Rectangle Area Calculator
Calculate the area and perimeter of a rectangle.
How to find the area of a rectangle
A rectangle's area is simply its length multiplied by its width, since it is a grid of unit squares that many rows deep and that many columns across. The same two measurements also give the perimeter, the distance around the outside, and the diagonal across the corners.
$$A = l \times w \qquad P = 2(l + w) \qquad d = \sqrt{l^2 + w^2}$$
The diagonal uses the Pythagorean theorem, because the length, width, and diagonal form a right triangle.
How to use this calculator
- Enter the length.
- Enter the width.
- Press Calculate to get the area, perimeter, and diagonal.
Worked example
A rectangle 4 long and 6 wide has an area of 4 × 6 = 24 square units. Its perimeter is 2 × (4 + 6) = 20 units, and its diagonal is the square root of 4 squared plus 6 squared, about 7.21 units.
How to use this calculator
- Enter the rectangle's length.
- Enter the rectangle's width.
- Read the area and perimeter instantly.
- Use the worked example to check your own calculation.
The formula explained
$$ A = l \times w,\quad P = 2(l + w) $$
- \(A\) = area
- \(P\) = perimeter
- \(l\) = length
- \(w\) = width
Step by step method
- Measure or choose the rectangle's length and width using the same unit.
- Multiply length by width to find the area.
- Add length and width, then multiply by 2 to find the perimeter.
- Compare your results with the calculator's instant output.
Worked example
Suppose a rectangle is 8 meters long and 5 meters wide.
- Compute the area with \(A = 8 \times 5 = 40\).
- Compute the perimeter with \(P = 2(8 + 5) = 2(13) = 26\).
Answer. Area = 40 square meters, perimeter = 26 meters
Tips and common mistakes
- Use the same unit for both measurements before calculating.
- Remember that area is in square units, while perimeter is in linear units.
- If you swap length and width, the area stays the same and the perimeter also stays the same.
- Double-check that you are multiplying for area and adding then doubling for perimeter.
Frequently asked questions
What is the difference between area and perimeter?+
Area is the space inside the rectangle, measured in square units. Perimeter is the distance all the way around the edge, measured in plain units. You would use area for flooring and perimeter for fencing.
How do I find the area of a square?+
A square is just a rectangle with equal sides, so enter the same value for length and width. Its area is the side length squared.
How is the diagonal calculated?+
The diagonal splits the rectangle into two right triangles, so the Pythagorean theorem applies. Square the length and width, add them, and take the square root to get the diagonal.
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