Room Volume Calculator
Room volume is the number every other home calculation starts from. Size an air conditioner, pick an air purifier that actually turns the air over, estimate paint, or check whether furniture will fit: the first input is always length times width times height. Most online tools stop there. They assume a perfect rectangle, work in feet only, and mention air changes per hour without ever computing the airflow. This calculator does the full job: rectangular and L-shaped rooms, feet, inches, meters or centimeters, the result in cubic feet, cubic meters and liters, the CFM needed for your target air-change rate, and a two-coat paint estimate from the wall area.
Paint estimate assumes standard latex coverage of about 350 sq ft per gallon per coat, two coats, no deductions for windows or doors. Buy the next full gallon up.
Quick Answer
A 15 by 12 foot bedroom with 9-foot ceilings holds 1,620 cubic feet (45.87 m³, 45,873 liters) of air. Its walls cover 486 sq ft, so two coats need 2.8 gallons of paint (buy 3), and at 6 air changes per hour the room needs 162 CFM (275 m³/h) of airflow. Type your dimensions above and you get all five numbers at once.
Key takeaways
- Volume is always length × width × height in the same unit. An L-shaped room is two rectangles added together, which the L-shape tab does for you.
- The CFM figure is what equipment specs are written in: a purifier or fan rated below it cannot hit your target air-change rate in that room.
- Volume is air space; paint is wall area. Never buy paint from the volume number.
- The US EPA's residential guidance and ASHRAE 62.2 treat 0.35 air changes per hour as the minimum whole-home ventilation rate, while portable air purifiers are sized around 5 ACH. They answer different questions, so keep them separate.
- Quick Answer
- How to use the room volume calculator
- The room volume formula and the unit conversions
- Worked example: the 15 by 12 bedroom
- What most room volume calculators miss
- Typical room volumes for reference
- What ACH should you target?
- Putting the volume to work
- Frequently asked questions
- Add This Calculator to Your Website
How to use the room volume calculator
Measure inside wall to inside wall, type the three numbers in, and click Calculate room volume. The results stay hidden until you calculate.
- Pick the room shape. Rectangular covers almost every bedroom, office and garage. Switch to L-shaped room for open-plan spaces and split the floor plan into two rectangles, A and B, measuring the longest straight runs of each.
- Enter length, width and ceiling height. All three must be in the same unit. For a sloped or vaulted ceiling, enter the average of the lowest and highest points: a ceiling that rises from 8 to 12 feet goes in as 10.
- Choose the unit. Feet, inches, meters and centimeters all work. The answers always come back in cubic feet, cubic meters and liters, so equipment specs in either system are covered.
- Set the target air changes per hour. This feeds only the airflow estimate. Five is the common purifier target (see the ACH table below); the volume itself does not depend on this field.
- Read the right line. Volume in three units for sizing equipment, wall-area paint for rectangular rooms, and the CFM / m³/h pair for fans and purifiers. Copy my results or Share results sends the full summary.
The room volume formula and the unit conversions
For a rectangular room, volume = length × width × height, with all three measurements in the same unit. For an L-shaped room, split the floor plan into two rectangles, compute each volume, and add them: V = (Alength × Awidth + Blength × Bwidth) × height. The airflow figure comes from CFM = (volume in cubic feet × target ACH) ÷ 60, and the metric equivalent is simply m³/h = volume in cubic meters × target ACH. The paint line uses wall area = 2 × (length + width) × height, then two coats at 350 sq ft per gallon per coat.
CFM = (volume in ft³ × ACH) ÷ 60 | m³/h = volume in m³ × ACH
Paint (gal) = 2 × (length + width) × height in ft² × 2 coats ÷ 350
| Conversion | Multiply by |
|---|---|
| Cubic feet to cubic meters | 0.0283168 |
| Cubic meters to cubic feet | 35.3147 |
| Cubic feet to liters | 28.3168 |
| Cubic feet to US gallons | 7.48052 |
If you need the volume of a box, tank or cylinder rather than a room, the cubic footage calculator covers those shapes with the same unit conversions.
Worked example: the 15 by 12 bedroom
The calculator opens pre-filled with this example. Room is 15 feet long, 12 feet wide, 9-foot ceilings, target 6 air changes per hour:
And an L-shaped check, using the second tab. Rectangle A is 20 × 15 ft, rectangle B is 10 × 12 ft, both with 9 ft ceilings: V = (20 × 15 + 10 × 12) × 9 = (300 + 120) × 9 = 3,780 cubic feet (107.04 cubic meters). Enter 20, 15, 10, 12 and 9 in the L-shape tab to reproduce it.
What most room volume calculators miss
We checked the tools currently ranking for this keyword. Nearly all of them share the same gaps:
- Feet only, no unit toggle. Several top tools accept feet and nothing else, telling metric users to convert first. This one accepts feet, inches, meters and centimeters directly.
- L-shaped rooms get a tip, not a tool. Competitors advise you to split the room and add the parts yourself. The L-shaped tab here does the two-rectangle math for you.
- ACH is mentioned but never computed. One leading page answers whether you can calculate air changes per hour with "yes, once you know the volume" and stops. The airflow estimate above turns your volume and target ACH into the CFM a fan, purifier or ventilation system must deliver.
