How to Calculate Square Footage

Square footage is the area of a flat surface measured in square feet. For rectangular rooms, it's simply length × width. For irregular shapes, break the space into simpler shapes and add them up.

Formula by shape

Rectangle or square

Area = Length × Width
Example: 12 ft × 15 ft = 180 sq ft

L-shaped room

Area = Rectangle 1 + Rectangle 2
Split into two rectangles, calculate each, add together.

Triangle

Area = (Base × Height) ÷ 2
Example: 8 ft base × 6 ft height ÷ 2 = 24 sq ft

Circle

Area = π × radius² (π ≈ 3.14159)
Example: 5 ft radius → 3.14159 × 25 = 78.5 sq ft

Quick reference — common room sizes

Room dimensions	Square footage
8×10 ft	80 sq ft
10×10 ft	100 sq ft
10×12 ft	120 sq ft
12×12 ft	144 sq ft
12×14 ft	168 sq ft
12×15 ft	180 sq ft
14×16 ft	224 sq ft
15×20 ft	300 sq ft
20×20 ft	400 sq ft

How to convert feet and inches

When your measurements include inches, convert to decimal feet:

Inches	Decimal feet
3”	0.25
4”	0.33
6”	0.50
8”	0.67
9”	0.75
10”	0.83

Example: 11 ft 6 in × 14 ft 9 in = 11.5 × 14.75 = 169.6 sq ft

Step-by-step for a full house

Sketch each room — rough floor plan with all dimensions
Measure each room — length and width in feet and inches
Calculate each room — length × width
Add all rooms together — total square footage
Add waste factor — 10% for flooring, 15% for tile

Subtract doors and windows for paint

For painting, measure wall area (not floor area):

Wall area = Perimeter × Wall height
Subtract: each door ≈ 20 sq ft, each window ≈ 15 sq ft
Use the Paint Calculator to automate this calculation

Frequently Asked Questions

How do I calculate square footage of a room?

For a rectangular room, multiply length (ft) × width (ft). For example, a 12×15 ft room = 180 square feet. For L-shaped or irregular rooms, break the space into rectangles, calculate each, and add them together.

How do I calculate square footage with inches?

Convert inches to feet first by dividing by 12. For example, a room that is 11 ft 6 in × 14 ft 3 in = 11.5 × 14.25 = 163.9 square feet.

How do I calculate square footage of an L-shaped room?

Divide the L-shape into two rectangles. Measure each rectangle and multiply length × width. Then add the two areas together. Example: 10×12 ft + 8×6 ft = 120 + 48 = 168 sq ft.

How do I calculate square footage for flooring?

Measure the room length and width, multiply them, and add 5–10% for waste and cuts. For multiple rooms, calculate each one separately and add. Use our Flooring Calculator to do this automatically.

How many square feet is a 10x10 room?

A 10×10 room is exactly 100 square feet. A 12×12 room is 144 sq ft, a 15×15 is 225 sq ft, and a 20×20 is 400 sq ft.