Fastener Size Guide
How to Read a Fastener Size
Fastener sizes look intimidating until you know what each part means. Here's the quick decoder.
Example: M8 × 30
- M - metric thread (ISO standard, used worldwide outside the US)
- 8 - thread diameter in millimetres (the diameter of the threaded shank, not the head)
- 30 - length in millimetres (measured from under the head to the tip, for hex and pan/csk heads)
So an M8 × 30 hex bolt has an 8mm-diameter thread and is 30mm long under the head.
Thread diameters we stock
| M-code | Diameter | Common uses |
|---|---|---|
| M3 | 3mm | Light electronics, small panel work |
| M4 | 4mm | Cabinet, electrical, light machinery |
| M5 | 5mm | General assembly, garden equipment |
| M6 | 6mm | Automotive, furniture, light structural |
| M8 | 8mm | Decking, fencing, machinery, marine |
| M10 | 10mm | Structural timber, heavy frames, marine |
| M12 | 12mm | Heavy structural, anchor bolting |
| M16 | 16mm | Industrial, heavy lifting fixtures |
Lengths
Length is in millimetres, measured from under the head to the tip for bolts and most screws. For countersunk heads (which sit flush), length is measured from the top of the head to the tip (so the full overall length).
Common lengths in our range: 8, 10, 12, 16, 20, 25, 30, 40, 50, 60, 75, 80, 100mm.
Nuts and washers
Nuts and washers only have a diameter - no length. An M10 nut fits an M10 bolt. An M10 washer has a 10mm internal hole.
Wood and self-tapping screws
Wood screws and self-tapping screws don't have a metric "M" thread spec - they're sized by nominal diameter and length:
- 4.0 × 30 - a 4mm-diameter wood screw, 30mm long
- 3.5 × 19 - a 3.5mm self-tapping screw, 19mm long
We file them under the closest M-code in our filters (e.g. 4.0 wood screws show under "M4" so they're easy to find alongside machine screws of the same diameter).
Our SKU codes
Every Simfix product has a SKU like 7985A203008ZPK10-SIM. Decoded:
- 7985 - the DIN standard (DIN 7985)
- A2 - stainless steel grade
- 03008 - M3 × 8mm size
- Z - zinc-finish flag (we only stock plain stainless, but the code is retained for industry compatibility)
- PK10 - pack of 10 (the DIY pack)
- SIM - Simfix brand prefix
The Trade pack of the same product changes PK10 to PK100 (or whatever the Trade pack quantity is).