Bathroom Fan Size Calculator
Bath fan CFM from room size or fixtures, HVI method.
What it does
Bath fans fight moisture with air changes, and size sets whether they win. HVI rates small baths at 1 CFM per square foot with a 50 CFM floor, so an 8 by 5 bath needs a 50 CFM fan. Bigger suites sum fixtures instead: toilet, shower, and tub at 50 each with jetted tubs at 100.
The tool above runs both HVI methods with an ACH cross-check. Enter area, ceiling height, and fixture counts, and it prints CFM, air changes an hour, and the 20-minute runtime rule. Defaults give 50 CFM at 9.4 ACH; a 240 sq ft suite with three fixtures needs 150.
The institute behind the numbers publishes them openly. HVI bathroom guidance states the 1 CFM rule with ~8 ACH, the 50 minimum, fixture sums past 100 sq ft, and 20 minutes of runtime. Energy Star adds certified efficacy floors with HVI test procedures behind the labels.
Ducting decides real performance. Long flex runs with elbows strangle rated CFM, so short smooth rigid duct to outside matters as much as the fan box. Vent through the roof or wall, never into attics, and let humidity sensors finish the job.
How to use
- Enter the floor area.
- Enter the ceiling height.
- Count toilets, showers, tubs, jetted tubs.
- Read CFM first, then the ACH check.
- Buy at or above the CFM number.
- Timer it 20 min past showers.
Formula
- sq ft
- Floor area in sq ft, 10 to 2,000.
- height
- Ceiling height for the ACH check.
- fixtures
- Fixture counts for rooms past 100 sq ft.
HVI bathroom guidance: 1 CFM per sq ft (~8 ACH), 50 minimum, fixture sums past 100 sq ft, 20 min runtime.
Worked example
Example: 8 x 5 ft bath
Defaults: 40 sq ft bath, 8 ft ceiling, toilet + shower.
- Inputs: 8 x 5 ft bath (40 sq ft), 8 ft ceiling.
- Rule: max(50, 40) = 50 CFM minimum.
- ACH check: 50 x 60 / (40 x 8) = 9.4.
- Runtime: 20 min after showers.
- Tool shows 50 CFM, 9.4 ACH, run 20 min after.
Reference table
| Bath | Rule | Pick |
|---|---|---|
| Under 50 sq ft | 50 CFM min | 50 CFM |
| 50-100 sq ft | 1 CFM / sq ft | 70 sq ft: 70 CFM |
| Over 100 sq ft | Fixture sum | 150 for 3 fixtures |
| Runtime | After showers | 20 min |
Limits
- HVI planning only. Duct losses cut rated flow.
- One zone assumed; compartments differ.
- Noise and efficacy need model labels.
- Makeup air in tight homes unmanaged.
Common mistakes
- Sizing under 50 CFM for small baths.
- Venting into attics instead of outside.
- Long flex duct strangling flow.
- No timer so fans run 2 minutes.
- Skipping the ACH sanity check.
Try it yourself
Quick checks. Answers sit tucked under each question.
70 sq ft bath, 8 ft ceiling?
max(50, 70) = 70 CFM. ACH 70 x 60 / (70 x 8) = 7.5.
240 sq ft suite, toilet + shower + tub?
Over 100: 50 + 50 + 50 = 150 CFM. ACH 150 x 60 / (240 x 8) = 4.7.
Questions
What CFM for a small bath?
1 CFM per sq ft to 100 sq ft, minimum 50. A 40 sq ft bath needs 50 CFM.
What CFM for a big bath?
Sum fixtures: toilet 50, shower 50, tub 50, jetted 100. A 240 sq ft suite with three needs 150 CFM.
How is ACH figured?
CFM x 60 over room volume. Here 50 x 60 / 320 = 9.4 ACH against the 8 target.
How long should it run?
Twenty minutes after bathroom use, per HVI. Timers or humidity sensors automate it.
Does Energy Star matter?
Yes. Certified fans list CFM per watt and sones; quiet efficient models run longer willingly.
Do ductless fans count?
No. Steam needs ducted exhaust; recirculating fans with filters do not remove moisture.
Sources
- HVI: Bathroom Ventilation (1 CFM/sq ft, fixtures, 20 min)
- HVI: Bathroom Exhaust Fans Consumer Guide (8 ACH, 50 min)
- Energy Star: Ventilation Fans Criteria