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CFM Calculator

Work out how much airflow, in cubic feet per minute, a room or system needs. Start from the room size and air changes per hour, or from the heat load and temperature difference.

Level: Grades 6–7

  • Common Core standard 6.G.A.2Find the volume of a right rectangular prism with fractional edge lengths by packing it with unit cubes of the appropriate unit fraction edge lengths, and show that the volume is the same as would be found by multiplying the edge lengths of the prism. Apply the formulas V = l w h and V = b h to find volumes of right rectangular prisms with fractional edge lengths in the context of solving real-world and mathematical problems.
  • Common Core standard 6.RP.A.3.dUse ratio reasoning to convert measurement units; manipulate and transform units appropriately when multiplying or dividing quantities.
  • Common Core standard 6.EE.A.2.cEvaluate expressions at specific values of their variables. Include expressions that arise from formulas used in real-world problems. Perform arithmetic operations, including those involving whole-number exponents, in the conventional order when there are no parentheses to specify a particular order (Order of Operations).
Printable worksheet

CFM Calculator

Know the answer you need? Pick an input to work backward.

Inputs
Work out CFM from

How many times per hour the room's volume of air should be replaced. Required rates depend on the room's use and the code or guideline that applies.

Result

Airflow

96 CFM

A 12 × 10 × 8 ft room at 6 air changes per hour needs 96 CFM (163.1 m³/h).

Airflow
163.1 m³/h
Airflow
45.3 L/s
Room volume
960 ft³

See the step-by-step solution ↓

Step-by-step solution

  1. Find the room's volume: length × width × height

    12×10×8=960 ft³

  2. Multiply the volume by the air changes per hour, then divide by 60 to turn 'per hour' into 'per minute'

    960×660=96 CFM

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out how much airflow a room or system needs, in CFM (cubic feet per minute). That number is behind almost every ventilation and forced-air decision. It tells you how big a fan to buy, how much air a register (vent) should deliver, and how large the duct feeding it must be.

There are two everyday ways to get it. One starts from the room: its volume and how many times an hour the air should be replaced. The other starts from the heat the air has to carry. It also needs how much warmer or cooler the supply air is than the room.

This CFM calculator does both. In room mode, it multiplies the volume by the air changes per hour and divides by 60 minutes. In load mode, it uses the sensible heat formula, Btu/h = 1.08 × CFM × ΔT, and solves it for the airflow. Sensible heat is the heat that changes the air's temperature. You can enter the load in Btu/h, watts or tons, and the temperature difference in °F or °C. Every result also comes in cubic metres per hour and litres per second.

Where it's used

Why this math matters outside the classroom:

  • Choosing a bath fan. A homeowner finds an 8 × 6 ft bathroom with an 8 ft ceiling at 8 air changes an hour needs 51.2 CFM.
  • Workshop ventilation. A woodworker sizes an exhaust fan for a 24 × 20 × 10 ft shop at 6 ACH, which works out to 480 CFM.
  • Checking a furnace. A technician finds a 60,000 Btu/h furnace with a 50 °F rise moves about 1,111 CFM.
  • Air-conditioner airflow. An HVAC student works out that 18,000 Btu/h of sensible cooling at an 18 °F difference takes about 926 CFM.
  • Unit conversion practice. Students turn 96 CFM into 163.1 m³/h by multiplying by 0.0283168 m³ per cubic foot and 60 minutes.

Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.G.A.2, 6.RP.A.3.d, 6.EE.A.2.c. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose whether to work from the room size and air changes, or from the heat load and temperature difference.
  2. For a room, enter the length, width and ceiling height in feet and the air changes per hour you need.
  3. For a load, enter the sensible load and the temperature difference between the supply air and the room air. Leave the factor at 1.08 unless you're well above sea level.
  4. Read the airflow in CFM, with m³/h and L/s alongside.

What the inputs mean

Air changes per hour
The right rate depends on the space and how it's used, and building codes or health guidance often set it. A bedroom, a kitchen, a workshop and a paint booth have very different needs.
Sensible heat load
Only the sensible part of the load belongs here. A cooling load also has a latent part, the moisture the coil removes from the air. This formula doesn't cover that.
Temperature difference (ΔT)
This is a difference, not a temperature. A 20 °F difference equals 11.1 °C, because each Celsius degree is 1.8 Fahrenheit degrees.
Sensible heat factor
The 1.08 comes from standard air: 0.075 lb per cubic foot × 60 minutes per hour × 0.24 Btu per pound per °F. Thinner air at altitude lowers it by the same proportion.

