Fan Affinity Laws Tables (CFM, Pressure and Power vs RPM)
What a change in fan speed does to airflow, pressure and power, worked for two example fans with the same arithmetic and rounding as the Equipment Tracker app. No account, nothing to install.
REFERENCE TOOL — NOT A GUARANTEE
This tool does the math for you, but it is not a substitute for your own professional judgment, code compliance check, or manufacturer specifications. Always verify results before doing any work — you are responsible for your own calculations.
Fan Affinity Law What-If Tool (All 3 Laws)
CFM₂ = CFM₁×(RPM₂/RPM₁) · ΔP₂ = ΔP₁×(RPM₂/RPM₁)² · BHP₂ = BHP₁×(RPM₂/RPM₁)³
The tables show the app's three results for each new speed: airflow in CFM, pressure in in. w.g. and the power ratio.
Base fan: 1000 RPM, 1200 CFM, 0.5 in. W.C.
| New RPM (RPM₂) | Airflow (CFM₂) | Pressure (ΔP₂, in. w.g.) | Power ratio (BHP₂ / BHP₁) |
|---|---|---|---|
| 600 | 720 | 0.18 | 0.22x |
| 700 | 840 | 0.24 | 0.34x |
| 800 | 960 | 0.32 | 0.51x |
| 900 | 1080 | 0.41 | 0.73x |
| 1000 | 1200 | 0.5 | 1x |
| 1100 | 1320 | 0.61 | 1.33x |
| 1200 | 1440 | 0.72 | 1.73x |
| 1300 | 1560 | 0.85 | 2.2x |
| 1400 | 1680 | 0.98 | 2.74x |
Base fan: 1725 RPM, 2000 CFM, 1 in. W.C.
| New RPM (RPM₂) | Airflow (CFM₂) | Pressure (ΔP₂, in. w.g.) | Power ratio (BHP₂ / BHP₁) |
|---|---|---|---|
| 1200 | 1391 | 0.48 | 0.34x |
| 1300 | 1507 | 0.57 | 0.43x |
| 1400 | 1623 | 0.66 | 0.53x |
| 1500 | 1739 | 0.76 | 0.66x |
| 1600 | 1855 | 0.86 | 0.8x |
| 1725 | 2000 | 1 | 1x |
| 1800 | 2087 | 1.09 | 1.14x |
| 1900 | 2203 | 1.21 | 1.34x |
| 2000 | 2319 | 1.34 | 1.56x |
Frequently asked questions
What are the three fan affinity laws?
As the Equipment Tracker app states them: new airflow is the old airflow times the speed ratio (RPM2 over RPM1), new pressure is the old pressure times the speed ratio squared, and new power is the old power times the speed ratio cubed.
What happens if I speed a fan up from 1000 to 1200 RPM?
For a fan that moves 1200 CFM at 0.5 in. W.C., the app gives 1440 CFM, 0.72 in. w.g. and a power ratio of 1.73x. A 20% speed increase takes about 1.73 times the power.
What happens if I slow the same fan to 900 RPM?
The app gives 1080 CFM, 0.41 in. w.g. and a power ratio of 0.73x.
How are the results rounded?
Airflow is rounded to a whole CFM; pressure and the power ratio are rounded to two decimals.
Does this cover impeller diameter, air density or the duct system curve?
No. The Equipment Tracker app's what-if tool works from the two speeds, the base airflow and the base pressure, so only those are used here.
Does this page store anything or need an account?
No to both. It does the math for you, but it is not a substitute for your own professional judgment, a code compliance check or the manufacturer's specifications.
Keep each fan's speed settings and service history in Equipment Tracker
Write each unit's blower settings and service history down once and they are there on the next service call, on your phone, with or without signal. The core of the app is free on the phone, and every first plan, Pro or Team, starts with a 7-day free trial.
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