bores are made to an H tolerance (ISO 4759-3), which has no minus side, so a good bore measures at or slightly above the nominal value: 8.4–8.6 mm on an M8 DIN 125 washer is normal.
- Outside diameter (d2): measure across the outer edge with the outside jaws, again in two directions. Stamped edges have a small burr on one face; measure clear of it.
- Thickness (s): use a micrometer or the caliper's outside jaws away from the edge. Thickness is the quickest way to separate two series whose bore and OD are close.
Write all three down before you look anything up. Most misidentified washers were identified on the bore alone.
2. Bore to bolt size (metric)
Metric washer bores follow the medium clearance series. The bore is not the bolt size: an M8 bolt does not take an 8.0 mm washer.
| Bore d1 | Bolt | Small OD DIN 433 / ISO 7092 |
Standard OD DIN 125 / ISO 7089 |
Large OD DIN 9021 / ISO 7093 |
|---|---|---|---|---|
| 2.2 | M2 | 4.5 × 0.3 | 5 × 0.3 | — |
| 2.7 | M2.5 | 5 × 0.5 | 6 × 0.5 | — |
| 3.2 | M3 | 6 × 0.5 | 7 × 0.5 | 9 × 0.8 |
| 4.3 | M4 | 8 × 0.5 | 9 × 0.8 | 12 × 1.0 |
| 5.3 | M5 | 9 × 1.0 | 10 × 1.0 | 15 × 1.2 |
| 6.4 | M6 | 11 × 1.6 | 12 × 1.6 | 18 × 1.6 |
| 8.4 | M8 | 15 × 1.6 | 16 × 1.6 | 24 × 2.0 |
| 10.5 | M10 | 18 × 1.6 | 20 × 2.0 | 30 × 2.5 |
| 13 | M12 | 20 × 2.0 | 24 × 2.5 | 37 × 3.0 |
| 15 | M14 | 24 × 2.5 | 28 × 2.5 | 44 × 3.0 |
| 17 | M16 | 28 × 2.5 | 30 × 3.0 | 50 × 3.0 |
| 21 | M20 | 34 × 3.0 | 37 × 3.0 | — (22 bore, see below) |
| 25 | M24 | 39 × 4.0 | 44 × 4.0 | — (26 bore, see below) |
| 31 | M30 | 50 × 4.0 | 56 × 4.0 | — |
| 37 | M36 | 58 × 5.0 | 66 × 5.0 | — |
Cells read outside diameter × thickness. "—" means that series is not made in that size, or uses a different bore.
Where the bore is not the same across series
Up to M16 the three series share one bore per bolt size. Above that they split, and this is where reorders go wrong:
- M18: DIN 433 uses a 19 mm bore; DIN 9021 uses 20 mm (OD 56, thickness 4.0).
- M20: DIN 125 and DIN 433 use 21 mm; DIN 9021 uses 22 mm (OD 60, thickness 4.0).
- M24: DIN 125 and DIN 433 use 25 mm; DIN 9021 uses 26 mm (OD 72, thickness 5.0).
So a washer with a 25 mm inner diameter is an M24 washer: 44 mm OD and 4 mm thick is DIN 125 / ISO 7089, 39 mm OD is DIN 433 / ISO 7092. A 26 mm bore with a 72 mm OD is the M24 DIN 9021 / ISO 7093 fender washer.
One more trap: grade C washers have a larger bore
ISO 7091 (DIN 126, GB/T 95) is the coarse, product-grade C version of the standard washer. Its outside diameter and thickness match ISO 7089, but the bore is larger: 9 mm on M8 against 8.4 mm. If your M8 washer measures 16 × 1.6 with a 9 mm bore, it is grade C, not a worn grade A part. The ISO washer standards guide explains the grades.
3. Inch washers (SAE and USS)
US washers are sized by bolt diameter in inches under ASME B18.21.1. SAE pattern washers have a narrow OD; USS pattern washers have a wide OD. Neither has a direct metric equivalent, and several sizes sit close enough to metric washers to be mixed up.
| Bolt | SAE bore / OD / thk | SAE in mm | USS bore / OD / thk | USS in mm |
|---|---|---|---|---|
| #10 | 0.219 / 0.500 / 0.049 | 5.6 / 12.7 / 1.24 | — | — |
| 1/4" | 0.281 / 0.625 / 0.065 | 7.1 / 15.9 / 1.65 | 0.312 / 0.734 / 0.065 | 7.9 / 18.6 / 1.65 |
| 5/16" | 0.344 / 0.688 / 0.065 | 8.7 / 17.5 / 1.65 | 0.375 / 0.875 / 0.083 | 9.5 / 22.2 / 2.11 |
| 3/8" | 0.406 / 0.812 / 0.065 | 10.3 / 20.6 / 1.65 | 0.438 / 1.000 / 0.083 | 11.1 / 25.4 / 2.11 |
| 1/2" | 0.531 / 1.062 / 0.095 | 13.5 / 27.0 / 2.41 | 0.562 / 1.375 / 0.109 | 14.3 / 34.9 / 2.77 |
| 5/8" | 0.656 / 1.312 / 0.095 | 16.7 / 33.3 / 2.41 | 0.688 / 1.750 / 0.134 | 17.5 / 44.4 / 3.40 |
| 3/4" | 0.812 / 1.469 / 0.134 | 20.6 / 37.3 / 3.40 | 0.812 / 2.000 / 0.148 | 20.6 / 50.8 / 3.76 |
| 1" | 1.062 / 2.000 / 0.134 | 27.0 / 50.8 / 3.40 | 1.062 / 2.500 / 0.165 | 27.0 / 63.5 / 4.19 |
Basic (nominal) dimensions from ASME B18.21.1. The size finder lists every size from #6 to 1".
