Free · Airflow & ductwork
HVAC CFM Calculator
Airflow from tonnage, from BTUs and temperature difference, from room size and air changes, or from a velocity reading in a duct. Plus a furnace temperature rise check that tells you whether the blower is set right.
System
Required airflow
1,200CFM
At 350 / ton
1,050 CFM
Humid climates
At 450 / ton
1,350 CFM
Dry climates, heat pumps
Nominal airflow by tonnage
| Tons | 350 / ton | 400 / ton | 450 / ton |
|---|---|---|---|
| 1.5 ton | 525 | 600 | 675 |
| 2 ton | 700 | 800 | 900 |
| 2.5 ton | 875 | 1,000 | 1,125 |
| 3 ton | 1,050 | 1,200 | 1,350 |
| 3.5 ton | 1,225 | 1,400 | 1,575 |
| 4 ton | 1,400 | 1,600 | 1,800 |
| 5 ton | 1,750 | 2,000 | 2,250 |
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The CFM formulas
The 1.08 is air density (0.075 lb per cubic foot) times its specific heat (0.24 BTU per pound per °F) times 60 minutes. It is a sea-level number; at 5,000 feet it drops to about 0.90, so airflow needs go up 20%.
Which mode to use
- By tonnage for setting blower speed on a new install. 400 CFM per ton is nominal; use 350 in humid climates to wring more moisture out of the air and 450 in dry climates or for heat pump heating.
- From BTU and ΔT when you have a room-by-room load and want the CFM each register needs to deliver. A 20°F cooling split is typical.
- Air changes for ventilation and exhaust: bathrooms, kitchens, shops, server rooms.
- Duct velocity to turn an anemometer reading into airflow when you do not have a flow hood.
- Furnace temp rise to verify blower setup. A rise outside the nameplate range means the airflow is wrong, and high rise is the number one cause of cracked heat exchangers.
Airflow problems show up everywhere else
Low airflow looks like low charge on the gauges, high temperature rise on the furnace, and iced coils in July. Before you touch the refrigerant, confirm CFM: clean filter, blower wheel and coil, correct speed tap, and a total external static under the equipment's rating. Then size the ducts that are choking it with the duct size calculator.
Frequently asked questions
How many CFM per ton of air conditioning?
400 CFM per ton is the nominal figure. Use 350 CFM per ton in humid climates where dehumidification matters more than sensible capacity, and 450 CFM per ton in dry climates or for heat pumps in heating mode. A 3-ton system therefore needs 1,050–1,350 CFM.
How do I calculate CFM from BTU?
Divide BTU per hour by 1.08 times the temperature difference in °F. A 36,000 BTU/h sensible load with a 20°F split needs 36,000 ÷ (1.08 × 20) = 1,667 CFM. For heating with a 50°F rise the same BTUs need only 667 CFM.
What temperature rise should a furnace have?
Whatever the nameplate says, commonly 35–65°F on 80% furnaces and 20–50°F or 30–60°F on condensing furnaces. Measure supply and return temperatures out of line of sight of the heat exchanger, subtract, and compare. High rise means low airflow.
How many air changes per hour do I need?
Bedrooms and offices 4–6, kitchens and bathrooms 8, commercial kitchens and shops 12 or more, and code minimums for whole-house ventilation are set by ASHRAE 62.2, which works out lower. The presets on this page are common design targets, not code values.
Can I measure CFM with an anemometer?
Yes, with a traverse. Take velocity readings across the duct in a grid, average them, and multiply by the duct area in square feet. A single center reading overstates flow because velocity drops near the duct walls.