Belt Length Calculator
Need to replace a belt? Enter pulley sizes and center distance — get the right belt length and slip risk in seconds.
Calculation Mode
Enter both pulley diameters and spacing → get the required belt length
Enter a known belt length → find how far apart the pulleys should sit
Input Parameters
V-Belt Specifications Reference
| Profile | Top Width (mm) | Height (mm) | Power Range |
|---|---|---|---|
| A | 13 | 8 | 0.4-3 HP |
| B | 17 | 11 | 2-7.5 HP |
| C | 22 | 14 | 7.5-15 HP |
| D | 32 | 19 | 15-50 HP |
| E | 38 | 23 | 50+ HP |
Belt Drive Diagram
Calculation Results Sample values — enter your own dimensions
About Belt Length Calculator
The Belt Length Calculator is a specialized tool for mechanical power transmission design. It quickly calculates the required belt length for open belt drive systems. Suitable for factory equipment maintenance, mechanical design, agricultural machinery, and any application requiring belt drive replacement or design.
Calculation Formulas
Open belt length formula: L = 2C + π(D+d)/2 + (D-d)²/(4C)
Where L is belt length, C is center distance, D is large pulley diameter, d is small pulley diameter.
Contact angle formula: θ = 180° - 2×arcsin[(D-d)/(2C)]
The small pulley wrap is smaller than the large one and is the key to judging slip risk.
Wrap angle too low? Try widening the center distance or reducing the diameter difference to reach the 150° safe line.
How to Use
- Select calculation mode: Forward (find belt length from pulley dimensions) or Reverse (find center distance from belt length)
- Enter the large pulley diameter D (outer diameter of the larger pulley in mm)
- Enter the small pulley diameter d (outer diameter of the smaller pulley in mm)
- Enter the center distance C or known belt length
- Select belt type (A/B/C/D/E) for standard size recommendation
- View results including belt length, contact angles, speed ratio, and slip risk assessment
Frequently Asked Questions
How do I choose the right V-belt profile?
Select based on power transmission: Type A for 0.4-3 HP, Type B for 2-7.5 HP, Type C for 7.5-15 HP, Type D for 15-50 HP, Type E for 50+ HP. Higher power requires wider belts.
What contact angle is safe?
A small pulley wrap angle ≥150° is the safe zone (low risk, good drive efficiency); 120° is the usable lower limit — below that, slipping is likely; below 90° is not recommended. Increase the center distance to raise the wrap angle.
What if the belt is too long or too short?
Adjusting the center distance is the most common solution. Alternatively, choose the closest standard belt length and fine-tune using the machine's tension adjustment mechanism.
What is the speed ratio used for?
Speed ratio = D/d indicates how many rotations the small pulley makes for each rotation of the large pulley. If the large pulley is the driver, the driven (small) pulley speeds up; vice versa for speed reduction.
Formula sources: Omni Calculator, tec-science | Last updated: November 2025
