The Room Calculator gives you every basic measurement of a rectangular room from its length, width and ceiling height: floor area (square footage), wall area, ceiling area, room volume, perimeter and floor diagonal.
You can subtract the area of doors and windows to get the net wall area for paint or wall coverings. Specialized room calculators — square footage in feet and inches, room size from area, room volume and BTU sizing — are linked further down.
Room Calculator
Everything about a rectangular room from its length, width and height — floor area, wall area, volume and perimeter.
Formula
- Floor area: A = L × W
- Perimeter: P = 2 × (L + W)
- Net wall area: P × H − openings
- Volume: V = L × W × H
- Floor diagonal: d = √(L² + W²)
Where L = length, W = width, H = ceiling height. Ceiling area equals floor area for a flat ceiling.
How the Calculation Works
Step 1: Measure length and width at floor level and the height from floor to ceiling.
Step 2: The calculator multiplies for area and volume, and uses the perimeter × height for the walls.
Step 3: Door and window areas you enter are subtracted from the wall area.
Variables Explained
| Output | Typical use |
|---|---|
| Floor area | Flooring, rugs, listing square footage |
| Net wall area | Paint, wallpaper, wall panels |
| Ceiling area | Ceiling paint, tiles, drywall |
| Volume | Heating, cooling, ventilation, dehumidifiers |
| Perimeter | Baseboard, crown molding, trim |
| Diagonal | Checking square, fitting furniture |
Example Calculation
Given: 15 × 12 ft room, 8 ft ceiling, no openings subtracted
- Floor area = 15 × 12 = 180 ft²
- Perimeter = 2 × 27 = 54 ft
- Wall area = 54 × 8 = 432 ft²
- Volume = 180 × 8 = 1,440 ft³
- Diagonal = √(225 + 144) ≈ 19.21 ft
A standard 3 × 6.67 ft (36 × 80 in) door is 20 ft²; entering it gives 412 ft² of net wall area.
Dimensions Explained
- For L-shaped or irregular rooms, split the floor into rectangles and add them (see the Odd-Shaped Room Calculator).
- Areas are the physical surfaces; material purchase amounts also depend on coverage, packaging and waste.
Frequently Asked Questions
How do I calculate the square footage of a room?
Multiply length by width in feet. A 12 × 14 ft room is 168 ft².
How do I calculate the wall area of a room?
Perimeter × ceiling height, minus doors and windows.
How do I calculate room volume?
Length × width × height; the result is in cubic feet or cubic meters.
Why is the floor diagonal useful?
If both diagonals are equal, the room is square-cornered; it also tells you the longest straight item that fits flat.
Can I use meters?
Yes — switch the units and the results follow.