Try the Bolt Torque, Preload & Strength Calculator
Calculate bolt tightening torque and preload for metric bolts M3–M36, property classes 4.6–12.9, with nut-factor options, proof load.
- Free to use
- No signup
- Nothing to install
- Runs in your browser
Illustration. Open the tool for the real thing.
What is the Bolt Torque, Preload & Strength Calculator?
The standard bolt-tightening relationship is T = K·d·F: the torque you apply equals the nut factor times the nominal diameter times the preload (clamp force) you get. This calculator turns it around for you: pick a metric size from M3 to M36 and a property class (4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9 or 12.9), choose the percentage of proof load you want as preload (75% for reusable joints, up to 90% for permanent ones) and the friction condition (K = 0.20 dry, 0.17 zinc plated or lightly oiled, down to 0.10 lubricated), and it returns the torque in N·m, lbf·ft, lbf·in and kgf·m, the preload and the proof load.
It also lists the minimum yield load and the Eurocode 3 design tension and shear resistances, and a full torque chart for every size and the four most common classes. The stress areas and proof strengths follow ISO 898-1 for coarse threads. Because the nut factor is empirical, the tool shows the realistic ±25% uncertainty in preload from torque control alone — for critical joints use a standard such as VDI 2230 and tension-indicating methods.
3 steps to your result
Open the tool and follow along. Nothing to install.
What you get
T = K·d·F with N·m, lbf·ft, lbf·in and kgf·m outputs.
Stress area and proof strength for M3–M36 and nine property classes.
Eurocode 3 tension and shear resistance and yield load.
All sizes and classes at 75% proof, dry steel.
AZRS tools are free to open and use.
It opens in your browser, so there is nothing to download.
Used for
Pick one to see how it fits.
How it compares
A plain look at this tool next to the usual alternative.
| This engineering calculator | Desktop software | |
|---|---|---|
| Install | Nothing to install | Usually an app or program to install |
| Account | No signup | Depends on the product |
| Cost | Free to use | Free or paid, depends on the product |
| Where it runs | In your browser tab | On your computer |
Questions people ask
What is the nut factor?
An empirical number that lumps thread and head friction; 0.20 is typical for plain dry steel.
What does 8.8 mean?
Minimum tensile strength 800 MPa and yield 640 MPa (the ratio is 0.8).
Is the chart for fine threads?
No — coarse threads; fine-thread bolts have larger stress areas.
Is it free?
Yes.
How do I use the Bolt Torque, Preload & Strength Calculator?
1. Choose bolt size and property class. 2. Set the preload percentage and the friction condition. 3. Read the tightening torque and preload. 4. Check the resistances and the torque chart.
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Tightening torque, clamp force, proof load and design resistances for M3–M36 bolts by. Free to use.
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