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Fan Laws Calculator (Airflow, Static Pressure & Horsepower)

Apply the fan laws: change fan speed and see the new CFM, static pressure and brake horsepower (∝ rpm, rpm², rpm³), find the rpm needed for a target airflow, the pressure and power that follow, the effect of air density, and pulley size to hit the new speed.

Current airflow

Current static pressure

Current fan brake horsepower

Current fan speed

Target airflow

Motor nameplate

Motor pulley pitch diameter (belt drive; 0 for direct)

Motor speed

Air density ratio (1.0 = standard 0.075 lb/ft³)

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How it works

CFM₂ = CFM₁ × (rpm₂/rpm₁); SP₂ = SP₁ × (rpm₂/rpm₁)² × (ρ₂/ρ₁); BHP₂ = BHP₁ × (rpm₂/rpm₁)³ × (ρ₂/ρ₁); fan pulley = motor pulley × motor rpm ÷ fan rpm

For a given fan in a given duct system, airflow is proportional to speed, pressure to speed squared and power to speed cubed. These fan laws let a technician predict what a pulley change or VFD setting will do without a new fan curve: 20% more air costs 44% more static pressure and 73% more horsepower, which is why so many "just speed it up" fixes trip motor overloads, and why trimming an oversized fan by 15% cuts its energy use by nearly 40%. Because the laws assume the system stays the same, they fail if dampers move or filters load; and thinner air at altitude or high temperature lowers both the pressure developed and the power drawn in proportion to density.

Worked example

Raising a 2,000 CFM fan at 1,050 rpm, 0.8″ static and 0.75 bhp to 2,400 CFM means 1,260 rpm: static pressure rises to 1.15″ and power to 1.30 bhp — over the 1 hp motor, so a 1½ hp motor is needed. On a 1,725 rpm motor with a 3.5″ pulley the fan pulley must shrink from 5.75″ to 4.79″.

Frequently asked questions

Do the fan laws apply to ECM blowers?

Yes for the physics; a constant-airflow ECM simply changes its own speed to hold CFM as static pressure changes, so its power rises with the system resistance — a dirty filter shows up as a higher watt draw.

Why did the static pressure not follow the square law when I sped up the fan?

Something in the system changed — a relief damper opened, a filter loaded, or the fan moved onto a different part of its curve. The laws hold only for the same system curve.

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Standard HVAC field formulas (sensible heat equation, fan laws, ASHRAE 62.2 and blower-door conventions) at sea-level air density unless noted. Use manufacturer data, a Manual J/D/S load calculation and a licensed HVAC technician for design and commissioning decisions.