Fan Laws Calculator — Free Online Calculator
Fan affinity laws tell you exactly what happens to airflow, static pressure, and brake horsepower when you change blower speed — essential when you're deciding whether to drop a tap to fix high TESP or predict performance after a motor speed change. Enter your known operating point and the new RPM; this tool applies the three affinity laws and shows predicted CFM, static pressure, and BHP instantly.
Enter baseline blower performance
Known airflow at the current blower speed.
TESP or fan static at the baseline speed.
From nameplate, fan curve, or measured amp draw.
Current blower speed (measured or nameplate).
Proposed blower speed — e.g. one tap lower on a multi-speed motor.
Predicted CFM at new speed
Static pressure
0.41 in. w.c.
Brake HP
0.55 BHP
Speed ratio (RPM₂ ÷ RPM₁)
0.90
What this means
Speed reduced — slower blower speed lowers CFM and static pressure proportionally.
See the breakdown
Affinity laws assume the same fan and duct system with only speed changing. Verify predicted CFM and TESP in the field after any blower speed adjustment.
Fan laws at common speed ratios
Baseline: 1,200 CFM · 0.50 in. w.c. · 0.75 BHP at 1,200 RPM. Same fan, same duct — only speed changes.
| Speed ratio | CFM | Static pressure | Brake HP |
|---|---|---|---|
| 0.80 (−20%) | 960 CFM | 0.32 in. w.c. | 0.38 BHP |
| 0.90 (−10%) | 1,080 CFM | 0.41 in. w.c. | 0.55 BHP |
| 1.00 (baseline) | 1,200 CFM | 0.50 in. w.c. | 0.75 BHP |
| 1.10 (+10%) | 1,320 CFM | 0.61 in. w.c. | 1.00 BHP |
Sources & standards: ACCA Manual D (residential duct design and blower performance) · Manufacturer fan curves and nameplate data · ASHRAE Handbook — HVAC Systems and Equipment (fan affinity laws).
The formula, explained in plain English
Fan affinity laws describe how a centrifugal blower behaves when you change only its speed on a fixed duct system. The speed ratio is the new RPM divided by the original RPM — every other quantity scales from that single number.
CFM is linear
Airflow tracks speed one-for-one. Drop RPM 10% and you lose 10% of your CFM — critical when checking whether a slower tap still delivers 400 CFM/ton.
Static pressure is squared
TESP falls faster than CFM when you slow down — a 10% speed cut drops static pressure 19%. That is why lowering blower speed is a common fix for borderline duct restrictions.
Power is cubed
Brake horsepower drops steeply at lower speeds — 0.9³ ≈ 0.73, so a 10% slowdown saves roughly 27% of blower power. Speeding up has the opposite effect and can overload the motor.
Same system only
These laws apply when the fan and duct geometry stay the same. Duct modifications, filter changes, or damper adjustments change the system curve — use fan laws for speed changes, not duct redesign.
Worked examples
Three blower speed changes from a 3-ton system baseline: 1,200 CFM, 0.50 in. w.c., 0.75 BHP at 1,200 RPM.
10% slowdown — 1,200 → 1,080 RPM
CFM₂ = 1,200 × 0.90 = 1,080 CFM
SP₂ = 0.50 × 0.90² = 0.41 in. w.c.
BHP₂ = 0.75 × 0.90³ = 0.55 BHP
Result: a common tap-down to relieve borderline TESP — airflow drops 120 CFM but static pressure falls from 0.50 to 0.41 in. w.c., often enough to get under a 0.5 nameplate limit.
20% slowdown — 1,200 → 960 RPM
CFM₂ = 1,200 × 0.80 = 960 CFM
SP₂ = 0.50 × 0.80² = 0.32 in. w.c.
BHP₂ = 0.75 × 0.80³ = 0.38 BHP
Result: aggressive speed reduction — TESP drops 36% but CFM falls to 960 (320 CFM/ton on a 3-ton system). Verify Delta T and comfort before leaving the blower at this tap.
10% speed-up — 1,200 → 1,320 RPM
CFM₂ = 1,200 × 1.10 = 1,320 CFM
SP₂ = 0.50 × 1.10² = 0.61 in. w.c.
BHP₂ = 0.75 × 1.10³ = 1.00 BHP
Result: more airflow but static pressure climbs 21% and power jumps 33% — likely over a 0.5 in. w.c. residential limit. Measure TESP and amp draw before running at this speed.
Frequently asked questions
Common questions about fan affinity laws, blower speed changes, and HVAC airflow.
What are the fan affinity laws?
The fan affinity laws relate blower speed (RPM) to airflow and power on a fixed duct system: CFM changes linearly with RPM, static pressure changes with the square of RPM, and brake horsepower changes with the cube of RPM. They assume the same fan, same duct geometry, and the same air density.
What happens to CFM if I slow the blower 10%?
CFM drops by the same percentage. A 10% RPM reduction (ratio 0.9) cuts airflow from 1,200 CFM to 1,080 CFM. Static pressure falls to 81% of the original (0.9²) and brake horsepower to about 73% (0.9³) — less power, but also less delivered airflow.
When should I use fan laws on an HVAC job?
Use them when you know performance at one blower tap or speed and need to estimate another — for example after changing blower speed to fix high TESP, sizing a ECM speed reduction, or predicting what a 10% slowdown does to airflow before you commit to a duct modification.
Do fan laws work if I change the duct system?
No — affinity laws assume the same fan on the same duct system with only speed changing. Adding a larger return, replacing flex with hard pipe, or opening dampers changes the system curve itself. Fan laws predict speed changes only, not duct redesign.
How do fan laws relate to static pressure?
Static pressure scales with the square of speed. Slowing the blower 10% drops TESP to 81% of the original — often enough to bring a borderline system back under the nameplate limit. Pair fan-law predictions with actual TESP readings from the static pressure calculator.
Can I use fan laws to size a new blower motor?
Fan laws estimate brake horsepower at a new speed from a known operating point — useful for checking whether a motor has headroom. They do not replace manufacturer fan curves, belt losses, or drive efficiency. Always verify amp draw and TESP in the field after any speed change.
Sized the blower change? Quote the repair in seconds.
Fan-law numbers tell you what the blower will do at the new speed — TradesQuote turns the repair into a detailed, line-item estimate in seconds. Describe the blower speed change, motor swap, or duct fix (or upload photos), and our AI builds quantities, unit prices, and totals, validated by a built-in quality control agent.
AI line-item estimates
Quantities, unit prices, and totals generated instantly.
Knowledge base
Upload past jobs so estimates reflect your real pricing.
Shareable & signable
Clients review, accept, and sign from a public link.
No credit card required · 14-day free trial · Cancel anytime