Belt Drive Calculator (Speed Ratio and Belt Length)
Size a two-pulley belt drive: speed ratio, output rpm, belt speed, approximate belt pitch length and wrap angle on the small pulley. Uses the standard open-drive approximation L = 2C + 1.57(D + d) + (D - d)^2 / (4C).
How to use this calculator
- Enter driver pulley diameter d, driven pulley diameter D and centre distance C.
- Enter driver rpm to get output rpm, belt speed and wrap angle.
- Use the belt length result to select the nearest standard belt size.
Frequently asked questions
How accurate is the belt length formula?
The approximation is accurate to well under one percent for typical centre distances. Exact arc-based formulas differ slightly, and belt manufacturers publish their own length tables, so always select the nearest standard belt from the manufacturer catalogue.
What wrap angle is acceptable?
Keep the wrap angle on the smaller pulley above about 120 degrees for a V-belt drive. Below that, slip and belt wear rise sharply and you should add an idler or increase centre distance.
What belt speed is too high?
Classical V-belts are usually limited to about 25 to 30 m/s. Above that, centrifugal tension reduces the effective grip and heat build-up shortens belt life.