Room Volume Calculator

How do you calculate the volume of a room in cubic feet?

Multiply length × width × height with every dimension in the same unit. A 12 ft × 10 ft room with an 8 ft ceiling holds 12 × 10 × 8 = 960 cubic feet. Measure floor length and width wall to wall at the baseboard, and take ceiling height from finished floor to finished ceiling — not to the top of the drywall or the underside of a joist. Convert inches to decimal feet first: 8 ft 6 in is 8.5 ft, not 8.6. Cubic feet divided by 27 gives cubic yards and multiplied by 0.0283168 gives cubic meters, so that same 960 ft³ is 35.6 yd³ or 27.2 m³.

Volume answers different questions than floor area. Area drives paint coverage, flooring quantity, and most published air-conditioner sizing charts; volume drives air changes per hour, duct and fan sizing, dust collection, radon mitigation, and the quantity of conditioned air a space actually contains. Two rooms with the same 300 ft² footprint hold 2,400 ft³ under an 8-foot ceiling and 3,000 ft³ under a 10-foot ceiling — a 25% difference the floor plan never shows. This calculator treats the room as a rectangular box and does not deduct furniture, cabinets, or soffits, which is the correct convention for air-volume work.

How do you convert cubic feet to cubic meters?

Multiply cubic feet by 0.0283168 to get cubic meters, or divide by 35.3147. The factor is not arbitrary: one inch is exactly 2.54 cm, so one foot is exactly 0.3048 m and one cubic foot is 0.3048³ = 0.028316846592 m³ exactly. Going the other way, 1 m³ = 35.3147 ft³. A 960 ft³ bedroom is 27.2 m³; a 2,700 ft³ living room is 76.5 m³. For cubic yards divide cubic feet by 27, and for US gallons multiply by 7.48052 — that 960 ft³ bedroom holds roughly 7,181 gallons of air.

Metric room work usually runs the other direction: measure in metres and multiply. A 3.6 m × 3.0 m room with a 2.4 m ceiling is 25.9 m³, which converts to about 916 ft³. Watch the classic unit trap, because cubing amplifies small linear errors: a 1% error in each of three dimensions compounds to roughly 3% in volume, and mixing feet with metres in a single multiplication produces an answer wrong by a factor of about 35. This tool converts every field to one internal unit before multiplying, so switching the unit selector rescales your values instead of reinterpreting them.

How do you calculate air changes per hour from room volume?

Air changes per hour (ACH) equals airflow in cubic feet per minute × 60 ÷ room volume in cubic feet. A 300 cfm supply into a 2,400 ft³ room delivers 300 × 60 ÷ 2,400 = 7.5 ACH. Run it backwards to size equipment: to reach 5 ACH in a 2,700 ft³ room you need 2,700 × 5 ÷ 60 = 225 cfm of delivered clean air. This is why volume, not floor area, is the honest denominator for ventilation and filtration questions, and why a vaulted room with the same footprint as a flat-ceiling room needs more airflow to hit the same air change rate.

EPA's portable air cleaner sizing chart rests on the same relationship. It pairs 100 ft² with a minimum clean air delivery rate of 65 cfm, 300 ft² with 195 cfm, and 600 ft² with 390 cfm, and states that the CADRs are calculated based on an 8-foot ceiling [1]. Convert each pair to a volume and every row lands at about 4.9 air changes per hour: 65 × 60 ÷ 800 = 4.88. EPA adds that with ceilings above 8 feet you should consider a unit sized for a larger area [1]. Once you know your real volume you can check any manufacturer's room-size claim in one line of arithmetic.

What size air purifier do I need for a 15 by 20 foot room?

Start with the geometry. A 15 ft × 20 ft room is 300 ft² of floor; with a 9 ft ceiling its volume is 15 × 20 × 9 = 2,700 ft³, or 76.5 m³. EPA's chart puts 300 ft² at a minimum CADR of 195 cfm, but that figure assumes an 8-foot ceiling [1]. Scale it by the height ratio: 195 × 9 ÷ 8 = 219 cfm. Rounding up, a unit rated at 220 to 250 cfm smoke CADR is the named target for this room, which works out to 219 × 60 ÷ 2,700 = 4.9 air changes per hour — the same rate EPA's own table implies.

If you want a higher target, solve for it directly: 2,700 ft³ × 8 ACH ÷ 60 = 360 cfm. Two smaller units can share the load, and EPA notes that where no single available product will serve the area you could consider using multiple air cleaners [1]. For an open plan, measure the whole connected space rather than the nominal room; a 2,700 ft³ living room that opens into an 1,800 ft³ dining area is a 4,500 ft³ problem. Remember that CADR is typically measured at the air cleaner's highest speed, so a quiet setting delivers fewer air changes than the label suggests [1].

How do you find the volume of a room with a sloped or vaulted ceiling?

Split the space into simple solids. For a shed-slope ceiling, average the low and high wall heights: a 14 ft × 16 ft room sloping from 8 ft to 12 ft has an average height of 10 ft and a volume of 14 × 16 × 10 = 2,240 ft³. For a symmetric cathedral ceiling, compute the box up to the wall plate and add the triangular prism above it, which is base area × (ridge height − plate height) ÷ 2. A 14 ft × 16 ft room with 8 ft walls and a 13 ft ridge is 1,792 ft³ plus 14 × 16 × 5 ÷ 2 = 560 ft³, for 2,352 ft³ total.

Tray ceilings, dormers, bay windows, and stairwell openings are the same exercise: measure each block, sum the pieces, and note which ones you counted so nothing appears twice. Enter the average or total-equivalent height in this calculator rather than the peak, because peak height would overstate a vaulted room by a third or more. Closets and alcoves that stay open to the room count; sealed soffits and dropped bulkheads do not. For HVAC and ventilation planning, include stairwells and open lofts in the connected zone, since air moves freely between them.

What is the volume of common room sizes?

Rectangular rooms only. Use average height for sloped ceilings and add dormers or bays as separate blocks.

How do cubic feet convert to cubic meters, cubic yards, and gallons?

Factors: 1 ft³ = 0.028316846592 m³ exactly, ÷27 for cubic yards, ×7.48052 for US gallons.

What CADR does EPA list for each room area?

EPA states its CADRs are calculated for an 8-foot ceiling; with higher ceilings choose a unit sized for a larger area.

Work these out first

• start from the floor areaSquare Footage Calculator

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