Bolt Torque Calculator - Free Online Fastener Torque Tool
Bolt Torque Calculator
Calculate the proper tightening torque for bolts based on size, material grade, and thread condition. Ensure safe and reliable fastener connections.
Bolt Specifications
Calculation Method
Leave blank for automatic
Quick Reference
Common K-Factors
| Dry Steel: | 0.20 |
| Lubricated: | 0.15 |
| Anti-seize: | 0.12 |
| Galvanized: | 0.25 |
Grade Strengths
| Grade 8.8: | 640 MPa |
| Grade 10.9: | 900 MPa |
| Grade 12.9: | 1080 MPa |
| SAE Grade 8: | 896 MPa |
How to use this calculator
🔩 How to Use This Calculator
- Select bolt system (Metric or Imperial)
- Choose bolt size and material grade
- Select thread condition (dry, lubricated, etc.)
- Set target preload percentage (typically 75%)
- Click calculate to get torque specifications
- Follow the guidelines for proper bolt tightening
📐 Torque Calculation Formula
Basic Formula
T = K × D × F
Where:
T = Torque (N⋅m)
K = Nut factor (friction coefficient)
D = Nominal bolt diameter (mm)
F = Preload force (kN)
Preload Calculation
F = σ × As
σ = Proof strength × % utilization
As = Tensile stress area
🌟 Practical Examples
M20 Grade 8.8 bolt, lubricated, 75% preload
Torque = 295 N⋅m (218 ft⋅lb)
1/2"-13 Grade 8 bolt, dry threads, 65% preload
Torque = 122 N⋅m (90 ft⋅lb)
M24 Grade 10.9 bolt, anti-seize, 85% preload
Torque = 680 N⋅m (502 ft⋅lb)
💡 Best Practices
- • Always use a calibrated torque wrench
- • Tighten bolts in a star or cross pattern for flanges
- • Apply torque in multiple passes (30%, 60%, 100%)
- • Re-torque after initial settling if required
- • Consider torque + angle method for critical joints
- • Document actual torque values for QA
- • Replace bolts that have been yielded
⚠️ Important Warnings
- • Never exceed 90% of proof load
- • K-factor can vary ±25% - test critical applications
- • Thread damage dramatically increases required torque
- • Temperature affects bolt preload (thermal expansion)
- • Galvanized bolts may gall - use appropriate lubricant
- • Always verify with manufacturer specifications
- • Consider joint relaxation and embedding
About this calculator
Calculate proper torque values for bolts based on size, grade, and lubrication. Essential for mechanical assembly and maintenance.
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