Formula and method

CFM = room volume (ft³) × ACH ÷ 60
Btu/h = 1.08 × CFM × ΔT (°F)   so   CFM = Btu/h ÷ (1.08 × ΔT)
1.08 = 0.075 lb/ft³ × 60 min/h × 0.24 Btu/(lb·°F)
1 W = 3.412142 Btu/h;  1 ton = 12,000 Btu/h;  1 CFM = 1.699011 m³/h = 0.471947 L/s

ACH (air changes per hour) counts how many roomfuls of air pass through each hour. Multiply the volume by that count to get cubic feet per hour. Then divide by 60 to turn it into cubic feet per minute.

In the second formula, Btu/h is the heat the air carries, in British thermal units per hour. ΔT (delta T) is the temperature difference between the supply air and the room. The formula is a heat balance for moving air. The heat carried equals how much air flows, times how much heat each pound holds per degree (its specific heat), times the temperature change. Using standard air and 60 minutes in an hour, the constants combine into 1.08. That works when airflow is in CFM, the difference is in °F and heat is in Btu/h.

Worked example

A 12 × 10 ft room with an 8 ft ceiling needs 6 air changes per hour.

  1. Room volume: 12 × 10 × 8 = 960 ft³.
  2. Airflow: 960 × 6 ÷ 60 = 96 CFM.
  3. In metric: 96 × 1.699011 = 163.1 m³/h, or 45.3 L/s.

Result: The room needs about 96 CFM of airflow to get 6 air changes per hour.

Understanding the result

The result is the airflow that actually reaches the space. A fan's rated CFM is usually measured with little in its way. Ducts, filters and grilles cut how much it moves. So pick a fan that delivers the result at the pressure it will actually face.

In load mode, a bigger temperature difference means less air for the same heat. Furnaces run large differences and need less air per Btu. Air conditioners run smaller ones and need more.

Assumptions and limitations

  • Room mode assumes a rectangular room where the air mixes completely.
  • Load mode uses standard air near sea level unless you change the factor. It covers sensible heat only.
  • It doesn't size the equipment or work out the heating or cooling load itself. Use a proper load calculation for that.
  • Required ventilation rates come from codes, standards and the equipment maker, not from this calculator. A qualified HVAC professional should design or approve the system.
  • Moisture (latent) loads, leaky ducts and fan performance curves are outside its scope.

Frequently asked questions

How do I calculate CFM for a room?

Multiply length × width × ceiling height to get the volume in cubic feet. Multiply that by the air changes per hour you need, then divide by 60. A 12 × 10 ft room with an 8 ft ceiling at 6 ACH needs 960 × 6 ÷ 60 = 96 CFM.

How do I convert BTU to CFM?

You need the temperature difference too. Divide the Btu/h by 1.08 × ΔT in °F. For 24,000 Btu/h with a 20 °F difference, that's 24,000 ÷ 21.6 = 1,111 CFM.

Where does the 1.08 in the sensible heat formula come from?

It's standard air density (0.075 lb/ft³) times 60 minutes per hour times the specific heat of air (0.24 Btu per pound per °F). It assumes air near sea level. At 5,000 ft the air is about 17% thinner, so the factor drops to roughly 0.90.

Is this the same as the 400 CFM per ton rule?

No. That rule of thumb sets a typical airflow for air-conditioning equipment. This calculator works from the sensible load and the actual temperature difference. That can give quite different numbers, especially in humid climates, where part of the system's capacity goes to removing moisture.

Sources

How this calculator is checked

Method: CFM = volume × ACH ÷ 60; sensible heat Btu/h = 1.08 × CFM × ΔT°F, with 1.08 = 0.075 lb/ft³ × 60 min/h × 0.24 Btu/(lb·°F) for standard air. Exact unit factors: 1 ft³ = 0.028316846592 m³, IT Btu = 1055.05585262 J, 1 ton = 12,000 Btu/h. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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