CFM & Airflow Calculator
Find the airflow a room needs from its volume and a target air-change rate: CFM = volume × ACH ÷ 60. Pick a space type for a typical ACH or enter your own. The presets are planning rules of thumb, not code minimums — where ventilation code governs, ASHRAE 62.1 computes outdoor air from occupants and floor area instead, and the real 62.1 rates are tabulated below and in the ventilation rate chart. Free, no login.
Room
Required Airflow
CFM = (room volume ft³ × ACH) ÷ 60. The ACH presets are typical rule-of-thumb design values, not code minimums — ASHRAE 62.1 CFM/person and CFM/ft² rates are the pending methods above.
How this calculator works
Air changes per hour describes how often the room’s full volume of air turns over. Converting a target ACH to fan airflow is one line:
CFM = room volume (ft³) × ACH ÷ 60
Two honest caveats. First, the space-type presets (4 ACH living space, 6 office, 30 commercial kitchen, and so on) are rules of thumb — typical design values, not code minimums, and published ranges are wide (offices span roughly 4–8 ACH). Second, ACH is not the same thing as code ventilation: ASHRAE 62.1 computes required outdoor air per occupancy category as Vbz = Rp × Pz + Ra × Az — a per-person rate (Rp) times occupants plus a per-square-foot rate (Ra) times floor area — while ACH counts total turnover, which may be mostly recirculated air. The calculator’s CFM/person and CFM/ft² methods are marked pending in the picker; where 62.1 governs, use the verified rates in the table below.
Worked example
A 20 × 15 ft room with a 9 ft ceiling, at the 4 ACH living-space preset (the calculator’s defaults):
- Volume: 20 × 15 × 9 = 2,700 ft³.
- Airflow: 2,700 × 4 ÷ 60 = 180 CFM.
- Contrast with 62.1: if that same 300 ft² room were an office with 2 occupants, the breathing-zone outdoor air would be Vbz = 5 × 2 + 0.06 × 300 = 28 CFM of outdoor air — a different quantity answering a different question (outdoor air for air quality vs total circulation).
Enter the same room above (20 × 15 × 9 ft, Living space) to reproduce it.
Reference: ACH presets and ASHRAE 62.1 outdoor-air rates
The calculator’s presets, with their basis — every row is a planning rule of thumb:
| Space type | Preset | Basis |
|---|---|---|
| Living space | 4 ACH | Typical residential living-space air changes |
| Office | 6 ACH | Typical office air changes |
| Kitchen (commercial) | 30 ACH | Typical commercial kitchen air changes |
| Bathroom | 8 ACH | Typical bathroom air changes |
| Warehouse | 2 ACH | Typical warehouse air changes |
| Classroom | 6 ACH | Typical classroom air changes |
| Retail | 8 ACH | Typical retail air changes |
ASHRAE 62.1 breathing-zone outdoor air (Vbz = Rp × Pz + Ra × Az) — the verified per-person and per-area rates behind the ventilation rate chart (unchanged across the 2007–2022 editions for these categories):
| Occupancy | Rp (cfm/person) | Ra (cfm/ft²) |
|---|---|---|
| Office space | 5 | 0.06 |
| Conference / meeting room | 5 | 0.06 |
| Classroom (age 9+) | 10 | 0.12 |
| Retail sales | 7.5 | 0.12 |
| Restaurant dining room | 7.5 | 0.18 |
| Hotel / dormitory bedroom | 5 | 0.06 |
| Health club / aerobics room | 20 | 0.06 |
| Corridors | — | 0.06 |
Frequently asked questions
How do I calculate CFM from room size?
Multiply the room volume in cubic feet by the target air changes per hour, then divide by 60: CFM = (L × W × H × ACH) ÷ 60. That's exactly what this calculator does. Example: a 20 × 15 ft room with a 9 ft ceiling is 2,700 ft³; at 4 ACH it needs 2,700 × 4 ÷ 60 = 180 CFM.
How many air changes per hour does a room need?
The presets here are common planning values: about 4 ACH for residential living space, 6 for offices and classrooms, 8 for bathrooms and retail, 2 for warehouses, and up to 30 for commercial kitchens. These are rules of thumb for total air turnover, not code minimums. Some occupancies — healthcare in particular — have ACH requirements set by code or standard, and commercial ventilation is governed by ASHRAE 62.1, so confirm against the requirement that actually applies to your space.
Is ACH the same as ASHRAE 62.1 ventilation?
No. ASHRAE 62.1's ventilation rate procedure computes required outdoor air from occupant count (CFM per person) plus floor area (CFM per ft²) by occupancy category — it isn't expressed as air changes. ACH describes how often the total room volume turns over, which may be mostly recirculated air. This tool currently calculates from ACH only; where 62.1 governs, run the per-person and per-area math from the standard itself.
How many CFM does a bathroom exhaust fan need?
Residential codes commonly require 50 CFM for intermittent (switch-controlled) bath exhaust or 20 CFM continuous — check the mechanical code adopted in your jurisdiction. The 8 ACH preset here will often land near or above that for a typical bathroom: a 5 × 8 ft bath with an 8 ft ceiling is 320 ft³, and 8 ACH works out to about 43 CFM, so the 50 CFM code floor governs. Use whichever number is larger.
Does the CFM a room needs depend on duct size?
No — the required airflow comes from the space and its use; the duct is then sized to deliver it. Get the CFM first from air changes (or a load calculation, since heating/cooling airflow is usually set by the load rather than by ventilation), then size the duct so it carries that CFM at a reasonable velocity and friction loss. If an existing duct can't deliver the target CFM, the fix is a bigger or shorter duct run, not a smaller target.
Method: CFM = volume × ACH ÷ 60 (exact conversion). The ACH presets are uncited planning rules of thumb, not code — published typical ranges by space type are tabulated on the air changes chart, where only the ASHRAE 170 healthcare rows are code-mandated. Where a ventilation standard applies, ASHRAE 62.1’s Ventilation Rate Procedure (Vbz = Rp × Pz + Ra × Az) governs outdoor air, and heating/cooling airflow is usually set by the load calculation rather than by air changes. Exhaust-driven spaces (baths, kitchens) also carry mechanical-code minimum exhaust rates — use whichever number is larger, and the adopted code and AHJ govern.
Pair with the ventilation rate chart for the full ASHRAE 62.1 rates, the air changes per hour chart for published ACH ranges by space, the air velocity chart for duct and grille velocities, or the Duct Size Calculator to carry the CFM into a duct size.