Belt Drive Pulley and Fan RPM Calculator
Work out a belt-driven blower’s speed from the motor and pulley sizes — or the pulley you need for a new speed — plus belt speed and belt length.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Fan speed
- 863rpm
The fan turns at about 863 rpm.
- Raising fan speed raises power with the cube of speed (fan laws). Check motor amps against the nameplate and the fan’s maximum speed before fitting a smaller fan pulley or larger motor pulley. Lock out power before working on a belt drive.
- Speed ratio (fan ÷ motor)
- 0.5
- Belt speed
- 1,581fpm
- Belt pitch length
- 46.7in
Show the working
- Fan speed n₂ = n₁ × d₁ ÷ d₂1,725 × 3.5 ÷ 7 = 863 rpm
- Belt speed = π × motor pulley × motor rpm ÷ 12π × 3.5 × 1,725 ÷ 12 = 1,581 ft/min
- Belt length = π·d₁/2 + π·d₂/2 + 2C + (d₂ − d₁)² ÷ 4C46.7 in
The pulley rule
With no slip, the belt moves at the same speed over both pulleys, so motor pulley diameter × motor rpm = fan pulley diameter × fan rpm. Rearranged, it gives the fan speed or either pulley size. Use pitch diameters (where the belt rides), and for an adjustable pulley its setting at the time.
Belt speed = π × motor pulley diameter × motor rpm ÷ 12, in feet per minute. Belt length for two pulleys at a known centre distance C is approximately π(d₁ + d₂) ÷ 2 + 2C + (d₂ − d₁)² ÷ 4C.
Before you change a pulley
- Fan power rises with the cube of speed: 10 % more speed needs about 33 % more power, so check motor amps against the nameplate afterwards
- Check the fan’s maximum rated speed
- Lock out and tag out power before touching a belt drive, and re-tension and align the belt after any change
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Find
- Fan speed from the two pulleys
- Motor speed
- 1,725 rpm
- Motor pulley pitch diameter
- 3.5 in
- Fan pulley pitch diameter
- 7 in
- Shaft centre distance (optional, for belt length)
- 15 in
Result
- Fan speed
- 863 rpm
- Speed ratio (fan ÷ motor)
- 0.5
- Belt speed
- 1,581 fpm
- Belt pitch length
- 46.7 in
The fan turns at about 863 rpm.
Assumptions
- No belt slip: motor pulley diameter × motor rpm = fan pulley diameter × fan rpm, using pitch diameters.
- Belt length uses the open-belt approximation L = π(d₁ + d₂)/2 + 2C + (d₂ − d₁)²/4C.
Limitations
- Real belts slip slightly, so the fan runs a little slower than calculated.
- Select the actual belt from the belt maker’s standard lengths and check the motor load after any speed change.
Frequently asked questions
How fast does my blower turn with a 3.5-inch motor pulley and a 7-inch fan pulley?
Half the motor speed: with a 1,725 rpm motor, about 862 rpm (a little less in practice because of belt slip).
How do I speed up a belt-driven blower?
Use a larger motor pulley, open an adjustable pulley to a larger pitch diameter, or fit a smaller fan pulley — then check motor amps, since power rises steeply with speed.
Is the belt length exact?
It is the standard open-belt approximation. Choose the nearest standard belt from the belt maker and adjust the motor base for tension.
Sources
- Belts — Pulley Diameters vs. Speed — The Engineering ToolBox. Single belt drive: d₁·n₁ = d₂·n₂, rearranged for either speed or either pulley diameter.
- Belt Transmissions — Speed and Length of Belts — The Engineering ToolBox. Belt length l = dₘπ/2 + d_fπ/2 + 2l_fm + (d_f − dₘ)²/(4l_fm); belt velocity v = π·dₘ·nₘ/12 ft/min.
- Improving Fan System Performance: A Sourcebook for Industry — U.S. DOE (Industrial Technologies Program) with AMCA International. Component pressure loss Δp = C·ρ·(V/1,097)² (in. wg, V in fpm, ρ = 0.075 lb/ft³ standard), giving a parabolic system curve; system curves can have a static component; sheave changes alter fan speed; airflow is linear with speed and power is cubed.
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