How to use this calculator
- Enter airflow. Use supply, return or coil airflow in CFM or m³/h.
- Enter temperature difference. Use the air temperature rise for heating or the air temperature drop across a cooling coil.
- Enter target sensible load. Use the target BTU/h or kW to calculate required airflow and required ΔT.
- Check CFM per ton. For cooling checks, compare the CFM/ton result with the coil and blower design target.
How it works
The calculator uses the sensible heat relation:
q = ρ · cₚ · Q · ΔT
where ρ is air density, cₚ is specific heat,
Q is airflow and ΔT is the temperature rise or drop.
With standard air in imperial units, this is the familiar shortcut:
BTU/h ≈ 1.08 · CFM · Δ°F
Required airflow is found by rearranging the same equation:
Q_req = q / (ρ · cₚ · ΔT)
and the required temperature difference is ΔT_req = q/(ρ·cₚ·Q).
Cooling tons are reported from 1 ton = 12,000 BTU/h.
Use the required airflow result as the starting point for the duct sizing calculator. After selecting duct and terminal sizes, the HVAC static pressure calculator keeps the duct, filter, coil and diffuser losses inside the blower budget.
Worked example
Verified against the live calculator
With 530 CFM of standard air and a 20°F coil
temperature drop, the sensible cooling is
1.08 × 530 × 20 ≈ 11,400 BTU/h, or about
0.95 tons. A target sensible load of 17,100 BTU/h
at the same 20°F drop would need roughly 790 CFM.
Frequently asked questions
What is the HVAC CFM to BTU formula?
For standard air, sensible BTU/h is commonly estimated as BTU/h = 1.08 × CFM × Δ°F. The 1.08 factor comes from air density, specific heat and minutes per hour.
Does this calculate total cooling load?
No. It calculates sensible load only: the dry-bulb temperature rise or drop carried by airflow. Latent load from moisture removal needs a psychrometric or Manual J/ASHRAE load calculation.
How many CFM per ton is typical?
Comfort cooling is often screened around 350-450 CFM per ton, with many systems near 400 CFM/ton. The right value depends on latent load, coil selection, blower capability, noise and manufacturer data.
Can I size HVAC equipment with this calculator?
Use it for airflow and sensible-capacity checks, not final equipment sizing. Residential equipment sizing should use ACCA Manual J/Manual S or the applicable commercial ASHRAE load method and manufacturer performance data. Once airflow is known, use the duct size calculator and grille size calculator for first-pass distribution checks.
Method & assumptions
- Calculates sensible heat only; latent/moisture load is not included.
- Defaults use dry air near room temperature: ρ = 1.2 kg/m³ and cₚ = 1.006 kJ/(kg·K).
- Final HVAC load, duct design and equipment sizing should use Manual J/Manual S, Manual D, ASHRAE load methods, psychrometrics and manufacturer performance data.