CFM calculation formula in hvac work starts with one relationship: CFM (cubic feet per minute) is how much air moves through a duct or space. Every hvac cfm calculator, air duct cfm calculator, and calculate cfm for room tool applies variations of the same physics.
Core CFM Formulas
From tonnage (system airflow):
CFM = Tons × 400 (typical residential)3-ton system → 1,200 CFM total system airflow.
From BTU and temperature change (sensible heat):
CFM = BTU/hr ÷ (1.08 × ΔT°F)Example: 36,000 BTU/hr with 20°F ΔT → CFM = 36,000 ÷ (1.08 × 20) = 1,667 CFM.
From room volume (ventilation):
CFM = Room volume (cu ft) × ACH ÷ 60Example: 1,200 cu ft room at 6 ACH → CFM = 1,200 × 6 ÷ 60 = 120 CFM.
Core CFM formulas side by side
CFM per Room
To calculate cfm for room cooling:
- Get room load from Manual J (BTU/hr)
- Assume system ΔT of 18–22°F across the coil
- CFM = Room BTU ÷ (1.08 × ΔT)
| Room load (BTU/hr) | CFM at ΔT=20°F |
|---|---|
| 4,000 | 185 |
| 6,000 | 278 |
| 8,000 | 370 |
| 10,000 | 463 |
Room CFM at a 20°F coil ΔT
Duct CFM and Velocity
An air duct cfm calculator also checks velocity:
Velocity (fpm) = CFM ÷ Duct area (sq ft)| Duct | Area (sq ft) | CFM at 700 fpm |
|---|---|---|
| 6 in round | 0.196 | 137 |
| 8 in round | 0.349 | 244 |
| 10 in round | 0.545 | 382 |
| 12 in round | 0.785 | 550 |
Keep branch velocity under ~900 fpm to avoid register noise.
Duct CFM capacity at 700 fpm
Measuring Actual CFM
Field methods:
- Anemometer traverse at supply register
- Flow hood over register (most accurate for single outlet)
- Static pressure + fan chart from equipment manufacturer
- T × A × V (temperature factor × area × velocity) in ducts
For static pressure diagnostics, see HVAC Static Pressure. For duct sizing from CFM, see Duct Size Chart.
Use the Free Calculator
HVAC CFM Calculator — hvac cfm calculator from tonnage, BTU, or room volume.
HVAC Duct Size Calculator — air duct cfm calculator to duct diameter.
FAQ
What is the cfm calculation formula in hvac?
The most common: CFM = BTU/hr ÷ (1.08 × ΔT). For whole systems: CFM ≈ tons × 400.
How do I calculate cfm for room?
Divide room cooling load (BTU/hr) by (1.08 × design temperature drop across the coil, typically 18–22°F).
What is a good hvac cfm calculator?
One that accepts tonnage, BTU, or room dimensions and returns CFM with velocity check for the selected duct size.
How much CFM per ton?
400 CFM per ton is the residential default. Range is 350–450 depending on equipment and climate (humid climates often want 350–400 for better dehumidification).