What is bolt torque?
Torque is a measure of the conversion of linear force to rotational force at some distance from the axis of rotation. When applied to a bolt, this force becomes tension in the bolt threads, which in turn applies a clamping force (or preload) between the two materials you are bolting together.
The amount of tension in the bolt is significant. For each application of a bolt, there will be an ideal bolt torque value. If you apply too little torque, vibrations will tend to loosen the bolt over time, which may result in joint failure. On the other hand, if you tighten up a bolt with too much torque, the bolt may stretch — reducing the clamping force — or even break during assembly or operation.
Bolt tightening torque calculation formula
The conversion from torque to clamping force depends on the bolt material, its diameter, and the level of lubrication. The standard bolt torque formula is:
T = K × D × F
- T — Tightening torque (N·m or lbf·ft)
- K — Nut factor (torque coefficient, dimensionless) — accounts for friction
- D — Nominal bolt diameter (m in SI; in for imperial)
- F — Target bolt clamp load / preload (N or lbf)
The nut factor K is not a pure friction coefficient; it combines thread friction, bearing friction, and geometric effects. Typical values by lubrication condition:
| Condition | K factor |
|---|---|
| Dry (as received) | 0.20 |
| Zinc plated / cadmium | 0.17 |
| Light oil (engine oil) | 0.15 |
| Moly grease / anti-seize | 0.13 |
| Copper paste (heavy anti-seize) | 0.12 |
Clamp load from proof load
The proof load is the maximum tensile load a bolt can withstand without permanent deformation. It is calculated as:
Fproof = As × Sp
- As — Thread stress area (mm² or in²) — the effective cross-sectional area of the threaded shank
- Sp — Proof strength of the bolt material (MPa or psi)
The recommended clamping force is typically 75% of the proof load, which ensures a safe margin below the elastic limit while still providing reliable joint clamping.
How to use this bolt torque calculator
- Select the unit system — Metric (SI) or US Imperial.
- Choose the bolt size — Standard metric sizes M4–M36 or UNC imperial sizes 1/4-20 through 1-1/2-6.
- Select the bolt grade — Metric grades 4.6 / 5.8 / 8.8 / 10.9 / 12.9 or A2/A4 stainless; SAE Grade 2 / 5 / 8 or ASTM A193 B7 for imperial.
- Choose the lubrication condition — Affects the nut factor K.
- Set the clamping force percentage — Default is 75% of proof load; adjust as needed.
- Optionally enter a custom clamping force if you already know the required preload.
- Click Calculate Torque to get the tightening torque in N·m, lbf·ft, and lbf·in simultaneously.
Example calculations of bolt torque
Example 1 — Metric M12 Grade 8.8 bolt, dry
- Bolt: M12, Grade 8.8 → Sp = 600 MPa, As = 84.27 mm²
- Proof load: F = 84.27 × 600 = 50,562 N
- Clamping force at 75%: F = 50,562 × 0.75 = 37,922 N
- K (dry) = 0.20, D = 12 mm
- T = 0.20 × 12 × 37,922 / 1000 = 91.0 N·m
Example 2 — Imperial 1/2-13 UNC Grade 5, light oil
- Bolt: 1/2-13, Grade 5 → Sp = 85,000 psi, As = 0.1419 in²
- Proof load: F = 0.1419 × 85,000 = 12,062 lbf
- Clamping force at 75%: F = 12,062 × 0.75 = 9,046 lbf
- K (light oil) = 0.15, D = 0.50 in
- T = 0.15 × 0.50 × 9,046 = 678 lbf·in = 56.5 lbf·ft
FAQs
What is the nut factor K?
The nut factor (also called the torque coefficient) combines the effects of thread friction, bearing surface friction, and thread geometry. It is not the same as the coefficient of friction. Lubrication dramatically reduces K — always lubricate per the joint specification.
Why 75% of proof load?
Using 75% provides a safety margin: the bolt is loaded well into its elastic range, producing a reliable and consistent clamp force, while staying below the proof load where permanent stretch begins. Some safety-critical applications use 70–90%; always check your design standard.
Metric vs. imperial bolt grades — what are the differences?
Metric grades (ISO 898) are expressed as property class numbers such as 8.8, 10.9, and 12.9, where the first number × 100 = tensile strength in MPa and the product of both numbers × 10 = yield strength in MPa. SAE/ASTM grades use simpler names (Grade 2, 5, 8) but the proof strengths are lower — always refer to the appropriate standard for your application.
Does lubrication really matter?
Yes — dramatically. Switching from dry (K = 0.20) to copper paste (K = 0.12) reduces the required torque by 40% for the same clamping force. Applying the wrong torque for the wrong lubrication condition is one of the most common causes of joint failure in mechanical assemblies.