Square Footage = L × W; Cubic Footage = L × W × H; BTU = sqft × 20Multiply length by width for floor area in square feet. Multiply again by ceiling height for volume in cubic feet. Convert square feet to square meters by multiplying by 0.0929; cubic feet to cubic meters by 0.0283. A rough AC sizing rule is 20 BTU per square foot of conditioned space.
Enter length, width, and ceiling height. Get square footage, cubic footage, perimeter, and AC BTU.
Enter your room length, width, and ceiling height to get square footage, cubic footage, metric equivalents, perimeter, and a rough air-conditioning BTU estimate. Useful for flooring quotes, paint and wallpaper orders, HVAC sizing, and rental listings.
Room size is the starting input for almost every home project: flooring, paint, wallpaper, furniture fit, AC sizing, even rental pricing. This calculator gives you the four most common measurements (square footage, cubic footage, perimeter, BTU) from a single set of inputs.
You came here because
Common situations
- Flooring and carpet orders: Get the square footage your installer needs for an accurate quote.
- Paint estimating: Use perimeter and ceiling height to calculate wall area for a paint order.
- AC and heater sizing: Match unit BTU to room load for proper cooling without short-cycling.
- Real estate listings: Calculate and verify the square footage figure used in listings.
Under the hood
How the calculation works
- 1Measure the room length and width along the floor.
- 2Measure the ceiling height from floor to ceiling.
- 3Enter all three values in feet.
- 4The calculator returns floor area, volume, perimeter, and AC BTU.
- 5Metric equivalents appear in the secondary results.
Show me
A real example
Example: 15 × 12 ft room with 8-foot ceiling
- 1Floor area = 15 × 12 = 180 ft²
- 2Volume = 180 × 8 = 1,440 ft³
- 3Perimeter = 2 × (15 + 12) = 54 ft
- 4AC sizing = 180 × 20 = 3,600 BTU
Watch out for
What can go wrong
- Measuring to the wrong reference: Floor measurements include the full distance between finished walls. Do not measure to baseboards or to drywall corners that are not square.
- Forgetting closets and nooks: L-shaped rooms need to be broken into rectangles. Sum the rectangle areas; do not measure as if the room is fully rectangular.
- Using 20 BTU/ft² in extreme climates: The 20 BTU rule applies to moderate climates. Hot, sunny rooms need 30 BTU/ft²; well-insulated cool rooms need 15 BTU/ft².
- Using listing square footage for material orders: Listing figures often include nooks, closets, or stair landings. Material orders need the actual covered floor area.
Glossary
Related concepts
| Term | Definition |
|---|---|
| Square footage | Floor area in square feet. Length times width for a rectangular room. |
| Cubic footage | Room volume in cubic feet. Floor area times ceiling height. Used for HVAC airflow and air-purifier sizing. |
| BTU | British Thermal Unit. A measure of heating or cooling energy. AC sizing uses BTU per hour as the unit. |
Make it better
Pro tips
- Measure with a laser tool: Laser measures are accurate to 1/16 inch and faster than tape measures for whole rooms. They pay for themselves on a single big project.
- Sketch the room first: Even a rough sketch with dimensions helps catch missed corners and gives your contractor a quick reference.
- Add 10% to flooring orders: Square footage gives you the net area. Add 10% for cutting waste, repairs, and pattern matching when ordering material.
- Verify AC sizing with a Manual J: The 20 BTU rule is a starting point. For a real HVAC install, request a Manual J load calculation that factors windows, insulation, and orientation.
Common questions
Frequently asked questions
Quick reference
Common Room Sizes
Standard rooms with 8-foot ceilings
| Room type | Dimensions | Square footage | AC BTU |
|---|---|---|---|
| Small bedroom | 10 × 10 ft | 100 ft² | 2,000 BTU |
| Standard bedroomtypical | 12 × 12 ft | 144 ft² | 2,880 BTU |
| Master bedroom | 14 × 16 ft | 224 ft² | 4,480 BTU |
| Living room | 15 × 20 ft | 300 ft² | 6,000 BTU |
| Large family room | 20 × 25 ft | 500 ft² | 10,000 BTU |
For related calculations, try the Flooring Calculator, Paint Calculator, or Concrete Calculator. Browse all Calculator Online calculators for the full catalog.
Methodology
This calculator uses the standard room size calculator formula. Results match those from established financial, scientific, and health references.
Reviewed by
Calculator Online Editorial Team. All formulas verified against authoritative sources before publication.
Last updated
2026-05-24