CFM Calculator (ACH to CFM for Room Ventilation)
Instant ventilation CFM: CFM = (ft³ × ACH) ÷ 60. Example: 10×12×8 ft at 6 ACH → 96 CFM. This is room air-change airflow—not air-compressor CFM or car A/C CFM. Reverse-solve ACH from known fan CFM when needed.
Input parameters
Defaults use US sq ft and feet. Quick: room size + ACH → CFM. Advanced: switch to CFM → ACH when you already know fan airflow.
Quick examples:
About this calculator
Estimates bulk airflow from geometry and ACH with CFM = (ft³ × ACH) ÷ 60 so you can compare fan duty across rooms, warehouses, and retrofit zones. For cooling and heating kW/ton block loads, use the HVAC Capacity Calculator. Browse all HVAC tools on the HVAC Calculators Hub.
Calculation results
What you are seeing: total airflow if the entire room volume were turned over at the ACH you typed (or ACH implied by known CFM). Not modeled: duct leakage, heat recovery, variable-volume turndown, zonal mixing, or minimum outdoor-air fractions. Next: size duct with HVAC duct size calculator, or use HVAC Capacity for tonnage/kW.
Engineering disclaimer
This calculator provides preliminary airflow estimates only. For final ventilation design, contaminant control, and compliance with local building codes, consult a licensed mechanical engineer or certified industrial hygienist.
ACH requirements by room type (screening)
Order-of-magnitude triage for early planning—not a CDC/ASHRAE code extract. Confirm with project criteria, ASHRAE 62.1 / healthcare standards, and local requirements. Example CFM assumes a 10×12×8 ft room (960 ft³) unless noted.
| Room type | ACH (screening) | Typical CFM (10×12×8) |
|---|---|---|
| Residential living | 2–3 | 32–48 |
| Office | 5–6 | 80–96 |
| Classroom (20×24×9) | 5–6 | 360–432 @ that volume |
| Restaurant dining | 8–10 | 128–160 |
| Healthcare patient room | ~6 | ~96 |
| Operating room | 20–25 | 320–400 |
| Isolation (AIIR) | ~12 | ~192 |
| Laboratory (chemical) | 6–12 | 96–192 |
| Restroom | 10–15 | 160–240 |
| Kitchen commercial | 15–20 | 240–320 |
| Light workshop / fab | 6–12 | 96–192 |
| Warehouse (rarely fully mixed) | 1–4 | 16–64 |
Formula & explanation
Primary (US customary): room volume V (ft³) = area (ft²) × height (ft), then
CFM = (V × ACH) ÷ 60.
Reverse: ACH = (CFM × 60) ÷ V.
Metric chain: Qm³/h = ACH × Vm³;
CFM ≈ Qm³/h ÷ 1.699;
L/s = Qm³/h ÷ 3.6.
This page vs HVAC Capacity: here you set how often the room volume turns over; there you estimate sensible/latent block energy (kW, tons). After CFM is known, use the HVAC duct size calculator for trunk diameter screening.
Minutes per air change (purge time)
Minutes per air change ≈ 60 ÷ ACH. At 6 ACH, one complete volume turn takes ~10 minutes (ideal mixing). Use as a ventilation intensity screen—not a pathogen removal guarantee.
ACH vs outdoor air (ASHRAE 62.1)
ACH is total supply air changes of the room volume. Outdoor-air rate (people + area components in ASHRAE 62.1) can be a smaller stream mixed into recirculated air. Do not treat ACH from this calculator as code OA compliance—use 62.1 worksheets after you know occupancy and zone type.
Worked examples
- 10×12×8 ft (960 ft³) at 6 ACH → CFM = (960 × 6) ÷ 60 = 96 CFM.
- 20×24×9 ft classroom (4,320 ft³) at 5 ACH → 360 CFM.
- 4×5×3 m lab (60 m³) at 8 ACH → 480 m³/h ≈ 44.4 L/s (≈ 282 CFM).
Last updated: 2026-07-29. Screening only—not outdoor-air code compliance.
References
Guides: HVAC sizing · How to calculate HVAC capacity · HVAC Calculators Hub
ACH to CFM conversion chart (examples)
Worked volumes at common ACH targets using CFM = (ft³ × ACH) ÷ 60. Click a Quick example above to load similar geometry.
| Room (L×W×H) | Volume | ACH | Required CFM |
|---|---|---|---|
| 10×12×8 ft bedroom | 960 ft³ | 6 | 96 CFM |
| 5×8×8 ft bathroom | 320 ft³ | 7 | 37 CFM |
| 12×12.5×8 ft kitchen | 1,200 ft³ | 8 | 160 CFM |
| 15×20×9 ft office | 2,700 ft³ | 6 | 270 CFM |
| High-bay note | If only the lower mixed height participates, reduce effective volume before trusting ACH-driven CFM. | ||
Frequently asked questions
How do you convert ACH to CFM?
Multiply room volume in cubic feet by ACH, then divide by 60: CFM = (ft³ × ACH) ÷ 60. Example: a 10×12×8 ft room (960 ft³) at 6 ACH needs (960 × 6) ÷ 60 = 96 CFM.
How much CFM for 6 ACH in a 10×12×8 ft room?
Volume is 960 ft³. At 6 ACH, CFM = (960 × 6) ÷ 60 = 96 CFM. Enter floor area and ceiling height above—the calculator applies the same formula automatically.
What does ACH mean in HVAC?
ACH counts how many times the room’s total air volume is replaced per hour under the scenario you assume. It helps communicate purge and general ventilation concepts but does not, by itself, define compliant minimum outdoor air.
How do I convert CFM back to ACH?
Use ACH = (CFM × 60) ÷ room volume in ft³. Switch the calculator to CFM → ACH, enter known fan CFM and room size, and read the resulting air changes per hour.
What ACH is typical for bedrooms, bathrooms, and offices?
Screening bands often cited for planning: bedrooms about 5–6, bathrooms 6–7, kitchens 7–8, and business offices 6–8. Treat any default ACH as a placeholder until standards and risk review lock it in—see the room chart above.
Does this calculator replace ASHRAE 62.1 outdoor air calculations?
No. Code and ASHRAE procedures combine people, area, zone type, and filtration. Use this tool only for ACH-driven bulk airflow from the volume you enter.
Is this a compressor CFM or car A/C CFM calculator?
No. Searches for “CFM calculator” often mean shop air-compressor free-air delivery or automotive HVAC. This page solves room ventilation CFM from ACH (and reverse ACH from fan CFM). For cooling tons use the HVAC capacity calculator; for duct diameter use the duct size / ductulator.
What is an air changes per hour calculator used for?
An air changes per hour calculator (this page) turns target ACH and room volume into supply/exhaust CFM for bathrooms, offices, labs, and high-bay screening—or reverse-checks ACH when you already know fan CFM.