HVAC CFM Calculator

Get the airflow a system or a single room actually needs. Calculate CFM from tonnage, from the sensible heat equation with your measured ΔT, or from room volume and air changes per hour, with CFM-per-ton and CFM-per-square-foot cross-checks. Free, no signup.

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CFM Calculator

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System capacity

Required airflow

1,200 CFM

3.0 tons × 400 CFM per ton

Inside the 350-450 CFM/ton window
Airflow
1,200 CFMTons × CFM per ton
System capacity
3.0 tons36,000 BTU/hr
CFM per ton
400 CFM/tonTarget window 350-450 (400 nominal)
CFM per sq ft
-Add a conditioned floor area to see this

This is total system airflow. Split it across rooms by load, then size the trunk and branches with the duct size calculator.

1.5 tons · 18,000 BTU/hr
600 CFM
2 tons · 24,000 BTU/hr
800 CFM
2.5 tons · 30,000 BTU/hr
1,000 CFM
3 tons · 36,000 BTU/hr
1,200 CFM
3.5 tons · 42,000 BTU/hr
1,400 CFM
4 tons · 48,000 BTU/hr
1,600 CFM
5 tons · 60,000 BTU/hr
2,000 CFM

At 400 CFM per ton.

Nominal airflow
400 CFM per ton
Humid / dry climate
350 / 450 CFM per ton
Sensible heat constant
1.08 = 0.075 × 0.24 × 60
Cooling coil ΔT (split)
16-22 °F
Furnace temperature rise
30-70 °F (rating plate)
Whole-home airflow
1.0-1.25 CFM per sq ft
Typical residential ACH
4-8 for most rooms

  • Pick how you know the load

    Switch between tonnage (or BTU/hr), a measured temperature split, and room air changes per hour, whichever number you already have in hand.

  • Enter the job numbers

    Capacity and climate, or sensible BTU/hr and ΔT, or room dimensions and a room-type ACH. Add site elevation to correct the 1.08 constant for thin mountain air.

  • Check the airflow against the benchmarks

    See required CFM plus CFM per ton against the 350-450 window and CFM per square foot against the whole-home rule of thumb, then take the CFM to the duct size calculator.

How it works

  1. 1

    Pick how you know the load

    Switch between tonnage (or BTU/hr), a measured temperature split, and room air changes per hour, whichever number you already have in hand.

  2. 2

    Enter the job numbers

    Capacity and climate, or sensible BTU/hr and ΔT, or room dimensions and a room-type ACH. Add site elevation to correct the 1.08 constant for thin mountain air.

  3. 3

    Check the airflow against the benchmarks

    See required CFM plus CFM per ton against the 350-450 window and CFM per square foot against the whole-home rule of thumb, then take the CFM to the duct size calculator.

How to calculate CFM for an HVAC system

There are three ways contractors arrive at CFM, and this calculator does all three. From capacity: airflow equals tons times a CFM-per-ton target, 400 CFM per ton nominal, 350 in humid climates where you want more moisture removal, 450 in dry climates where sensible cooling matters more. From a measured temperature split: airflow equals sensible BTU/hr divided by 1.08 times ΔT, where 1.08 is air density (0.075 lb/ft³) times specific heat (0.24 Btu/lb·°F) times 60 minutes per hour. From ventilation: airflow equals room volume times air changes per hour divided by 60.

CFM is the input to everything downstream, so get it right before you cut metal. Establish the load first with the HVAC Load Calculator or the BTU Calculator, use this tool to convert that load into airflow, then take the CFM into the Duct Size Calculator to size trunks and branches, and into the Static Pressure Calculator to confirm the blower can actually move it. One note on the 1.08 constant: it assumes sea-level air. Above roughly 2,000 feet, thinner air carries less heat per cubic foot, so the same load needs more CFM, enter your site elevation and the calculator scales the constant by the ASHRAE air-density factor.

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Worth asking before you book

CFM Calculator FAQs

400 CFM per ton is the nominal design point, so a 3-ton system needs about 1,200 CFM and a 5-ton system about 2,000 CFM. Drop to 350 CFM per ton in humid climates to pull more moisture out of the air, and go up to 450 CFM per ton in dry climates. Below 350 the coil runs cold enough to risk freezing; above 450 you get almost no dehumidification.
CFM = sensible BTU/hr ÷ (1.08 × ΔT). The 1.08 constant is air density (0.075 lb/ft³) × specific heat of air (0.24 Btu/lb·°F) × 60 minutes per hour. For example, a 12,000 BTU/hr sensible load across a 20°F split needs 12,000 ÷ (1.08 × 20) = 556 CFM. A healthy cooling coil splits 16-22°F; a gas furnace rises 30-70°F, check the rating plate.
Multiply length × width × ceiling height to get room volume in cubic feet, multiply by the recommended air changes per hour for that room type, then divide by 60. A 12 × 14 ft room with a 10 ft ceiling at 6 ACH needs 1,680 × 6 ÷ 60 = 168 CFM. Bedrooms run 5-6 ACH, living rooms 6-8, kitchens 7-8, and restaurant kitchens 15-20.
Yes, for the ΔT method. The 1.08 constant assumes sea-level air density, and thinner air carries less heat per cubic foot, at 5,000 ft the density factor is about 0.83, so the constant drops to roughly 0.90 and the same load needs about 20% more airflow. The tonnage and air-change methods are volumetric, so elevation does not change those answers.
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