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.

AMCA 203-90 Fig. L.1 (approx.) To-scale diagram Free, no sign-up

Drive data

Diameters are the pitch (effective) diameters of the pulleys, not the outside diameters.

Motor power kW, standard IEC rating

Driver (motor) speed rpm

Pulley sizing

Driver pulley diameter mm, auto-suggested by section

Driven pulley diameter mm

Center distance mm

Belt section ISO 4184

Number of belts parallel, your choice

Fill in the drive data to size the belt.

Get a full drive datasheet

Link copied — it reopens with these exact inputs.

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 →

Pulley ratio, belt length, wrap angle and torque are exact mechanics. Drive efficiency is an approximation of AMCA 203-90 Figure L.1, itself described by AMCA as a range affected by belt tension, alignment, sheave diameter and belt section — not a precise single value. This calculator does not select a specific belt cross-section or belt count; that requires the belt manufacturer's power-rating tables for the exact belt type chosen.