How to use it
- Pick a shape.
- Choose the unit of your measurements.
- Enter the dimensions.
- Read the area, the perimeter and the converted square feet or meters.
Area formulas
| Shape | Area | Perimeter |
|---|---|---|
| Square | A = a² | P = 4a |
| Rectangle | A = l × w | P = 2(l + w) |
| Triangle | A = b × h / 2 | sum of sides |
| Triangle (3 sides) | Heron's formula | a + b + c |
| Right triangle | A = a × b / 2 | a + b + c |
| Parallelogram | A = b × h | – |
| Trapezoid | A = (a + b) / 2 × h | – |
| Rhombus | A = p × q / 2 | 2√(p² + q²) |
| Circle | A = πr² | C = 2πr |
| Ellipse | A = π × a × b | – |
Pick the shape and the calculator shows the formula it uses, the area in your unit and the conversion to square feet and square meters. It all runs in your browser.
Worked examples
- Room: 12 ft × 15 ft = 180 sq ft, perimeter 54 ft.
- Triangle: base 10 in, height 6 in: 10 × 6 / 2 = 30 sq in.
- Trapezoid: parallel sides 8 ft and 12 ft, height 5 ft: (8 + 12) / 2 × 5 = 50 sq ft.
- Circle: radius 4 ft: π × 16 ≈ 50.27 sq ft.
- Ellipse: semi-axes 5 ft and 3 ft: π × 15 ≈ 47.12 sq ft.
Heron's formula for three sides
If you know all three sides of a triangle but not its height, use Heron's formula:
s = (a + b + c) / 2, then A = √(s(s − a)(s − b)(s − c))
Example: sides 3, 4 and 5: s = 6, A = √(6 × 3 × 2 × 1) = √36 = 6.
The sides must satisfy the triangle inequality (any two sides add up to more than the third); otherwise the calculator tells you no such triangle exists. The triangle calculator also finds the angles.
Area unit conversions
| Unit | Equals |
|---|---|
| 1 sq ft | 144 sq in |
| 1 sq yd | 9 sq ft |
| 1 acre | 43,560 sq ft |
| 1 sq mi | 640 acres |
| 1 m² | 10.764 sq ft |
| 1 sq ft | 0.0929 m² |
Convert lengths before you multiply, or let the calculator do it: enter measurements in inches and it shows the result in square inches, square feet and square meters.
Irregular shapes and practical use
For an L-shaped room, a yard or any irregular area, split it into rectangles, triangles and circles, find each area and add them up. Subtract cutouts such as a kitchen island. For flooring, paint and room measurements with waste allowances and costs, the square footage calculator handles several pieces at once.
Measure to the nearest half inch or so and keep units consistent; mixing feet and inches is the most common source of errors. For 3D shapes, use the volume calculator or the surface area calculator.
Common mistakes
- Using the slanted side as the height. For triangles, parallelograms and trapezoids, the height is measured at a right angle to the base.
- Forgetting to halve. Triangle and rhombus formulas divide by 2.
- Radius vs. diameter in circle and ellipse formulas.
- Square units. A 3 ft × 3 ft square is 9 sq ft, which is 1 sq yd, not 3.
- Mixing inches and feet. A 30-inch by 6-foot hallway runner is 2.5 ft × 6 ft = 15 sq ft, not 180.
- Rounding measurements too much. Rounding 11 ft 8 in up to 12 ft on both sides of a room adds about 8% to the area.
Frequently asked questions
How do I calculate the area of a rectangle?
Multiply length by width. A 12 ft by 15 ft room is 180 square feet.
How do I find the area of a triangle?
Multiply the base by the height and divide by 2. If you only know the three sides, use Heron's formula.
How many square feet are in an acre?
43,560 square feet.
How do I convert square feet to square meters?
Multiply by 0.0929, or divide by 10.764.
Can I enter measurements in inches?
Yes. Choose inches as the unit; the calculator also converts the result to square feet and square meters.
Last updated: November 12, 2026