- Paint is conflated with volume. Volume is air space; paint needs wall area. The tool reports wall surface area and a two-coat paint estimate as separate, clearly labeled figures, and skips the paint line entirely for L-shaped rooms rather than guessing.
Typical room volumes for reference
Use these to sanity-check your measurement before buying equipment. If your bedroom computes to 5,000 cubic feet, remeasure.
| Room | Typical size (ft) | Volume |
|---|---|---|
| Standard bedroom | 15 × 12 × 8 | 1,440 cu ft |
| Master bedroom | 20 × 14 × 9 | 2,520 cu ft |
| Living room | 18 × 14 × 10 | 2,520 cu ft |
| Kitchen | 12 × 12 × 8 | 1,152 cu ft |
| Bathroom | 9 × 5 × 8 | 360 cu ft |
| Two-car garage | 20 × 20 × 8 | 3,200 cu ft |
| Home office | 15 × 15 × 8 | 1,800 cu ft |
What ACH should you target?
Air changes per hour is where most people go wrong, because two different standards use the same unit. Keep them apart: the EPA and ASHRAE 62.2 minimum of 0.35 ACH is whole-home ventilation, the baseline fresh-air rate a house needs. The 5 ACH figure is what portable air purifiers are sized around, and it is the CDC's recommended clean-air target for rooms. Rooms that generate moisture and odors need more.
| Room | Typical ACH target | Why |
|---|---|---|
| Bedrooms | 5–6 | The standard purifier sizing target; matches the CDC clean-air guidance |
| Living rooms | 5–6 | Same occupancy-driven target as bedrooms |
| Offices | 6–7 | Higher occupancy and equipment heat load |
| Kitchens | 7–8 | Cooking odors, grease and combustion particles |
| Bathrooms | 8–9 | Moisture control to stop mold and mildew |
Once you have the CFM figure from the calculator, compare it against the equipment spec. A portable air purifier lists a CADR (clean air delivery rate) in CFM; it should meet or beat your number. Our air volume calculator goes deeper on the ventilation side, with a dedicated ACH-to-CFM tab and room-type presets.
Putting the volume to work
Volume drives three common purchases. For cooling, feed your result into an AC size calculator to find the right BTU rating. For painting, the wall-area figure pairs with our paint calculator when you want to deduct windows and doors. For boxes, tanks and cylinders rather than rooms, the cubic footage calculator covers those shapes with the same unit conversions.
Frequently asked questions
Divide the floor plan into two rectangles, multiply length times width times ceiling height for each, and add the two volumes. The L-shaped tab above does this in one step: enter both rectangles and one ceiling height.
Use whichever matches your equipment specs. US HVAC and air purifier ratings use cubic feet and CFM; most of the rest of the world uses cubic meters and cubic meters per hour. The calculator reports both, and one cubic meter equals 35.3147 cubic feet.
Use the average ceiling height: measure the lowest and highest points, add them, and divide by two. For a vaulted ceiling that rises from 8 to 12 feet, enter 10 feet. For complex shapes, split the room into sections as with an L-shape.
Air purifiers are commonly sized for about 5 air changes per hour, which is also the CDC's recommended clean-air target for rooms. Bathrooms and kitchens need higher rates because of moisture and odors. The EPA's separate 0.35 ACH figure is a minimum whole-home ventilation rate, not a purifier target.
Both measure airflow. CFM is cubic feet per minute, the standard for US equipment; m3/h is cubic meters per hour, used with metric specs. They convert at 1 CFM = 1.699 m3/h. The calculator shows both so you can match whichever your equipment lists.
No. It assumes bare walls at 350 sq ft per gallon per coat, two coats. The estimate rounds up to whole gallons, which usually covers normal window and door area. For a precise figure with deductions, use the paint calculator linked above.
Liters are handy for smaller spaces and for equipment that specs metric volumes, such as humidifiers, grow tents and chemical dosing. One cubic foot equals 28.3168 liters, so a 1,620 cubic foot bedroom holds 45,873 liters of air.
Add This Calculator to Your Website
Embed this calculator on your own site with the code below. Pick a size to preview it live, then copy the code.
Powered by Calculator Zilo
References
- U.S. Environmental Protection Agency. How much ventilation do I need in my home to improve indoor air quality? (ASHRAE Standard 62.2-2016 residential minimum of 0.35 air changes per hour). Accessed October 23, 2026.
- Centers for Disease Control and Prevention (NIOSH). How Much Ventilation Is Enough? (5 air changes per hour clean-air target). Accessed October 23, 2026.
- American Industrial Hygiene Association. EPA Updates Ventilation Guidance to Prevent Virus Spread (5 ACH goal for indoor spaces). Accessed October 23, 2026.
- H2X Engineering. How to Calculate Air Changes Per Hour (ACH = CFM x 60 / volume; worked office example). Accessed October 23, 2026.
- Lawrence Berkeley National Laboratory, Indoor Air Quality Science. Air Cleaning Supporting Information (AHAM CADR guidance for portable air cleaners). Accessed October 23, 2026.
- The Tech Edvocate. How to calculate CFM for a room (room-by-room ACH recommendations; CFM = volume x ACH / 60). Accessed October 23, 2026.