FreeHVACCalculators

CFM Calculator

Find the airflow in cubic feet per minute to move a given load at a given temperature difference — or to change a room’s air a set number of times an hour.

Method reviewed October 22, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Use the sensible (temperature) part of the load, not the total.

Typically 15–22 °F for cooling and 40–70 °F for furnace heating.

Thinner air at altitude carries less heat per CFM. Leave at 0 for sea-level (standard-air) results.

Result

Required airflow
1,111CFM

Moving 24,000 BTU/h of sensible heat with a 20 °F temperature difference takes about 1,111 CFM.

Sensible heat factor used
1.08
Show the working
  1. Sensible heat factor (1.08 × air-density ratio)
    1.08 × 1 = 1.08
  2. CFM = Q ÷ (factor × ΔT)
    24,000 / (1.08 × 20) = 1,111 CFM

Airflow from a heat load

Air carries heat according to the sensible heat equation, Q = 1.08 × CFM × ΔT. Rearranged, CFM = Q ÷ (1.08 × ΔT). The 1.08 comes from 60 minutes per hour × 0.075 lb per cubic foot of standard air × 0.24 BTU per pound per °F.

At altitude air is thinner, so each cubic foot carries less heat; the calculator scales the factor by the air-density ratio for the elevation you enter.

Airflow from air changes

For ventilation, exhaust or air cleaning, airflow is often specified in air changes per hour: CFM = room volume × ACH ÷ 60. A 15 × 12 × 8 ft room has 1,440 ft³, so 6 ACH needs 144 CFM.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Calculate airflow from
Heat load and temperature difference
Sensible heat load
24,000 BTU/h
Supply-to-room temperature difference
20 °F
Site elevation
0 ft

Result

Required airflow
1,111 CFM
Sensible heat factor used
1.08

Moving 24,000 BTU/h of sensible heat with a 20 °F temperature difference takes about 1,111 CFM.

Assumptions

  • The 1.08 factor is 60 min/h × 0.075 lb/ft³ (standard air) × 0.24 BTU/lb·°F, and is scaled by the standard-atmosphere air-density ratio at the site elevation.
  • Air-change calculations use the gross room volume.

Limitations

  • Use sensible load only; latent (moisture) load does not change the airflow given by this equation.
  • Air-change targets vary by room type and by code — check the requirement that applies to your space.

Frequently asked questions

How many CFM do I need per room?

For heating and cooling, it is the room’s sensible load divided by 1.08 × the supply temperature difference — so it follows the room-by-room load calculation, not floor area. For ventilation, it follows the air-change or code requirement for that room type.

Should I use total or sensible load?

Sensible. The equation relates airflow to the temperature change of the air. Latent (moisture) removal happens at the coil and does not appear in this equation.

Sources

  1. Regional Line Card — air-side formulas — Trane. Sensible = 1.08 × CFM × ΔT; latent = 0.68 × CFM × Δgr; total = 4.5 × CFM × Δh, with the derivation of each constant.
  2. Cooling and Heating Equations — The Engineering ToolBox. Sensible and latent heat equations for air in IP units.
  3. PsychroLib — psychrometric functions after ASHRAE Handbook—Fundamentals (2017) ch. 1 — PsychroLib (open source, MIT licence). Humidity ratio W = 0.621945·pw/(p − pw), wet-bulb, enthalpy and standard-atmosphere equations as published by ASHRAE.
  4. Air Change Rates in Rooms — The Engineering ToolBox. ACH = CFM × 60 / room volume.

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