Quick answer
Airflow in CFM = duct area (ft²) × air velocity (ft/min), or room volume × air changes per hour ÷ 60. A 1.5 ft² duct with 800 fpm air speed moves 1,200 CFM (566 L/s).
Choose a mode, then enter the area and velocity or the volume and air changes. The calculator returns CFM and litres per second.
Related tools: Window Air Conditioner Size Calculator: Cooling capacity (BTU/h) for a room, from the ENERGY STAR room air conditioner sizing chart. Air Changes per Hour Calculator: Air changes per hour from airflow and room volume. Power-to-Weight Ratio Calculator: Power per unit weight of a vehicle.
How to use the CFM Calculator
- Choose use: Area × velocity and Room volume × air changes per hour.
- Optionally, enter duct area (ft²) (above 0).
- Optionally, enter air velocity (ft/min) (above 0).
- Optionally, enter room volume (ft³) (above 0).
- Optionally, enter air changes per hour (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Use | Choose one: Area × velocity; Room volume × air changes per hour | Starts at Area × velocity |
| Duct area (optional) | A number, in ft² | > 0 |
| Air velocity (optional) | A number, in ft/min | > 0 |
| Room volume (optional) | A number, in ft³ | > 0 |
| Air changes per hour (optional) | A number | > 0 |
Formula and method
Formula
CFM = area × velocity; CFM = volume × ACH ÷ 60
IEEE-754 double precision; shown to the decimals indicated.
1 CFM = 0.4719474 L/s.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Use = Area × velocity; Duct area = 1.5; Air velocity = 800.
| Result | Value |
|---|---|
| Airflow | 1,200 CFM |
| Liters per second | 566.34 |
Example 2
Inputs: Use = Room volume × air changes per hour; Room volume = 2400; Air changes per hour = 6.
| Result | Value |
|---|---|
| Airflow | 240 CFM |
| Liters per second | 113.27 |
Example 3
Inputs: Use = Area × velocity; Duct area = 0.8; Air velocity = 500.
| Result | Value |
|---|---|
| Airflow | 400 CFM |
| Liters per second | 188.78 |
The first line is the headline result (airflow). Below it the cFM Calculator lists liters per second, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Two modes
Area × velocity measures flow in a duct. Volume × ACH ÷ 60 sizes ventilation for a room: 2,000 ft³ at 6 ACH needs 200 CFM. Only the fields for the chosen mode are used.
Typical values
Ducts in homes run at 600 to 900 fpm. A bathroom fan moves 50 to 110 CFM, and a kitchen hood 100 to 400. 1 CFM is 0.4719 L/s.
How the result changes with duct area
Holding every other input at the first worked example, this table changes only duct area. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Duct area | Airflow | Liters per second |
|---|---|---|
| 0.75 | 600 CFM | 283.17 |
| 1.5 | 1,200 CFM | 566.34 |
| 3 | 2,400 CFM | 1,132.67 |
| 6 | 4,800 CFM | 2,265.35 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Duct area: left empty shows "Enter the duct area." beside that field and no result.
- Air changes per hour: left empty shows "Enter the air changes per hour." beside that field and no result.
Choosing between the CFM Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Window Air Conditioner Size Calculator](/engineering/window-air-conditioner-size-calculator/) | room area, area unit and sunlight | Recommended capacity |
| [Air Changes per Hour Calculator](/science/air-changes-per-hour-calculator/) | airflow and room volume | Air changes per hour |
| [Power-to-Weight Ratio Calculator](/engineering/power-to-weight-ratio/) | power, power unit and weight | Power-to-weight |
| [Gallons per Minute Calculator](/engineering/gallons-per-minute/) | use | Flow |
| [Engine Torque Calculator](/engineering/engine-torque-calculator/) | calculate and engine speed | Torque |
Background: structural and wiring estimates
Structural and electrical results are only as good as their assumptions. Bending stress is the bending moment divided by the section modulus, and an allowable stress is the yield strength divided by a safety factor that depends on the governing design code and the type of load. For a single-phase or DC run the voltage drop uses the round-trip length: V = 2 × L × R per unit length × I.
Recommended limits, such as keeping branch-circuit voltage drop around 3%, differ between sources and code editions. These pages are estimates: follow the applicable code and a qualified engineer or electrician for real work.
Assumptions and limitations
- Only the fields for the chosen mode are used
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for duct area and air changes per hour to confirm this.
Frequently asked questions
How many CFM do I need for a room?
Volume times target ACH divided by 60.
What is fpm?
Feet per minute, a measure of air speed.
How do I convert CFM to m³/h?
Multiply CFM by 1.699 to get cubic metres per hour; 1,200 CFM is about 2,039 m³/h.
What does the CFM Calculator calculate?
Airflow in cubic feet per minute from duct velocity or air changes. It returns Airflow and Liters per second.
What formula does the CFM Calculator use?
CFM = area × velocity; CFM = volume × ACH ÷ 60. 1 CFM = 0.4719474 L/s.
What do I need to enter in the cFM Calculator?
You enter Use. Optional: Duct area (ft²), Air velocity (ft/min), Room volume (ft³) and Air changes per hour.
What does the “Use” option change?
It sets how the cFM is worked out. The choices are Area × velocity and Room volume × air changes per hour.
What is an example of a cFM result?
With use Area × velocity, duct area 1.5 and air velocity 800, the airflow is 1,200 CFM.
Where does the method come from?
The method follows: OpenStax, University Physics Volume 1 (Rice University): mechanics; 1 ft³ = 28.316846592 L (exact, from the international foot; NIST Special Publication 811 Appendix B).
Can I rely on this calculator for a decision?
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them. For this calculator in particular: Only the fields for the chosen mode are used.
Related calculators
- Window Air Conditioner Size Calculator — Cooling capacity (BTU/h) for a room, from the ENERGY STAR room air conditioner sizing chart.
- Air Changes per Hour Calculator — Air changes per hour from airflow and room volume.
- Power-to-Weight Ratio Calculator — Power per unit weight of a vehicle.
- Gallons per Minute Calculator — Flow in gallons per minute from pipe size and velocity, or volume and time.
- Engine Torque Calculator — Convert between engine power and torque at a given rpm.
- Transmission Gear Ratio Calculator — Vehicle speed from engine rpm, gear ratio, final drive and tire size.
Sources
- OpenStax, University Physics Volume 1 (Rice University): mechanics
- 1 ft³ = 28.316846592 L (exact, from the international foot; NIST Special Publication 811 Appendix B)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.