Belt drive & pulley ratio calculator
Enter the motor speed and power plus both pulley diameters and the center distance, and get the speed ratio, belt length, wrap angle, belt speed, shaft torque and an estimated drive efficiency — with a to-scale drive diagram and a drive-loss chart.
How to use this calculator
- Enter the motor's rated power and driver (motor) speed.
- Enter both pulley diameters, or let the driven pulley auto-suggest from the speed ratio you want.
- Enter the center distance between the pulleys.
- Pick the belt section and number of belts.
- Read the speed ratio, belt length, wrap angle, belt speed, torque and estimated efficiency.
How this calculator works
Pulley ratio, belt length, wrap angle, belt speed and shaft torque are exact mechanics — no external source needed:
Ratio i = D2 / D1 N2 = N1 / i
Belt speed v = π·D1·N1 / 60 (m/s, D1 in m)
Belt length L = 2C + (π/2)(D1+D2) + (D2−D1)²/(4C)
Wrap angle θ = π − 2·asin((D2−D1)/2C) (small pulley)
Torque T = 9550 · P / N (N·m, P in kW)
Drive efficiency is different — it isn't a formula, it's read off a chart: AMCA 203-90 Annex L, Figure L.1, "Estimated Belt Drive Loss", which plots drive loss (% of motor power output) against motor power output for standard V-belts, as a range plus a typical curve, not a single number. The curve used here is a log-log interpolation through points read off that chart, anchored to the standard's own worked example (13.3 hp motor output → 5.1% drive loss). Treat it as an estimate, not a digitized dataset — for a specific belt and sheave combination, use the drive manufacturer's own rating tables.
The computed belt length is rounded to the nearest standard datum length for the chosen belt section, per ISO 4184:1992 ("Belt drives — Classical and narrow V-belts — Lengths in datum system"), Table A.1 for classical sections and Table 1 for narrow sections — so the length you get is one you can actually order. ISO 4184 covers belt lengths only; it does not cover power ratings, so this page can't derive the exact belt count from first principles — that depends on the specific belt manufacturer's rating tables.
What it does do is a basic plausibility check: set the "Number of belts" field to however many parallel belts you're planning, and the tool divides the motor power across them and checks the resulting load per belt against the section's typical single-belt power range — a Y-section belt asked to carry 55 kW single-handed will warn, and it'll suggest roughly how many belts (or a larger section) brings it back into range. Those ranges are the common rule-of-thumb figures used to pick a starting section before consulting a manufacturer's chart, not a rating table, so treat a "no warning" result as "plausible", not "confirmed".
This page checks one drive at a time. CloudAir's selection programs surface drive and efficiency data for every fan in your catalogue automatically. See how CloudAir handles it →