How to calculate square footage
Square footage is the area of a surface measured in square feet. For a rectangular room, measure the length and the width in feet and multiply them: a 20 ft by 15 ft room is 20 × 15 = 300 square feet.
This square footage calculator does the same for nine shapes. Pick a shape, enter each measurement in whichever unit you measured it in, and every value is converted to feet before the area is worked out. If you have several identical areas, the quantity multiplies the result.
Area formulas by shape
| Shape | Area |
|---|---|
| Rectangle | length × width |
| Rectangle border | outer length × outer width − (outer length − 2 × border) × (outer width − 2 × border) |
| Circle | π × (diameter ÷ 2)² |
| Ring | π × ((outer diameter ÷ 2)² − (outer diameter ÷ 2 − ring width)²) |
| Triangle (3 sides) | √(s × (s − a) × (s − b) × (s − c)), where s = (a + b + c) ÷ 2 |
| Triangle (base & height) | base × height ÷ 2 |
| Trapezoid | (top side + bottom side) ÷ 2 × height |
| Sector | angle ÷ 360 × π × radius² in degrees, or angle ÷ 2 × radius² in radians |
| Parallelogram | base × height |
Height always means the perpendicular distance between two sides, not the length of a slanted edge. For the rectangle border and the ring, the border is measured inward from the outer edge.
Unit conversions and cost
Lengths are converted with exact factors: 1 yard = 3 feet, 1 inch = 1/12 foot, 1 mile = 5,280 feet, and 1 foot = 0.3048 meter. The results table then shows the same area in square yards (÷ 9), square inches (× 144), acres (÷ 43,560), square meters (× 0.09290304), and the other units.
If you enter a price, the total cost is the area in the price's unit multiplied by that price. For example, 300 square feet at $4.50 per square foot costs $1,350, and the same area at $36 per square yard is 33.33 sq yd × $36 = $1,200.
Frequently asked questions
How do I calculate the square feet of a room?
Measure the length and width of the floor in feet and multiply them. For an L-shaped or irregular room, split the floor into rectangles, triangles, and circles, calculate each one, and add the results.
How do I convert inches to square feet?
Divide each measurement in inches by 12 to get feet, then multiply. If you already have square inches, divide by 144. A 96 in by 30 in countertop is 8 ft × 2.5 ft = 20 square feet.
How much extra flooring or tile should I buy?
The calculator gives the exact area. Flooring, tile, and similar materials need extra for cuts and breakage, commonly 5% to 15% depending on the material and layout, so check the manufacturer's recommendation before ordering.
How many square feet are in an acre?
One acre is 43,560 square feet, which is 4,840 square yards. To convert square feet to acres, divide by 43,560: a 10,890 square foot lot is 0.25 acre.
Limitations
The result is only as accurate as your measurements and assumes a flat surface with the exact shape you picked. Total cost covers the price per unit area only, not labor, waste, tax, or delivery.
What's changed
- Launched.
References
- NIST Special Publication 811, Appendix B.8: Factors for Units Listed Alphabetically
length and area unit conversion factors