Washer Size Finder: Identify a Washer's Standard and Part Number from Its Inside Diameter, Outside Diameter and Thickness

• ID: inside jaws, rocked gently until the reading is largest. A slightly oval hole reads small one way.
• OD: outside jaws, square across the face. Take two readings 90° apart and use the larger one.
• Thickness: a micrometer is better than calipers. Measure in two or three places away from burrs. Plating typically adds a few ten-thousandths of an inch per side.

• Split lock washer: thickness is the section thickness of the coil, not the height of the twisted washer.
• Tooth lock washer: measure thickness on the flat ring, not across twisted teeth. OD of an external-tooth washer is across the tooth tips.
• Belleville, curved and wave washers: thickness is the material thickness. Lay the washer on a flat plate and measure its height too; enter it in the optional Height box.

Convert nothing: switch the unit button and type what your calipers read. The finder searches inch and metric standards together. A 0.256 in hole that fits nothing in inch tables may be an M6 washer (6.4 mm).

Your washer could be at the low end of its standard, the high end, or slightly outside after wear, plating or being squeezed in a joint. A finder that only matched nominal sizes would miss it. So each candidate gets its own tolerance band from its standard, and the allowance widens that band on both sides.

• Spec limits only: the strictest test. Good for a new washer from a bag.
• ±0.010 in / ±0.25 mm: the default. Covers most used washers.
• ±0.020 to ±0.030 in: for worn, bent or painted parts. Expect more candidates.

Where a standard gives only one value (most ISO and DIN tables here list the nominal size), the band is that single value, and the allowance does all the work. The ISO and DIN tables list the hole as a minimum and the OD as a maximum; a real washer measures a little bigger in the hole and a little smaller outside, so the finder doubles the allowance on those open sides.

Many standards share sizes. NAS1149 may replace AN960, "SAE" washers in trade usage are ASME Type A narrow, and ISO 7089 took over DIN 125. Measuring cannot tell them apart. Choose by what the joint needs:

• Aircraft or military hardware: buy the part number on the drawing, with certs. Do not substitute a commercial washer.
• Material: check for magnetism (carbon steel and 400-series CRES are magnetic; 300-series, brass and aluminum are not) and color.
• Status: a cancelled standard (grey badge) is often still stocked, but its replacement is easier to buy. See card 03.

Format AN960 + coating letter + material letter + size + thickness letter, e.g. AN960XC616LL.

• AN, MS standards: read by hand from the scanned standard sheets on everyspec.com: AN960 Rev 21, AN970 Rev 6, MS15795G, MS9549, MS35338L, MS35333K and MS35335G.
• ISO, DIN, ASME: the nominal size tables in the fasten.it norm database. These standards are sold, not free.
• Cross-check: the ASME Type A plain washer sizes match MS15795 row for row, and the ASME regular lock washer sizes match MS35338.

• Rows with a red note in the detail panel differ from what the source prints, or the source disagrees with itself. Check them against the standard.
• Most ISO and DIN rows give nominal sizes only, not full tolerances.
• Not yet included: NAS1149, NAS620, NAS1515, ISO 10673, JIS B 1256, BS 4320, inch Belleville washers and commercial fender washers (no standard sizes).