Ventilation Calculator — Air Changes per Hour to Airflow

Inputs

• The space: its volume in m³ or ft³, or its floor area (m² or ft²) and average clear height (m or ft).
• Your rate: air changes per hour (ACH), or a required airflow in m³/h, CFM or L/s.
• What the airflow is for — general ventilation, process or hazardous exhaust, smoke extraction (fire mode), or healthcare and infection control. It changes the cautions shown, never the arithmetic.
• Where the rate comes from — a code or standard named in the design basis, a design brief, a supplier's or authority's requirement, a measurement — or that it is not yet sourced; optionally the clause, table or document.
• Optional: the hours a day the system runs, for the volume of air moved per day.

Method

1. Volume (m³) = the volume entered, or floor area × average clear height (1 ft = 0.3048 m; 1 ft² = 0.09290304 m²; 1 ft³ = 0.028316846592 m³).
2. With an air-change rate: airflow (m³/h) = volume × ACH.
3. With an airflow: it is converted to m³/h (1 CFM = 1.6990107955 m³/h; 1 L/s = 3.6 m³/h), and ACH = airflow ÷ volume.
4. CFM = m³/h ÷ 1.6990107955; L/s = m³/h ÷ 3.6; m³/s = m³/h ÷ 3,600.
5. Air moved per day = m³/h × running hours, when hours are given.
6. Sensitivity: the airflow and the air changes are recomputed with the rate 10% and 20% either side.

Assumptions

• The rate you enter is the one your design basis requires for this space and purpose. The tool has no rate of its own — none for offices or stores, and none for hazardous exhaust, infection control or smoke extraction.
• The volume is the air volume the system serves; large fixed contents such as racking, tanks or machines are not deducted unless you deduct them.
• Air changes describe dilution of the whole volume. Air that short-circuits from supply to exhaust, or zones the air does not reach, get less than the number suggests.

Limitations

• Converting an air-change rate does not establish that the rate is required, or safe, for the space.
• Process, fume and hazardous exhaust is sized from the substance, how fast it is released and the standard or authority that governs it — usually by capture at the source — not from an air-change rate.
• Smoke extraction (fire mode) needs its own design basis under the fire code applied in your state and the fire authority's requirements. It is a separate design from normal ventilation, with its own fans, power supply, controls and make-up air.
• Healthcare air-change, pressure and filtration requirements come from the clinical brief and the standard the project follows.
• No fan selection — duct and filter losses, static pressure, noise and make-up air are separate checks — and the heating or cooling load of the outdoor air is not calculated.