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Bolt Tightening Torque Calculator
Calculate the recommended tightening torque for bolts and screws based on size, grade, surface condition and target clamp load. Uses the short-form torque equation T = K × F × d per common engineering practice. Supports Metric (ISO 898-1) and Imperial (SAE J429 / ASTM).
Calculator
Standard
Bolt SizeCoarse thread
Bolt GradeISO 898-1
Surface ConditionTorque coefficient K
Electroplated zinc, no lube
Clamp Load% of proof load
75%
Typical: 75% for general, 90% for structural. Never exceed 100%.
Torque Coefficient (K Factor) Reference
The K factor accounts for friction in threads and under the bolt head. It is the single most important variable affecting torque accuracy.
| Surface Condition | K | Description |
|---|---|---|
| Black oxide (as-received) | 0.30 | Unlubricated, dry, as-manufactured |
| Zinc plated (dry) | 0.22 | Electroplated zinc, no lube |
| Zinc plated + oil | 0.18 | Zinc plated with light oil |
| Cadmium plated (dry) | 0.18 | Cadmium plating without lube |
| Hot-dip galvanized | 0.25 | HDG per ASTM A153 / ISO 10684 |
| Machine oil | 0.15 | Standard machine oil lubrication |
| Moly paste (MoS₂) | 0.13 | Molybdenum disulfide anti-seize |
| Anti-seize compound | 0.12 | Copper / nickel anti-seize |
| Wax / PTFE coating | 0.10 | Wax or Teflon-based friction reduction |
Source: VDI 2230, Machinery's Handbook, Fastenal Technical Reference. Actual K factors vary — always verify with calibrated equipment for critical joints.
Bolt Grade Proof Load Reference
Metric — ISO 898-1
| Grade | Proof (MPa) | UTS (MPa) |
|---|---|---|
| 4.6 | 225 | 400 |
| 4.8 | 310 | 400 |
| 5.6 | 280 | 500 |
| 5.8 | 380 | 500 |
| 8.8 | 580 | 800 |
| 9.8 | 650 | 900 |
| 10.9 | 830 | 1000 |
| 12.9 | 970 | 1200 |
Imperial — SAE / ASTM
| Grade | Standard | Proof (ksi) |
|---|---|---|
| Grade 2 | SAE J429 | 55 |
| Grade 5 | SAE J429 | 85 |
| Grade 8 | SAE J429 | 120 |
| A325 | ASTM F3125 | 85 |
| A490 | ASTM F3125 | 120 |
Formula & Notes
T = K × F × d
where:
T = tightening torque (N·m or ft·lbf)
K = nut factor / torque coefficient (dimensionless)
F = target clamp force (N or lbf) = proof load × clamp %
d = nominal bolt diameter (m or ft)
- This calculator uses the short-form torque equation (VDI 2230, Machinery's Handbook). For critical joints, use the full VDI 2230 method.
- 75% of proof load is the most commonly recommended target for general-purpose joints. Structural bolting (A325/A490) typically uses 70%.
- The K factor can vary ±25% even for the same surface condition. Always calibrate with actual fasteners when possible.
- Torque is not a direct measure of clamp force. Roughly 90% of input torque is consumed by friction; only ~10% produces actual clamping.
- For prevailing-torque locknuts, add the prevailing torque to the calculated tightening torque.