Technically reviewed by Vincent, Founder at HEXVARO
Last reviewed
Quick answer
Quick answer: bolt length allowance by size
Add the allowance below to your grip, then round up to a standard length. It covers one DIN 125 washer, one DIN 934 nut and two thread pitches past the nut.
| Size | Allowance (mm) | Inquiry |
|---|---|---|
| M8 | 10.6 | Quote this size |
| M10 | 13 | Quote this size |
| M12 | 16 | Quote this size |
| M16 | 20 | Quote this size |
| M20 | 24 | Quote this size |
| M24 | 29 | Quote this size |
Bolt length calculator
Enter the total thickness of the parts being clamped. The result is for a through bolt with a nut.
- Minimum length
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- Next standard length
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- Thread past nut
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Bolt length formula
Minimum length = grip + washer thickness + nut height + thread protrusion. Bolt length is measured from under the head to the tip, so the head is not included.
Worked example: an M12 bolt, 30 mm grip and one washer. Add 30 mm + 2.5 mm + 10 mm + 2 × 1.75 mm = 46 mm, then round up to 50 mm.
Allowance by bolt size (mm)
Nut height follows DIN 934 and washer thickness follows DIN 125. Protrusion is two coarse thread pitches, a common workshop rule. Follow your project rule if it differs.
| Size | DIN 125 washer | DIN 934 nut | 2 pitches | Allowance (mm) | Inquiry |
|---|---|---|---|---|---|
| M6 | 1.6 | 5 | 2 | 8.6 | Quote this size |
| M8 | 1.6 | 6.5 | 2.5 | 10.6 | Quote this size |
| M10 | 2 | 8 | 3 | 13 | Quote this size |
| M12 | 2.5 | 10 | 3.5 | 16 | Quote this size |
| M14 | 2.5 | 11 | 4 | 17.5 | Quote this size |
| M16 | 3 | 13 | 4 | 20 | Quote this size |
| M20 | 3 | 16 | 5 | 24 | Quote this size |
| M24 | 4 | 19 | 6 | 29 | Quote this size |
| M30 | 4 | 24 | 7 | 35 | Quote this size |
| M36 | 5 | 29 | 8 | 42 | Quote this size |
Common standard bolt lengths
Commercial lengths are not available in every millimetre. Common steps below are in mm, and the range depends on the size.
- Up to 80: 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80.
- From 80 to 160: steps of 10.
- From 160 to 200: 180 and 200.
- Longer or non-standard lengths are made to order.
Checks before you order
- Coatings: hot-dip galvanized nuts and spring washers can change the stack.
- Partial-thread bolts: check that the thread runs far enough into the grip for the nut to tighten.
- Lock nuts and double nuts: use the real nut height and add the second nut.
- Tapped holes: this calculator does not apply. Use the engagement length from your design.
- Structural bolts: use the allowance tables in your project code.
Order these sizes from HEXVARO
Send your size list with grade, finish and quantity — we reply within 2 working hours with price, lead time and shipping terms.
- Reply: within 2 working hours
- MOQ: 500 pcs per specification
- Lead time: 10–15 days stock items
- Payment: T/T · L/C · PayPal
- Shipping ports: Tianjin · Ningbo · Shanghai · Shenzhen
- Incoterms: EXW · FOB · CFR · CIF · DAP
- Experience: Founder with 10+ years in the fastener industry
Frequently asked questions
How do you calculate bolt length?
Add the grip, washer thickness, nut height and thread protrusion, then round up to the next standard length.
How much thread should show past the nut?
A common workshop rule is about two thread pitches. Project codes can set a different rule, so check yours first.
Does bolt length include the head?
No. Hex bolt length is measured from under the head to the tip of the bolt.
What length bolt do I need for a 30 mm grip?
With an M12 bolt, one washer and one nut, you need at least 46 mm, so order 50 mm.
What is grip length?
Grip is the total thickness of the parts clamped between the bolt head and the washer or nut.