Air Volume Calculator
Before you can size a fan, an exhaust vent or an air purifier, you need one number: the air volume of the room. The Air Volume Calculator below finds it from the room’s dimensions in feet, metres or inches, then goes one step further on the second tab and converts that volume into the ventilation airflow, in CFM, that the space actually needs, using air-changes-per-hour presets for common room types.
The two tabs are one workflow, not two separate tools. The volume tab reports both cubic feet and cubic metres whatever unit you measured in, and a single button carries the result into the ventilation tab, so the number is never retyped and never transcribed wrong. If your next question is what that air weighs rather than how much space it fills, the air mass calculator takes the volume from here and returns kilograms.
The ACH method is the standard quick approach for fan sizing and ventilation planning. Full heating and cooling design uses a room-by-room Manual J load calculation; treat the CFM figure as a planning target and confirm final designs against local code and ASHRAE 62.1 or 62.2.
Quick Answer
Key takeaways
- Air volume is length x width x height, with all three dimensions in the same unit. The result is in cubic feet if you measured in feet, cubic metres if you measured in metres.
- Ventilation airflow is CFM = (volume x ACH) / 60. The division by 60 converts air changes per hour into cubic feet per minute.
- Room presets use the middle of each published ACH range, and the range is printed on the label. Use Custom when a code or a designer specifies an exact value.
- Ceiling height matters. Two rooms with the same floor area and different ceilings hold different volumes of air and need different fans.
From dimensions to air volume
The volume tab multiplies the room’s three internal dimensions. Everything after that is unit discipline: measure all three in the same unit, and the calculator converts the result into both cubic feet and cubic metres, because fan and purifier specifications are written in both.
Measure the internal dimensions. Length, width and ceiling height, inside wall to inside wall. For an L-shaped room, split it into two rectangles, calculate each volume and add them.
Calculate the volume. Volume = length x width x height. A 20 x 15 x 8 ft room gives 2,400 ft³. Metre and inch inputs are converted to feet internally (metres x 3.28084, inches / 12) and the answer is reported in both ft³ and m³.
Carry the volume into the ventilation tab. One click on “Use volume calculated above” moves the figure across. No retyping, no transcription error between the two halves of the job.
Pick the room type or enter a custom ACH, then calculate CFM. The result is the minimum continuous airflow a fan or system must deliver to hit that air-change rate in that room.
Air changes per hour: what each room actually needs
ACH states how many times the room’s full volume of air should be replaced every hour. Moisture and contaminants drive the number up: bathrooms and kitchens sit high because they generate steam and cooking effluent, bedrooms sit low because a sleeping occupant is a gentle load. The ranges below are the ones the presets are built from, in line with the ventilation guidance summarised by the Health and Safety Executive in its ventilation guidance and the room-by-room method used across the HVAC trade.
| Room type | Typical ACH range | Preset used by the calculator |
|---|---|---|
| Bedroom | 4 to 6 | 5 |
| Living room | 6 to 8 | 7 |
| Kitchen | 7 to 10 | 9 |
| Bathroom | 8 to 10 | 9 |
| Office | 6 to 8 | 7 |
| Workshop / garage | 15 to 20 | 17 |
Two honest notes about this table. First, presets are midpoints, stated openly: a designer working to the cautious end of a range should pick the range’s top figure through the Custom option instead of accepting the middle. Second, whole-dwelling ventilation rates in building regulations are often far lower than room extract rates: the UK’s Approved Document F, for example, works from whole-house rates and boost extract rates per wet room, which is a different framing of the same problem. The ACH table here is for sizing a fan or checking a room’s airflow, the job this page exists for; the standards framework behind it is catalogued by ASHRAE in its standards and guidelines listings, where Standard 62.1 and 62.2 live.
Worked example: a 20 by 15 foot bedroom
The calculator is pre-filled with this case. A bedroom of 20 ft x 15 ft with an 8 ft ceiling: volume = 20 x 15 x 8 = 2,400.00 ft³, and the metric line reads 67.96 m³, which cross-checks directly (2,400 x 0.0283168 = 67.96). Switch to the ventilation tab, where the same volume is waiting, choose Custom and set 6 ACH, the top of the bedroom range for a room that feels stuffy: CFM = (2,400 x 6) / 60 = 240.0 CFM, or 407.8 m³/h. Leave the bedroom preset selected instead and the same room at the range midpoint of 5 ACH needs 200.0 CFM (339.8 m³/h).
A metric check confirms the unit handling: a room of 6 m x 4.5 m x 2.4 m entered in metres returns 64.80 m³, matching the direct multiplication 6 x 4.5 x 2.4 = 64.8. And a second room type for scale: a 10 x 6 x 8 ft bathroom holds 480 ft³, and at the bathroom preset of 9 ACH it needs 72.0 CFM, which is why the standard 80 CFM bathroom fan is the natural pick for it.
