Last updated July 28, 2026
Work out the bolt length you need: enter the grip — the total thickness of the plies being bolted — and the calculator adds the allowance for the nut, hard flat washers and thread protrusion, then rounds up. The allowances come from the table further down, which has been used in the Australian steel fabrication industry.
Structural bolt length calculator
Metric structural bolts to grade 4.6 or 8.8, with a nut and hard flat washers. Uses the allowance table published below.
A guide only. The allowance covers the nut, hard flat washers and thread protrusion for standard metric structural bolts. Always check against your own shop standard and the bolt lengths your supplier actually stocks.
The allowance table
These are the values the calculator uses. Add the allowance for your bolt size and grade to the grip, then round up to a length your supplier stocks.
| Bolt size | Grade 4.6 | Grade 8.8 |
|---|---|---|
| M12 | 19 | n/a |
| M16 | 23 | 27 |
| M20 | 26 | 31 |
| M24 | 31 | 37 |
| M30 | 35 | 43 |
| M36 | n/a | 49 |
This information is a guide only, and has been used in the Australian steel fabrication industry. Check it against your own shop standard before issuing drawings.
What makes up the length
The bolt has to pass through everything being joined and still have enough thread beyond the nut. Working from the head: the grip (all the plies), then the washers, then the nut, then the thread protruding past it.
Working in inches?
The calculator and the table above are metric, and the allowances in them are Australian fabrication practice. There is no honest way to convert them: an imperial bolt assembly is a different nut, a different washer and a different thread pitch, so the allowance is a different number rather than the same number in other units.
For high-strength bolts to the American series, the equivalent allowances are tabulated in the AISC Steel Construction Manual, in the bolt and threaded part tables in Part 7. The method is identical — grip plus allowance, then round up to a stocked length — only the numbers change.
The one habit that carries across both systems: round up to a length your supplier actually stocks, not to the length the arithmetic gives you. A bolt 5 mm longer that is on the shelf beats an exact one on a six week lead time.
More steel detailing references
- Standard hole & slot sizes for bolts
- Minimum bolt edge distances
- AWS weld symbols chart
- Printable steel detailing chart
References
- Standards Australia — AS 4100:2020, Steel structures — the code behind the metric bolt series
- AISC Steel Construction Manual, Part 7 — the imperial equivalent of the allowance table above
- Minimum bolt edge distances
- Standard hole and slot sizes for bolts
- AWS weld symbols chart