SettingsSettings for this calculationUS
Different rooms have different recommended ventilation rates.
Bathrooms and kitchens need much more air exchange than bedrooms or living rooms due to moisture and cooking odors/grease.
The length of the room.
Measure along the floor.
Tools needed: Tape measure
The width of the room.
Measure perpendicular to the length.
Tools needed: Tape measure
Floor-to-ceiling height.
Standard residential ceilings are commonly 2.4 m (8 ft).
Tools needed: Tape measure
Recommended airflow (CFM)
85.3 CFM
Based on general air-changes-per-hour targets, not a code-mandated minimum for your specific jurisdiction — check local building code for required bathroom/kitchen exhaust minimums.
- Room volume
- 640 ft³
- Target air changes/hour
- 8 ACH
- Same airflow in L/s
- 40.27 L/s
They open the calculator with your figures already in it
CFM Ventilation Calculator: 85.33 CFM — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Formula: CFM = (room volume in cubic feet x air changes per hour) / 60
- Typical air-changes-per-hour targets vary by room type — kitchens and bathrooms need substantially more than bedrooms due to moisture and odor
Inputs used
- Room Type
- Bathroom
- Room Length
- 10 ft
- Room Width
- 8 ft
- Ceiling Height
- 8 ft
Intermediate steps
- Room volume
- 640 ft³
- Target air changes/hour
- 8 ACH
- Same airflow in L/s
- 40.27 L/s
Confidence note: Based on general air-changes-per-hour targets, not a code-mandated minimum for your specific jurisdiction — check local building code for required bathroom/kitchen exhaust minimums.
What this calculation does not cover
- This sizes the airflow the room needs, not the fan you buy. Manufacturer CFM ratings are measured against a static pressure far lower than a real installation imposes, and duct length, elbows, flexible duct, the interior grille and the exterior cap all cut delivered flow. None of that is modelled here.
- This is not a code compliance check. Codes commonly set a flat minimum exhaust rate for bathrooms and kitchens, plus separate whole-dwelling ventilation rates driven by floor area and occupancy, and a small bathroom or WC will compute below those minimums. Size on whichever figure is larger and confirm it against the code your work is inspected under.
- Makeup air is not accounted for. Air pulled out has to be replaced, so a fan sized from this figure with no door undercut, transfer grille or trickle vent will move less than the number says, and in a sealed room it can depressurise the space enough to backdraft an open-flued gas or solid-fuel appliance. Combustion safety is a separate check, not something this result covers.
- The air change rate is one fixed value per room type, and only four room types exist. It does not move for the number of occupants, shower versus bath use, gas versus electric cooking, cooktop or hood width, an openable window already in the room, local humidity, or whether the fan runs continuously or on a timer.
- Room volume is length x width x height and nothing else. A sloped or vaulted ceiling, an L-shaped room, a dropped bulkhead, or a space open to an adjoining one such as an ensuite off a bedroom or a kitchen open to the dining area has no honest set of three numbers to enter, and the volume has to be worked out separately.
Add the equipment this sizes
This result is a specification — 85.3 CFM — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Part of a bigger job
Or plan the space itself: measure it once, doors and windows included, and this figure comes back worked out on that space, with what goes with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations2
- Formula: CFM = (room volume in cubic feet x air changes per hour) / 60
- Typical air-changes-per-hour targets vary by room type — kitchens and bathrooms need substantially more than bedrooms due to moisture and odor
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