Room airflow
calculator.
Find the airflow needed to change a room's air a chosen number of times per hour, or how many air changes a fan or air cleaner provides. Results in CFM, m³/h and L/s, with the time to clear airborne particles.
At 5 air changes per hour, this room needs about 200 CFM (339.8 m³/h).
- Airflow, metric
- 339.8 m³/h
- Airflow, L/s
- 94.389 L/s
- Room volume
- 2,400 ft³ (67.96 m³)
- Time to clear 99% / 99.9%*
- 55 / 83 min
CFM = volume (ft³) × ACH ÷ 60Air changes,
in plain numbers.
- CFM = room volume (ft³) × ACH ÷ 60. In metric, m³/h = room volume (m³) × ACH.
- ACH = CFM × 60 ÷ room volume. You choose the ACH; the calculator does not recommend one.
- Clearance time = ln(100 ÷ remaining %) ÷ ACH × 60 minutes, assuming perfect mixing and no ongoing source (CDC).
- This is an air-exchange estimate. It is not a heating or cooling load (Manual J), equipment sizing (Manual S), duct design (Manual D) or a code ventilation rate.
How do you calculate CFM from air changes per hour?
Multiply the room's length, width and height to get its volume, multiply by the air changes per hour (ACH), then divide by 60 to turn an hourly volume into cubic feet per minute. A 20 × 15 ft room with an 8 ft ceiling is 2,400 ft³. At 5 ACH it needs 2,400 × 5 ÷ 60 = 200 CFM.
volume = length × width × height
CFM = volume (ft³) × ACH ÷ 60
m³/h = volume (m³) × ACH, L/s = m³/h ÷ 3.6What ACH should I use?
It depends on the purpose, and the calculator deliberately leaves it to you. Published typical ranges include roughly 1–2 ACH for homes, 3–4 for offices and 6–20 for classrooms (Engineering ToolBox), while the CDC/NIOSH “Aim for 5” guidance suggests 5 or more air changes per hour to reduce airborne virus particles, adding that it is a rough guide and does not guarantee safe air. Hospital rooms, laboratories and kitchens have their own standards.
How long does it take to clear the air?
CDC infection-control guidance gives the time to remove 99% or 99.9% of an airborne contaminant once the source stops, assuming the air mixes perfectly. At 6 ACH that is about 46 and 69 minutes; at 12 ACH, 23 and 35 minutes. Real rooms mix imperfectly, so treat these as best-case figures.
minutes to remove 99% = ln(100) ÷ ACH × 60 ≈ 276 ÷ ACH
minutes to remove 99.9% = ln(1000) ÷ ACH × 60 ≈ 414 ÷ ACHAir cleaners and equivalent air changes
A portable air cleaner's clean air delivery rate (CADR) in CFM can be entered as the airflow to estimate equivalent air changes (eACH) for particles, a concept the CDC uses alongside ventilation. It describes filtered air, not fresh outdoor air.
Why this is not HVAC sizing
Heating and cooling equipment is sized from a room-by-room load calculation that accounts for insulation, windows, climate and occupants. The industry body ACCA is blunt that rules of thumb are not a substitute. Commercial ventilation codes such as ASHRAE 62.1 also set outdoor-air rates by occupancy and floor area rather than ACH. Use this calculator to understand airflow, and a qualified professional for design or compliance.
Worked examples.
A living room at 5 ACH
A 20 × 15 ft room with an 8 ft ceiling, at 5 air changes per hour.
200 CFM (340 m³/h). 99% of airborne particles clear in about 55 minutes with perfect mixing.
Load this exampleA metric office
A 5 × 4 m office with a 2.5 m ceiling at 4 ACH.
200 m³/h, which is 55.6 L/s or about 118 CFM.
Load this exampleWhat does my fan achieve?
A 250 CFM ventilation fan serves a 20 × 15 ft room with an 8 ft ceiling.
6.25 air changes per hour.
Load this exampleClearance time at 12 ACH
A room ventilated at 12 air changes per hour.
About 23 minutes to remove 99% and 35 minutes to remove 99.9%, matching CDC's published table.
Load this exampleAn air cleaner in a bedroom
An air cleaner with a CADR of 200 CFM in a 12 × 12 ft bedroom with an 8 ft ceiling.
About 10.4 equivalent air changes per hour for particles.
Load this exampleWhat this calculator
does not do.
- It does not size heating or cooling equipment or ducts, and it is not a Manual J, S or D calculation.
- It does not determine code-required ventilation (for example ASHRAE 62.1, 62.2 or 170, or local building codes).
- It assumes a simple rectangular room and perfect mixing. Real airflow paths, leakage and furniture change results.
- It does not say what ACH is safe or sufficient for any purpose, including infection control.
Common questions.
How many CFM do I need for a room?
Multiply the room volume in cubic feet by your chosen air changes per hour and divide by 60. For heating or cooling capacity you need a load calculation instead.
What is a good number of air changes per hour?
It depends on the use. CDC's “Aim for 5” suggests 5 or more ACH to reduce airborne particles in many spaces; homes are often 1–2 and classrooms 6 or more. Codes and specialist standards take precedence.
How do I convert CFM to m³/h?
Multiply by about 1.699 (1 CFM = 1.69901 m³/h). The calculator shows CFM, m³/h and L/s together.
Can I use an air purifier's CADR?
Yes, as an estimate of equivalent air changes for the particle sizes the CADR describes. It does not add fresh outdoor air.
- CDC — Guidelines for Environmental Infection Control, Appendix B: Air (Table B.1) — air changes per hour and time to remove airborne contaminants
- CDC/NIOSH — Aim for 5 — 5+ ACH as a rough guide; eACH
- Engineering ToolBox — Air change rates in typical rooms
- ACCA — Manual J load calculations vs. rules of thumb
- CaptiveAire — Room volume and CFM — notes that ASHRAE 62.1 uses the ventilation rate procedure, not ACH
Exact unit factors (1 ft = 0.3048 m). Tests reproduce all nine rows of CDC Table B.1, check CFM ↔ m³/h ↔ L/s invariants, airflow ↔ ACH round trips in both unit systems, and input validation.
Built and reviewed by CalculatorLab against the sources above; every worked example on this page is re-checked by our automated test suite before each release. How we build and check calculators. Found a problem? Tell us.