Rectangle Calculator
A rectangle 3 units long and 4 units wide has an area of 12, a perimeter of 14, and a diagonal of 5. This rectangle calculator finds the area, perimeter, and diagonal from length and width in any unit.
Quick answer
Area equals length times width.
What this tells you
- •Area equals length times width.
- •Perimeter equals 2 times the length plus 2 times the width, or twice the sum of the two sides.
- •The diagonal uses the Pythagorean theorem: the square root of length squared plus width squared.
How to Use
- 1Enter the length of the rectangle.
- 2Enter the width in the same unit.
- 3Click Calculate to get the area, perimeter, and diagonal.
How It Works
Formula
Area = L x W. Perimeter = 2 x (L + W). Diagonal = sqrt(L^2 + W^2).The area of a rectangle is the product of its length and width. The perimeter is the distance around the rectangle, or twice the sum of the two sides. The diagonal is the straight line from one corner to the opposite corner, found using the Pythagorean theorem with the length and width as the two legs of a right triangle.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
3 by 4 rectangle
Area = 3 x 4 = 12. Perimeter = 2 x (3 + 4) = 14. The diagonal is sqrt(9 + 16) = 5, which is the classic 3-4-5 right triangle.
5 by 5 square
A square is a rectangle with equal sides. Area = 5 x 5 = 25. Perimeter = 2 x (5 + 5) = 20. Diagonal = sqrt(25 + 25) = sqrt(50) = 7.0711.
Rectangle Sizes and Their Measurements
Common length-by-width pairs and the resulting area, perimeter, and diagonal.
| L x W | Area | Perimeter | Diagonal |
|---|---|---|---|
| 3 x 4 | 12 | 14 | 5 |
| 5 x 5 | 25 | 20 | 7.07 |
| 6 x 8 | 48 | 28 | 10 |
| 10 x 12 | 120 | 44 | 15.62 |
Common mistakes
- Confusing area with perimeter. Area measures the space inside the rectangle in square units, while perimeter measures the distance around it in linear units.
- Mixing units. Both length and width must be in the same unit before calculating.
- Using the diagonal formula on a non-rectangle shape. The formula diagonal equals the square root of L squared plus W squared works only for rectangles.