Metric and inch washers that look alike
- M10 DIN 125 vs SAE 3/8": bore 10.5 vs 10.3 mm, OD 20 vs 20.6 mm. The calipers barely separate them; thickness does: 2.0 mm metric against 1.65 mm SAE.
- M12 DIN 125 vs SAE 1/2": bore 13 vs 13.5 mm, thickness 2.5 vs 2.41 mm. Here the OD decides: 24 mm metric against 27 mm SAE.
- M20 DIN 125 vs SAE 3/4": bore 21 vs 20.6 mm, OD 37 vs 37.3 mm. Thickness again: 3.0 mm metric against 3.4 mm SAE.
When two readings are this close, measure several pieces and average them. One worn or burred washer is not a sample.
4. What to send when you reorder
Once you have a name for the washer, the rest of the order is short:
- Standard and size, for example "DIN 125 A M8" or "ISO 7093 M10". If you could not match a standard, send bore, OD and thickness with the tolerance you need. A non-standard washer is a normal order for us.
- Grade: 304 (A2) or 316 (A4). The 304 vs 316 guide covers which one your environment needs.
- Quantity and packing. Our washers come in one finish, plain stainless tumble-polished in sawdust; if your drawing calls for another surface, say so.
The full checklist is in How to Specify Stainless Steel Washers for an RFQ.
5. Where HUIHUI fits
We make 304, 316 and 316L flat washers to DIN, ISO and ASME dimensions, and to customer drawings, at our plant in Taozhuang, Jiashan (in production since 2008, ISO 9001:2015, certificate 74326Q00174R001). Standard sizes run on coil-fed JH21 presses; short runs of non-standard sizes are laser-cut so no die is needed. If you send a sample photo with the three measurements, we will tell you which standard it matches before you order. Export orders start at about 600 kg in total, with 1 carton per standard size and 100 kg per custom item.
What we will not do is guess a standard from a single measurement. If the bore, OD and thickness do not agree with one row of a table, we quote it as a drawing part and say so.
FAQ
What size bolt fits a washer with a 25 mm inner diameter?
An M24 bolt. DIN 125 / ISO 7089 and DIN 433 / ISO 7092 washers for M24 have a 25 mm bore; the DIN 125 version is 44 mm OD × 4 mm thick and the DIN 433 version is 39 mm OD × 4 mm. The M24 DIN 9021 / ISO 7093 fender washer has a 26 mm bore and a 72 mm OD.
Why is a washer's inner diameter bigger than the bolt?
Washer bores follow the medium clearance series so the washer slides over the thread and sits square under the head. An M8 washer has an 8.4 mm bore, M10 has 10.5 mm and M12 has 13 mm. Bores are toleranced on the plus side only, so a measured bore slightly above these values is normal.
How do I tell DIN 125 from DIN 433 and DIN 9021?
Up to M16 all three share the same bore, so measure the outside diameter. On M8: 15 mm OD is DIN 433 / ISO 7092, 16 mm is DIN 125 / ISO 7089 and 24 mm is DIN 9021 / ISO 7093. From M18 upward DIN 9021 also has a larger bore (20, 22 and 26 mm for M18, M20 and M24).
Is an SAE 1/2" washer the same as an M12 washer?
No. An SAE 1/2" washer has a 13.5 mm bore, 27 mm OD and 2.41 mm thickness. An M12 DIN 125 washer has a 13 mm bore, 24 mm OD and 2.5 mm thickness. The outside diameter is the quickest way to tell them apart.
My M8 washer has a 9 mm bore. Is it out of tolerance?
Probably not. If the OD is 16 mm and the thickness 1.6 mm, it is most likely an ISO 7091 / DIN 126 (product grade C) washer, which has the same OD and thickness as ISO 7089 but a 9 mm bore on M8.
Can HUIHUI make a washer that matches no standard?
Yes. Send the bore, outside diameter, thickness, grade and quantity, or a drawing. Short runs of non-standard sizes are laser-cut, so no die is needed; larger volumes are stamped. The minimum for a custom item is 100 kg, and export orders start at about 600 kg in total. Contact [email protected].
Not sure which washer you have?
Send a photo with the bore, outside diameter and thickness. We will name the standard, or quote it as a drawing part, within one business day.
Email: [email protected] · WhatsApp: +86 153 5662 8355