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Views: 0 Author: Site Editor Publish Time: 2026-08-29 Origin: Site
Washer size | Inner diameter (ID) | Outer diameter (OD) | Thickness (t) | Fits bolt |
|---|---|---|---|---|
M3 | 3.2 mm | 7 mm | 0.5 mm | M3 |
M4 | 4.3 mm | 9 mm | 0.8 mm | M4 |
M5 | 5.3 mm | 10 mm | 1.0 mm | M5 |
M6 | 6.4 mm | 12 mm | 1.6 mm | M6 |
M8 | 8.4 mm | 16 mm | 1.6 mm | M8 |
M10 | 10.5 mm | 20 mm | 2.0 mm | M10 |
M12 | 13.0 mm | 24 mm | 2.5 mm | M12 |
M16 | 17.0 mm | 30 mm | 3.0 mm | M16 |
M20 | 21.0 mm | 37 mm | 3.0 mm | M20 |
M24 | 25.0 mm | 44 mm | 4.0 mm | M24 |
Values per ISO 7089 (flat washers, normal series, tolerance A). ISO 7090 is the same but with a chamfered (rounded) outer edge; ISO 7091 is a lighter series.
Type | Standard | Job | Typical use |
|---|---|---|---|
Flat washer | ISO 7089 / DIN 125 | Spread load, protect surface | General assembly |
Spring washer | DIN 127 / GB/T 93 | Prevent loosening from vibration | Machinery, vehicles |
Fender washer | DIN 9021 / ISO 7093 | Very large OD for soft materials | Sheet metal, roofing, plastics |
Split lock washer | DIN 127 (split) | Extra locking under the nut | Light-to-medium vibration |
Tooth lock washer | DIN 6797 | Locking into the mating surface | Small screws, electrical |
Sealing washer | — | Seal against water/dust | Outdoor, plumbing, solar mounting |
Match the ID to the bolt — an M8 washer (8.4 mm ID) fits an M8 bolt. If you need to size from the bolt, the ID is always the bolt diameter plus a small clearance.
Choose the OD by surface — normal series for standard steel joints; fender washers (large OD) when the bolt head would otherwise pull through soft material like aluminium or wood.
Stacking washers — using two flat washers is acceptable for alignment, but washers do not increase joint strength. If the bolt is too long, use a shorter bolt or add a spacer — not a stack of washers.
Material matters — zinc-plated steel for dry indoor use; stainless 304 for moisture; 316 for marine or chemical exposure. Never use a zinc washer with a stainless bolt in a wet environment (galvanic corrosion).
Inch flat washers are specified differently: 5/16" washer for a 5/16" bolt, with ID/OD in inches. Metric and inch washers are not interchangeable — an M8 washer (8.4 mm ID) is slightly larger than a 5/16" washer (7.9 mm ID) and slightly smaller than a 3/8" washer (9.5 mm ID). Always match the washer system to the bolt system.
Q1: What size washer do I need for an M8 bolt? An M8 washer: 8.4 mm inner diameter, 16 mm outer diameter, 1.6 mm thick (ISO 7089).
Q2: Should the washer ID match the bolt exactly? No — the washer ID must be slightly larger than the bolt diameter so it sits flat, not on the thread's radius.
Q3: What's the difference between a flat washer and a spring washer? Flat washers spread load and protect surfaces; spring washers resist loosening from vibration.
Q4: Can I use a washer as a spacer? Not ideally. Multiple washers can align a joint, but a proper spacer or a shorter bolt is the correct fix for length issues.
Q5: What is a fender washer? A washer with a much larger outer diameter, used to prevent pull-through on soft materials.
Q6: Do I need stainless washers with stainless bolts? Yes — mixing zinc-plated steel washers with stainless bolts causes galvanic corrosion in wet environments.
Ningbo Weifeng Fastener Co., Ltd. supplies flat washers (ISO 7089/7090, DIN 125, GB/T 97.1/97.2), spring washers, fender washers and sealing washers in M3–M64, zinc plated, hot-dip galvanized or stainless steel (304/316) — browse our flat washer range or send us your specification for a quote within 24 hours.
Sources: ISO 7089/7090 (flat washers), ISO 7091, DIN 125/127/9021, GB/T 97.1/93/96, ANSI/ASME B18.22.1 (inch washers).
