Area Calculator
Rectangle: A = length × width = 5 × 3 = 15. Circle: A = π × r² = π × 5² ≈ 78.54.
How to use
- Pick the shape.
- Enter its dimensions in the same unit, then choose that unit from the list.
- The area appears in square units, with the perimeter where it can be found from the inputs, and conversions to m² and ft².
How it works
Rectangle: length × width. Triangle: base × height ÷ 2. Circle: π × radius². Trapezoid: (side a + side b) ÷ 2 × height. Ellipse: π × a × b, where a and b are the two semi-axes.
Perimeter is shown for the rectangle (2 × (length + width)), the circle (2πr) and the ellipse.
An ellipse has no exact simple perimeter formula, so Ramanujan's approximation is used. It is accurate to many digits for everyday shapes.
The area is converted using the exact definition of each length unit, for example 1 ft = 0.3048 m.
Shape formulas
| Shape | Inputs | Area |
|---|---|---|
| Rectangle | length l, width w | l × w |
| Triangle | base b, height h | b × h ÷ 2 |
| Circle | radius r | π × r² |
| Trapezoid | parallel sides a and b, height h | (a + b) ÷ 2 × h |
| Ellipse | semi-axes a and b | π × a × b |
FAQ
What does pi r 2 mean?
π r² is the area of a circle: pi (about 3.14159) times the radius squared. A radius of 5 gives π × 25 ≈ 78.54 square units.
How do I find the area from length and width?
For a rectangle or square, multiply the length by the width. A room 5 m long and 3 m wide covers 5 × 3 = 15 m².
What is the height of a triangle?
The perpendicular distance from the base to the opposite corner, not the length of a slanted side. If you only know the three sides, use the Triangle calculator.
Do I enter the radius or the diameter of a circle?
The radius, which is half the diameter. Halve a diameter before you enter it.
What are the parallel sides of a trapezoid?
The two sides that never meet, often called the bases. The height is the distance between them, measured at a right angle.
How do I convert square feet to square metres?
Pick feet as the unit and read the m² line. One square foot is 0.09290304 m², and one square metre is about 10.764 ft².