Sizing fans, purifiers and cooling from the CFM figure
The CFM result is a purchasing and specification target, and it feeds three common decisions. Bathroom and kitchen exhaust fans are sold by CFM: buy a fan rated at or above your number, and oversize slightly without worry, since most fans have speed settings and a duct run with bends delivers less than the free-air rating. Room air conditioners are sized by BTU rather than CFM, but the same volume feeds that choice too: the AC tonnage calculator matches a room to a cooling size, and the HVAC size calculator takes over for whole-system planning.
Air purifiers are the third case, and the one most often bought wrong. Purifiers are rated by CADR, clean air delivery rate, also in CFM. The common rule of thumb is a CADR of at least two-thirds of the room’s floor area in square feet, but that rule quietly assumes an 8 ft ceiling. With taller ceilings the volume, and therefore the air that needs cleaning, is larger, so compare the purifier’s CADR against the CFM target from the ventilation tab instead of the floor-area rule. At industrial scale the same volume-to-flow thinking prices the air itself: the air consumption cost calculator turns CFM into an annual electricity figure for compressed-air systems, and the CDC’s NIOSH ventilation guidance explains why hitting the target matters for airborne contaminant control, not just comfort.
What competitors miss
We checked the top-ranking air volume and CFM tools before rebuilding this page. The field splits into tools that stop at volume and tools that start at CFM, and the reader is left to bridge the gap.
Winning move 1: volume and ventilation in one workflow. Brave Calculator’s air volume page multiplies three dimensions in feet and stops: no CFM, no ACH, no fan number. Housecall Pro’s CFM calculator goes the other way and never states the room’s air volume as a result in its own right. Here both halves live on one card and the figure carries across with one click.
Winning move 2: units in both directions. Brave and Jobber are feet-only. This tool accepts feet, metres or inches and prints ft³ and m³ on the volume tab and CFM and m³/h on the ventilation tab, so a metric user and an imperial user read the same result without converting anything by hand.
Winning move 3: presets with their ranges printed. Rival preset pickers hide the chosen ACH or pick unexplained values. Here every preset is labelled with its range and the article table states that the preset is the midpoint, with Custom one click away for a code-specified figure. The h2x engineering guide to calculating air changes per hour teaches the same formula well but offers no room-volume-first tool at all.
Frequently asked questions
For a rectangular space, volume = length x width x height, with all three dimensions in the same unit. The result is in cubic units: cubic feet if you measured in feet, cubic metres if you measured in metres. This calculator accepts feet, metres or inches and reports both ft³ and m³.
Multiply the room volume in cubic feet by the desired air changes per hour (ACH) and divide by 60: CFM = (volume x ACH) / 60. ACH depends on the room type: bedrooms typically need 4 to 6, bathrooms 8 to 10. The ventilation tab applies the formula with room presets or your own custom ACH.
One CFM is one cubic foot of air moved per minute. It equals 1.699 cubic metres per hour (m³/h) or 0.472 litres per second. The calculator reports both CFM and m³/h on the ventilation tab so no conversion is needed.
A 12 ft x 12 ft room with an 8 ft ceiling holds 1,152 ft³ of air. At 6 air changes per hour it needs (1,152 x 6) / 60 = 115.2 CFM. At the bedroom preset of 5 ACH it needs 96.0 CFM. Enter the dimensions on the volume tab, carry the figure across, and the ventilation tab returns the number for either rate.
Yes. Purifier guidance based on floor area assumes a standard 8 ft ceiling, so it understates the air to be cleaned in rooms with taller ceilings. Base the purchase on the CFM target from the ventilation tab, which uses the room’s real volume, and choose a purifier whose CADR meets or exceeds it.
No. The ACH method is the standard quick approach for ventilation and fan sizing, but full heating and cooling design uses a room-by-room Manual J load calculation that accounts for insulation, windows, occupancy and climate. Treat the CFM result as a planning target, not a final engineering specification.
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References
- ASHRAE, “Standards and Guidelines” – the catalogue that includes Standard 62.1 (ventilation for acceptable indoor air quality, commercial) and 62.2 (residential), the standards behind ACH-based ventilation practice. ASHRAE: Standards and Guidelines. Accessed October 2026.
- Health and Safety Executive, “Ventilation in the workplace” – regulator guidance on fresh-air supply, ventilation rates and maintaining systems. HSE: Ventilation. Accessed October 2026.
- GOV.UK, “Ventilation: Approved Document F” – the building-regulations guidance for ventilation rates in dwellings and other buildings in England. GOV.UK: Approved Document F, Ventilation. Accessed October 2026.
- Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, “Ventilation” – why ventilation rates matter for controlling airborne contaminants indoors. CDC NIOSH: Ventilation. Accessed October 2026.
- h2x Engineering, “How to Calculate Air Changes Per Hour” – the ACH formula in both directions, ACH = (CFM x 60) / volume and CFM = (volume x ACH) / 60, with design targets from 0.35 to over 20 ACH. h2x: How to Calculate Air Changes Per Hour. Accessed October 2026.