Fan Laws Calculator
Free fan laws (affinity laws) calculator for HVAC blowers. Enter original RPM, CFM, static pressure, and BHP plus a new RPM to see how airflow, static pressure, and motor power change.
How the fan laws work
The fan laws (affinity laws) describe how a blower’s output changes when you change its speed — by swapping a pulley, changing a belt, or moving a speed tap or VFD setting:
| Law | Relationship |
|---|---|
| 1. Airflow | CFM₂ = CFM₁ × (RPM₂ ÷ RPM₁) |
| 2. Static pressure | SP₂ = SP₁ × (RPM₂ ÷ RPM₁)² |
| 3. Brake horsepower | BHP₂ = BHP₁ × (RPM₂ ÷ RPM₁)³ |
Worked example
Raise a blower from 800 to 900 RPM (ratio 1.125):
- Airflow: 1,000 CFM × 1.125 = 1,125 CFM (+12.5%)
- Static pressure: × 1.125² = +27%
- Brake horsepower: × 1.125³ = +42%
A modest speed bump nearly halves the headroom on the motor.
The catch: power scales with the cube
Because brake horsepower follows the cube of the speed ratio, doubling RPM doubles airflow but raises power draw 8×. Always verify the motor’s amp draw and nameplate rating before raising fan speed — and remember these laws assume a constant system curve, so real-world CFM gains run a little under the prediction as duct resistance rises.
Use this with the CFM & Tonnage Calculator to target the right airflow, and the Static Pressure Calculator to check the result.
How FieldPad helps in the field
Project the change here, then log the before-and-after readings in FieldPad against the equipment and the job. Keep your blower-setup notes on the same record as the invoice, and turn airflow corrections into client-ready estimates — offline, on your iPhone.
Stop Re-Running These Numbers by Hand
FieldPad is the all-in-one iPhone CRM for solo HVAC techs — clients, scheduling, estimates, invoices with signatures, inventory, and equipment history. Keep your readings on the job, the client, and the equipment, all in one offline-ready app. EPA Section 608 and Subpart C compliance included.
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