CFM Calculator
With room length 16 ft, room width 12 ft, ceiling height 8 ft, cfm comes to 102 CFM — airflow needed. It is reached in 4 steps, the last of which is (1536 * 4) / 60, and each one is printed on the page with its numbers filled in. The formula is the one published by ASHRAE Standard 62.2, not an approximation fitted to it.
Cubic feet per minute a room needs, from its volume and how many times an hour the air has to be changed.
Formula and sources checked · How we check
Room length 16 ft, Room width 12 ft, Ceiling height 8 ft, What the room is Living room or bedroom — 4 changes/hour
102 CFM
Airflow needed for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.
It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.
- Room volume
16 * 12 * 81,536 cu ft- Airflow needed
(1536 * 4) / 60102.4 CFM- Bathroom rule of thumb, 1 CFM per sq ft
max(50, 16 * 12)192 CFM- Through a 6 in duct at 700 ft/min
102.4 / 1370.747 ducts
Ask about this in the chatCFM by Room Size Chart
Worked example
A 16 × 12 ft living room with an 8 ft ceiling holds 1,536 cubic feet. Four changes an hour is 6,144 cubic feet an hour, or 102 CFM. A 6 in duct carries about 137 CFM at ordinary velocity, so one run does it.
How to work it out yourself
- 1.Multiply length by width by ceiling height for the volume in cubic feet.
- 2.Multiply by the air changes per hour the room is for.
- 3.Divide by 60 — the answer is per minute, the change rate is per hour.
- 4.For a bathroom, compare against the simpler rule of one CFM per square foot with a 50 CFM floor, and fit whichever is larger.
CFM for a room 8 ft high by floor area
| Room length (ft) | Airflow needed | Room volume | Bathroom rule of thumb, 1 CFM per sq ft |
|---|---|---|---|
| 8 | 51 CFM | 768 cu ft | 96 CFM |
| 10 | 64 CFM | 960 cu ft | 120 CFM |
| 12 | 77 CFM | 1,152 cu ft | 144 CFM |
| 16 | 102 CFM | 1,536 cu ft | 192 CFM |
| 20 | 128 CFM | 1,920 cu ft | 240 CFM |
| 24 | 154 CFM | 2,304 cu ft | 288 CFM |
Length in feet at a fixed 12 ft width and 8 ft ceiling, at four air changes an hour.
The formula
- Room volume
16 * 12 * 8 - Airflow needed
(1536 * 4) / 60 - Bathroom rule of thumb, 1 CFM per sq ft
max(50, 16 * 12) - Through a 6 in duct at 700 ft/min
102.4 / 137
Source: ASHRAE Standard 62.2 — ventilation in residential buildings, US EPA — indoor air quality
Questions people actually ask
- How do you calculate CFM for a room?
- Volume times air changes per hour, divided by 60. A 1,536 cubic foot room at four changes an hour needs 102 CFM. The division by 60 is the step people miss: the change rate is per hour and the answer is per minute.
- What CFM does a bathroom fan need?
- One CFM per square foot, never less than 50 — so a 6 × 9 ft bathroom takes a 54 CFM fan and a small one still takes 50. A bath with a shower over it is usually specified at 8 air changes an hour instead, which for the same room comes out higher, and the larger figure is the one to fit.
- What are air changes per hour?
- How many times the whole volume of air in the room is replaced in an hour. Four is comfortable for a living room, eight for a kitchen or bathroom where moisture and smells have to leave quickly, twelve where there are fumes. It is a rate, not a quantity, which is why a tall room needs more air than a short one of the same floor area.
- Does duct size limit the CFM?
- Yes, and it is the commonest reason a fan underperforms its box. Air moves at about 700 ft per minute in residential ducting, so a 6 in round duct carries roughly 137 CFM and a 4 in one about 60. A 110 CFM fan on 4 in flexible duct with two bends delivers a fraction of its rating.
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