🔩 Bolt Torque Calculator
Calculate bolt torque and preload
Typically 0.15-0.25 for steel
Required clamp force or preload
How to Use This Calculator
Enter Bolt Diameter
Input bolt diameter in inches. Common sizes: 1/4" (0.25), 3/8" (0.375), 1/2" (0.5), 5/8" (0.625), 3/4" (0.75).
Select Bolt Grade
Choose bolt grade (Grade 2, Grade 5, Grade 8, or ASTM A193 B7). Grade 5 is standard for most applications.
Enter Friction Coefficient
Enter friction coefficient (typically 0.15-0.25 for steel, 0.2 is standard). Higher friction requires more torque for same preload.
Calculate and Review
Click "Calculate Bolt Torque" to see torque in inch-pounds and foot-pounds, preload, proof load, and tensile strength. Use this for fastener design.
Formula
Torque: T = K × D × F
where: K = friction coefficient, D = bolt diameter, F = clamp force
Preload ≈ 75-90% of proof load for most applications
Example 1: Clamp Force (D = 0.5", K = 0.2, F = 5,000 lbs)
Step 1: Torque = K × D × F = 0.2 × 0.5 × 5,000 = 500 in-lbs
Step 2: Torque (ft-lbs) = 500 / 12 = 41.7 ft-lbs
Example 2: Proof Load (D = 0.5", Grade 5, K = 0.2)
Step 1: Proof Load ≈ 10,500 × (0.5/10)² = 26,250 lbs
Step 2: Torque = K × D × P = 0.2 × 0.5 × 26,250 = 2,625 in-lbs
Step 3: Torque (ft-lbs) = 2,625 / 12 = 218.8 ft-lbs
About Bolt Torque Calculator
The Bolt Torque Calculator is an essential tool for mechanical engineers, technicians, and builders who need to calculate bolt torque and preload for fastener design. This calculator implements standard bolt torque formulas based on friction coefficient, bolt diameter, and clamp force, helping you achieve proper preload and ensure fastener integrity.
When to Use This Calculator
- Fastener Design: Calculate bolt torque for proper preload
- Preload Analysis: Determine preload from torque specifications
- Material Selection: Compare different bolt grades
- Safety Analysis: Ensure proper fastener installation
- Educational Use: Learn and understand bolt torque calculations
Why Use Our Calculator?
- ✅ Accurate Formulas: Uses standard bolt torque equations
- ✅ Multiple Bolt Grades: Supports Grade 2, 5, 8, and ASTM A193 B7
- ✅ Complete Analysis: Shows torque, preload, proof load, and tensile strength
- ✅ Dual Units: Shows torque in inch-pounds and foot-pounds
- ✅ Time Savings: Instant calculations eliminate manual math
Understanding Bolt Torque
Basic Principle: Bolt torque (T) is calculated using T = K × D × F, where K is friction coefficient, D is bolt diameter, and F is clamp force (preload). Friction coefficient typically ranges from 0.15-0.25 for steel, with 0.2 being standard. Higher friction requires more torque for the same preload.
Bolt Grades: Bolt grades indicate tensile strength. Grade 2 = 74,000 psi, Grade 5 = 120,000 psi, Grade 8 = 150,000 psi, ASTM A193 B7 = 125,000 psi. Proof load is typically 85% of tensile strength. Higher grades require more torque for same preload percentage.
Preload: Preload (clamp force) should be 75-90% of proof load for most applications. Too little preload causes loosening, too much causes yielding. The calculator shows proof load for reference.
Tips for Best Results
- Accurate Diameter: Measure bolt diameter accurately for precise torque calculations
- Correct Friction: Use appropriate friction coefficient (0.2 is standard for steel)
- Check Bolt Grade: Verify bolt grade for correct proof load calculation
- Verify with Standards: Check manufacturer specifications for exact values
- Apply Safety Factors: Use appropriate safety factors for critical applications
Frequently Asked Questions
How do I calculate bolt torque?
Bolt torque is calculated using T = K × D × F, where K is friction coefficient (typically 0.2), D is bolt diameter in inches, and F is clamp force in pounds. For example, 0.5" bolt, K = 0.2, F = 5,000 lbs: T = 0.2 × 0.5 × 5,000 = 500 in-lbs (41.7 ft-lbs). The calculator does this automatically.
What is friction coefficient?
Friction coefficient (K) is a factor that accounts for friction between bolt threads and mating surfaces. For steel bolts, K typically ranges from 0.15-0.25, with 0.2 being standard. Lubricated surfaces have lower friction (K ≈ 0.15), while dry surfaces have higher friction (K ≈ 0.25).
What is preload?
Preload (clamp force) is the tension force applied to a bolt when tightened. Preload should be 75-90% of proof load for most applications. Too little preload causes loosening under vibration, while too much preload causes yielding. The calculator shows proof load for reference.
What is the difference between bolt grades?
Bolt grades indicate tensile strength: Grade 2 = 74,000 psi, Grade 5 = 120,000 psi, Grade 8 = 150,000 psi, ASTM A193 B7 = 125,000 psi. Higher grades have higher strength and proof load, requiring more torque for same preload percentage. Grade 5 is standard for most applications.
How much torque should I apply?
Torque depends on bolt diameter, grade, friction, and desired preload. Use the calculator to determine torque for your specific application. For critical applications, verify with manufacturer specifications and local codes. Always use calibrated torque wrenches for accurate application.