What Is Square Footage and How Is It Calculated?
Square footage is the area of a flat surface measured in square feet — the number of 1 ft × 1 ft squares that fit inside it. For a rectangular room it equals the length in feet multiplied by the width in feet, so a 12 ft × 15 ft bedroom is 180 square feet.
The basic square footage formula is Area = Length × Width. Other shapes use their own area formulas: a triangle is ½ × base × height, a circle is π × (diameter ÷ 2)², and a trapezoid is (side a + side b) ÷ 2 × height. Irregular rooms are split into these shapes, each piece is calculated, and the results are added together. Areas that don't count, such as a kitchen island or a fireplace hearth, are subtracted.
Since the 1959 international yard and pound agreement, one foot is exactly 0.3048 metres, so one square foot is exactly 0.09290304 square metres and one square metre is about 10.764 square feet. One acre is 43,560 square feet and one square yard is 9 square feet.
How to Use This Square Footage Calculator
Pick the unit you measured in, then describe your space as one or more areas. Results update as you type:
- Measurement unit: feet, inches, yards, metres or centimetres. Everything is converted to square feet and square metres automatically, so you never need to convert inches to feet first.
- Area shape: rectangle/square (length × width), triangle (base and height), circle (diameter), or trapezoid (two parallel sides and the height between them).
- Add or subtract: set an area to Subtract for cut-outs — a staircase opening, built-in cabinets, or an island you won't floor under.
- Add another area: build L-shaped, T-shaped or multi-room layouts by adding a rectangle for each section.
- Price per square foot (optional): enter a material, installation or rental price to get an instant total cost.
Measure each wall to the nearest inch (or centimetre), and for whole-home figures measure every room separately and add them. The share link stores every area, so you can send the exact layout to a contractor.
Square Footage Formulas by Shape
| Shape | Formula | Example |
|---|---|---|
| Rectangle / square | L × W | 12 × 15 ft = 180 sq ft |
| Triangle | ½ × b × h | ½ × 10 × 12 ft = 60 sq ft |
| Circle | π × (d ÷ 2)² | 14 ft diameter ≈ 153.94 sq ft |
| Trapezoid | (a + b) ÷ 2 × h | (8 + 12) ÷ 2 × 10 ft = 100 sq ft |
| L-shaped room | (L₁ × W₁) + (L₂ × W₂) | 20 × 14 + 10 × 8 = 360 sq ft |
Common Square Footage Conversions
| From | To | Multiply by |
|---|---|---|
| Square feet | Square metres | 0.092903 |
| Square metres | Square feet | 10.7639 |
| Square feet | Square yards | ÷ 9 |
| Square inches | Square feet | ÷ 144 |
| Square feet | Acres | ÷ 43,560 |
| Square feet | Roofing “squares” | ÷ 100 |
Units differ by country. The US quotes floor area in square feet; the UK, Canada and India commonly list both square feet and square metres; Australia, New Zealand and most of Europe use square metres. In the US and Australian roofing and building trades, a “square” is 100 square feet (about 9.29 m²).
Frequently Asked Questions About Square Footage
How do you calculate the square footage of a room?
Measure the length and width of the room in feet and multiply them. A 12 ft by 15 ft room is 180 square feet. For an L-shaped or irregular room, split it into rectangles, calculate each one, and add the results together.
How do you calculate the square footage of a house?
Under the ANSI Z765-2021 standard maintained by Home Innovation Research Labs, US home square footage is measured from the exterior walls and counts only finished, heated, above-grade living space. Garages, unfinished areas and basements are excluded from the main figure and reported separately.
How do I convert inches to square feet?
Multiply length by width in inches to get square inches, then divide by 144, because one square foot is 12 × 12 = 144 square inches. For example, 120 in × 96 in = 11,520 square inches, which is 80 square feet.
Do closets and hallways count in square footage?
For a whole-home figure, yes: finished closets, hallways and stairways that are part of the heated living space are included. For a single-room project such as flooring, include a closet only if the new material will run into it.
How accurate is this square footage calculator?
The math is exact for the shapes you enter, so accuracy depends on your measurements. Measure to the nearest inch, and when ordering flooring, tile or carpet add 5–15% for cuts and waste on top of the calculated area.