Form B Flat Washers
Form B flat washers are the lighter-gauge flat washer pattern to BS 4320, thinner than the common Form A at many sizes. We supply Form B washers in steel and stainless steel A2 and A4 across a wide range of metric sizes.
Also known as: BS 4320 form B, light flat washers.
Form B Washers dimensions
| Bolt Size | Inside Diameter | Outside Diameter | Thickness |
|---|---|---|---|
| M5 | 5.3mm | 10mm | 0.8mm |
| M6 | 6. 4mm | 12.5mm | 0.8mm |
| M8 | 8.4mm | 17mm | 1.0mm |
| M10 | 10.5mm | 21mm | 1.25mm |
| M12 | 13mm | 24mm | 1.6mm |
| M14 | 15mm | 28mm | 1.6mm |
| M16 | 17mm | 30mm | 2.0mm |
| M18 | 19mm | 34mm | 2.0mm |
| M20 | 21mm | 37mm | 2.0mm |
| M22 | 23mm | 39mm | 2.0mm |
| M24 | 25mm | 44mm | 2.5mm |
| M27 | 28mm | 50mm | 2.5mm |
| M30 | 31mm | 56mm | 2.5mm |
| M33 | 34mm | 60mm | 3.0mm |
| M36 | 37mm | 66mm | 3.0mm |
| M39 | 40mm | 72mm | 3.0mm |
Materials & strength
Light pattern
Thinner than the equivalent Form A washer, reducing stack height in the joint.
Steel & stainless range
Steel covers general work; A2 and A4 stainless suit corrosive environments.
What is a Form B washer?
Form B is the light-pattern flat washer from BS 4320: same general proportions as Form A but a thinner section, used where a lighter washer is specified or where stack height matters.
When would I use Form B rather than Form A?
Choose Form B where the drawing calls for it or where a thinner washer suits the joint; Form A remains the general-purpose default. Check the dimensions tables when substituting one form for the other.
What materials are Form B washers available in?
Form B washers are available in steel and stainless steel A2 and A4 across a wide range of metric sizes.
Need Form B Washers in a non-standard size or material?
Send us your drawing — specials quoted within 24 hours.