CFM Calculator

Which CFM Method to Use

CFM, cubic feet per minute, is the volume of air a fan, duct or register moves. There is no single CFM formula because the right answer depends on why you need the air.

• Air changes per hour: for exhaust, fresh air, workshops, server closets and other spaces where you want to replace the room’s air a set number of times per hour.
• BTU load and temperature difference: for heating and cooling supply air. This is how much air it takes to carry a load at a given supply temperature, and it is how room CFM is set in a Manual J and Manual D design.
• Duct size and velocity: for checking how much air an existing duct carries when you measure the velocity, or what a duct can carry at a design velocity.
• Ventilation code: for the continuous fresh air a house needs (ASHRAE 62.2 or the IRC) and the minimum bathroom and kitchen exhaust.

Air Changes per Hour vs. Load-Based Airflow

Many CFM calculators give a target ACH for each room, like 5 to 6 for a bedroom or 7 to 9 for a kitchen, and multiply by the room volume. Those targets are rules of thumb. No residential code sets supply airflow by air changes per hour.

For heating and cooling, a room needs the airflow that carries its share of the load. A south-facing bedroom with big windows needs more air than an interior bedroom of the same size. Use ACH for exhaust and ventilation targets you have been given, and the load method for supply air.

On a whole system, the cooling airflow usually lands near 400 CFM per ton of capacity. Humid climates often run closer to 350 for better moisture removal, and dry climates 450 or more.

CFM Formulas

All four methods are simple ratios. The 1.08 constant is the sensible heat of standard air at sea level: 60 minutes × 0.075 lb per cubic foot × 0.24 BTU per pound per °F.

L, W, H

Room length, width and ceiling height in feet

ACH

Air changes per hour

ΔT

Temperature difference between supply and room (or return) air, °F

A

Duct cross-section in square feet (rectangular: W × H ÷ 144)

V

Average air velocity in feet per minute

Use only the sensible part of a cooling load in the BTU formula. Total capacity includes the latent load (moisture), which does not change the air temperature.

Worked Example: Bedroom at 6 Air Changes

A 12 by 15 ft bedroom with 8 ft ceilings, at a target of 6 air changes per hour.

1. Volume: 12 × 15 × 8 = 1,440 cubic feet.
2. Airflow: 1,440 × 6 ÷ 60 = 144 CFM.

Result: 144 CFM.

Worked Example: Airflow for a 3-Ton Air Conditioner

A 3-ton (36,000 BTU/h) system with a sensible capacity of 27,000 BTU/h and a 20°F difference between return and supply air.

1. Airflow: 27,000 ÷ (1.08 × 20) = 1,250 CFM.
2. Per ton: 1,250 ÷ 3 = 417 CFM per ton, close to the usual 400.

Result: 1,250 CFM.

Worked Example: Furnace Airflow From Temperature Rise

A 60,000 BTU/h input furnace at 96% AFUE (about 57,600 BTU/h output) with a measured temperature rise of 50°F.

1. Airflow: 57,600 ÷ (1.08 × 50) = 1,067 CFM.

Result: about 1,067 CFM. If the rise were 70°F, the same furnace would be moving only 762 CFM, a sign of a dirty filter, a closed damper or a low blower speed.

Worked Example: Whole-House Ventilation

A 2,000 sq ft house with 3 bedrooms.

1. ASHRAE 62.2: 0.03 × 2,000 + 7.5 × (3 + 1) = 60 + 30 = 90 CFM.
2. IRC: 0.01 × 2,000 + 7.5 × (3 + 1) = 20 + 30 = 50 CFM.

Result: 50 CFM continuous meets the IRC formula; 90 CFM meets ASHRAE 62.2, which some energy programs and states require.

CFM per Ton Chart

Total system airflow for common air conditioner and heat pump sizes at three airflow targets.

Round Duct CFM by Velocity

Airflow a round duct carries at three common design velocities. Size for friction too: a small duct can reach the velocity long before it reaches the CFM.

Field Tips

Practical advice from Green Building Advisor, Greenheck, Contracting Business and HVAC School, checked against the code.

1. 1

   Bath fans rarely move their rated CFM

   Bath fans are rated at 0.1 in. wg of static pressure. Testing by Steven Winter Associates found typical installs at 0.30 to 0.45 in. wg, and most fans lose airflow steadily above 0.1. Short, straight, full-size duct and a fan rated at higher static pressure close the gap.

2. 2

   Big range hoods may need makeup air

   The IRC requires makeup air for kitchen exhaust over 400 CFM when the house has atmospherically vented appliances that could backdraft, such as a natural-draft water heater. The makeup air has to roughly equal the exhaust rate.

3. 3

   Read the register’s free area, not its face size

   A register’s open area is smaller than its face. When you measure with an anemometer, average a grid of readings across the face and multiply by the manufacturer’s free area (Ak) factor, not the nominal size.

4. 4

   Don’t trust a flow hood blindly

   Lawrence Berkeley Lab testing found residential flow hood readings off by about 20% to 30%, mostly because of uneven airflow off the grille. Use the readings to compare rooms and find problems, not as exact numbers.

5. 5

   Use temperature rise to check blower airflow

   On a furnace, CFM = output ÷ (1.08 × rise). A 57,600 BTU/h output furnace with a 50°F rise is moving about 1,067 CFM. A rise near the top of the nameplate range usually means low airflow.

Sources: Greenheck: Specifying ceiling exhaust fans to meet code, Green Building Advisor: Makeup air for kitchen exhaust, Contracting Business: How to take an accurate supply register airflow reading, Green Building Advisor: Simple methods for measuring air flow, HVAC School: Furnace air temperature rise