Bolt Size Calculator
Enter the load, whether it pulls along the bolts or shears across them, the bolt grade, the number of bolts and a safety factor. The calculator picks the smallest metric bolt that carries it and gives the stress area, how much of the bolt is used and a tightening torque.
- Bolt size
- Stress area
- Tightening torque
Shear here means the load crosses the bolt shank. Threads in the shear plane reduce strength further.
A bolt's strength comes from its stress area and its grade. Grade 8.8 bolts have a proof strength of about 600 MPa. A 20 kN tension load with a safety factor of 2 needs 67 mm² of stress area, so an M12 is the smallest that works. In shear the same load needs an M14.
Metric bolt chart
Coarse thread, grade 8.8. Search for a size.
| Bolt | Stress area (mm²) | Safe tension (kN) | Safe shear (kN) |
|---|---|---|---|
| M3 | 5.03 | 1.5 | 0.9 |
| M4 | 8.78 | 2.6 | 1.6 |
| M5 | 14.2 | 4.3 | 2.6 |
| M6 | 20.1 | 6 | 3.6 |
| M8 | 36.6 | 11 | 6.6 |
| M10 | 58 | 17.4 | 10.4 |
| M12 | 84.3 | 25.3 | 15.2 |
| M14 | 115 | 34.5 | 20.7 |
| M16 | 157 | 47.1 | 28.3 |
| M20 | 245 | 73.5 | 44.1 |
| M24 | 353 | 105.9 | 63.5 |
| M30 | 561 | 168.3 | 101 |
| M36 | 817 | 245.1 | 147.1 |
Safe tension at a safety factor of 2.
Load and bolt size
Grade 8.8, tension, safety factor 2. Slide the load.
The bolt is drawn in the size the load needs.
How to size a bolt
Load to diameter. Step through it.
Load
Per bolt.
Use a hardened washer under the head and nut on soft materials so the bolt does not sink in.
Grade 4.6, 8.8 or 10.9
Bolt grades. Compare them.
Mild steel.
High tensile.
Very high.
Never mix high-grade bolts with low-grade nuts. Use the matching nut class.
Before you assemble
Bolt checks. Tick each item.
Tick each item as you go.
Torque bolts in a cross pattern so the joint pulls down evenly.
What you get
Bolt sizing. Each tile explains one result.
Metric.
Needed.
Per cent.
N·m.
Per bolt.
The stress area is a little smaller than the shank, because of the threads.
Frequently Asked Questions
No question matches that search.
How do I choose a bolt size?
Divide the load times the safety factor by the bolt's allowable stress to get the stress area, then pick the bolt with at least that area.
What does grade eight point eight mean?
A tensile strength of eight hundred megapascals, with yield at eighty per cent of that.
Are bolts weaker in shear?
Yes, about sixty per cent of their tensile strength.
What is stress area?
The effective area of the threaded part that carries the load.
How tight should a bolt be?
Enough to preload it to about three quarters of its proof load.
What safety factor should I use?
Two for steady loads, three to four for shock or vibration.
Do two bolts carry twice the load?
Roughly, if the joint shares the load evenly.
Strong joints
Size bolts for the real load with a sensible margin, tighten them properly and the joint holds.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.