Ventilation and Airflow CFM Calculator
Estimate airflow by air changes per hour or direct source rate, plus preliminary round-duct area and diameter at an editable velocity.
How to use the ventilation cfm calculator
- Measure the actual project. Break irregular work into separate sections and record finished dimensions consistently.
- Enter the core measurements. The quick view focuses on project geometry; add or subtract sections where the calculator supports multiple areas.
- Review purchasing assumptions. Open Advanced inputs to replace example prices, waste, coverage, density, pack size, minimum order, or delivery assumptions with current project information.
- Calculate and verify. Review the formula, limitations, whole-unit rounding, product instructions, and supplier requirements before purchasing.
Key inputs covered: Airflow calculation mode, Target air changes, Required source airflow, Planning duct air velocity, Room length, Room width, Room height, Identical rooms.
Result coverage: The ventilation cfm calculator reports 6 separate quantities so the primary estimate and its supporting purchase or planning values can be checked together.
How the estimate works
ACH mode uses room volume x air changes / 60; direct mode uses the entered source rate. CFM divided by entered velocity gives free area, converted to an equivalent round diameter.
A 960 ft3 room at 6 ACH requires 96 CFM before any system-effect or balancing calculation.
Assumptions, rounding, and limitations
Dimensions are treated as finished measurements. Only deductions and allowances entered above are included. Decimal measurements display to two places; purchase quantities round up when a whole container or piece is required.
- This calculator does not determine a required code airflow, certify indoor-air quality, size a complete duct system, or account for pressure loss, fittings, filters, fan curves, noise, balancing, makeup air, or combustion safety.
- ASHRAE 62.1/62.2, mechanical codes, local amendments, room use, source control, and equipment instructions may prescribe different calculations and verification.
- Have a qualified ventilation/HVAC professional select and commission the system.
Sources and review
These references support unit conversions and selected planning assumptions. Product instructions and local requirements control the actual project.
Last reviewed: 2026-10-09
Editorial process
BuildPlanCalc publishes the formula, editable assumptions, limitations, and supporting references so the estimate can be checked rather than treated as a black box. See the calculation and review methodology, or report a correction.
Frequently asked questions
Are the results exact?
No. Results are planning estimates based on the dimensions and assumptions entered. Confirm product coverage, site conditions, and ordering rules with the supplier.