Energy & Workshop Math
CFM Calculator
Convert cooling capacity to airflow, or size room airflow from volume and ACH.
Enter your measurements
Results update as you type.
About this calculator
Convert a room volume and chosen ACH into airflow, or compare cooling capacity with the explicit 400 CFM/ton planning assumption. Confirm actual airflow against equipment and duct specifications.
How to measure
- HVAC mode: read the equipment's rated capacity in BTU/h.
- Room mode: measure internal length, width and height, and choose the design ACH.
- Airflow is a planning figure — verify with a balancing contractor.
How to use this calculator
- Pick the mode for the number you already have.
- For equipment, enter the cooling capacity in BTU/h.
- For a room, measure internal dimensions and enter the design ACH.
- Use the airflow to compare equipment or plan ventilation; confirm duct and register capacity separately.
Formula
HVAC mode: CFM = BTU/h ÷ 12,000 × 400
Room mode: CFM = room volume ft³ × ACH ÷ 60
Room volume ft³ = length × width × height
Assumptions
- The HVAC mode uses an explicit planning assumption of 400 CFM per refrigeration ton; actual equipment airflow varies.
- Room mode uses your selected air changes per hour; it does not prescribe a ventilation safety target.
Common mistakes
- Using room capacity without a chosen ACH or measured flow basis.
- Treating 400 CFM/ton as a universal equipment requirement.
- Ignoring duct and filter pressure losses when selecting actual equipment.
Worked example
Try it: press “Load example” above the form.
24,000 BTU/h is two nominal tons; at 400 CFM per ton the planning airflow is 800 CFM.
Limitations
Delivered airflow depends on duct static pressure, filter and coil pressure drop, and register coverage. This is not a duct design or commissioning calculation.
Frequently asked questions
What does CFM measure?
CFM means cubic feet per minute. Room mode multiplies room volume in ft³ by your selected ACH and divides by 60. The result describes a volumetric flow.
Is 400 CFM per ton a requirement?
No. It is an explicit planning assumption for the capacity comparison mode. Manufacturer specifications, sensible and latent loads and duct resistance determine an installation’s actual airflow.
What does the worked example show?
24,000 BTU/h is two nominal tons; at 400 CFM per ton the planning airflow is 800 CFM.
Which assumptions should I check before using the result?
Delivered airflow depends on duct static pressure, filter and coil pressure drop, and register coverage. This is not a duct design or commissioning calculation.
Sources
Sources explain the methods and any named product presets. Check your product label where coverage, yield or density differs.
- Omni: flow rate
Q = volume/time and area × mean velocity. ACH is airflow per hour divided by room volume; no required ventilation rate is recommended.
Calculation documentation · Updated 2026-11-08 · Version 1.6.4. Estimates are planning aids. Confirm quantities, coverage and prices with your supplier before purchasing. Found an error? Report it.
Related calculators
All Energy & Workshop Math calculatorsBTU Calculator
Estimate room cooling BTU per hour from area and simple adjustments.
Open →AC Size Calculator
Estimate room air-conditioner capacity from floor area and conditions.
Open →Air Changes Per Hour Calculator
Compute ACH from room volume and airflow, in cubic metres per hour or CFM.
Open →Flow Rate Calculator
Convert a measured volume and time, or a pipe diameter and velocity, into flow rate.
Open →Watts to Amps Calculator
Convert real power in watts to load current for DC, AC single-phase or balanced three-phase circuits.
Open →Amps to Watts Calculator
Convert load current in amps to real power in watts for DC, AC single-phase or balanced three-phase circuits.
Open →