How do I calculate CFM for a room?

Multiply the room length, width, and ceiling height in feet to get the volume in cubic feet, multiply by the recommended air changes per hour (ACH), and divide by 60. For example, a 10 x 12 ft bathroom with an 8 ft ceiling at 8 ACH needs (10 x 12 x 8 x 8) / 60 = 128 CFM.

How many air changes per hour do I need?

It depends on the room. Kitchens need the most at about 15 ACH, bathrooms and laundry rooms about 8, living rooms about 8, bedrooms and offices about 6, garages about 7, and basements about 4. These are common ventilation guidelines; local codes may require specific minimums.

How do I calculate CFM from BTU?

Use CFM = BTU/h divided by (1.08 x temperature difference in degrees F). The 1.08 constant combines the density and specific heat of standard air with the minutes-per-hour conversion. A 60,000 BTU/h furnace with a 20 degree rise needs 60,000 / (1.08 x 20) = about 2,778 CFM.

How many CFM per ton of air conditioning?

The standard rule of thumb is 400 CFM per ton of cooling, where 1 ton equals 12,000 BTU/h. The acceptable range is roughly 350 to 450 CFM per ton; drier climates often run higher airflow while humid climates run lower to improve moisture removal.

CFM Calculator
(Airflow / Air Changes)

Calculate required airflow in CFM from room size and air changes per hour, or from heating and cooling load in BTU/h and temperature rise.

Back to Calculators

How much airflow does your room need?

Enter the room dimensions and pick a room type. We preset the recommended air changes per hour (ACH) for you.

Room Length (ft)

Room Width (ft)

Ceiling Height (ft)

Room Type

Enter your room dimensions to see the required CFM.

How this calculator works

Room ventilation (ACH):CFM = (Length × Width × Height × ACH) ÷ 60. Multiply the room volume in cubic feet by the recommended air changes per hour, then divide by 60 to convert from air changes per hour to cubic feet per minute. A 10 × 12 ft bathroom with an 8 ft ceiling at 8 ACH needs (10 × 12 × 8 × 8) ÷ 60 = 128 CFM.

Airflow from load:CFM = BTU/h ÷ (1.08 × ΔT). The 1.08 constant is the sensible heat factor for standard air (0.075 lb/cu ft density × 0.24 BTU/lb-°F specific heat × 60 min/h). A 60,000 BTU/h furnace with a 20°F temperature rise needs 60,000 ÷ (1.08 × 20) = 2,778 CFM.

Cooling rule of thumb: air conditioning systems typically move about 400 CFM per ton of cooling (1 ton = 12,000 BTU/h), with an acceptable range of 350-450 CFM per ton depending on climate and humidity.

These are standard industry estimating formulas. For code compliance and whole-home ventilation design (ASHRAE 62.2), consult a qualified HVAC professional.

Size the job, then run it in WorkZen

Quote it, schedule it, invoice it, and keep every client detail in one place. WorkZen is free to start for HVAC pros.

See WorkZen Pricing

CFM Calculator(Airflow / Air Changes)

How much airflow does your room need?

How this calculator works

Size the job, then run it in WorkZen

CFM Calculator
(Airflow / Air Changes)