Quick start
- Enter the length and the width (or height).
- For a trapezoid, also enter the shorter parallel side in the optional box.
- Read the area, perimeter and diagonal.
Formulas
| Shape | Area | Other |
|---|---|---|
| Rectangle | length × width | Perimeter 2(l + w), diagonal √(l² + w²) |
| Square | side² | Perimeter 4s, diagonal s√2 |
| Trapezoid | (a + b) ÷ 2 × h | a and b are the parallel sides |
Example: flooring a room
A room 4.5 m by 3.2 m has an area of 14.4 m². Flooring is usually bought with about 10% extra for cutting, so you would buy around 15.8 m². The perimeter, 15.4 m, tells you how much skirting board you need.
Trapezoid example
A garden bed has parallel sides of 6 m and 4 m and is 3 m deep. Its area is (6 + 4) ÷ 2 × 3 = 15 m². Averaging the two parallel sides turns it into a rectangle of the same area.
Keep units consistent
Use the same unit for every length. If one side is in metres and another in centimetres, convert first. The area comes out in square units, such as m² or ft².
Common questions
What is the difference between area and perimeter?
Area is the space inside a shape. Perimeter is the distance around its edge.
How do I find the area of an L-shaped room?
Split it into two rectangles, find each area, and add them.
How do I convert m² to ft²?
Multiply by about 10.764, or use the Unit Converter.