How to find the perimeter of a rectangle
The perimeter of a rectangle is twice the sum of its sides: P = 2 × (l + w). For a rectangle 10 long and 4 wide, P = 2 × (10 + 4) = 28 units. The area is 10 × 4 = 40 square units — see the rectangle area calculator.
The doubling comes from the fact that a rectangle has two pairs of equal sides. You could equally write it as l + w + l + w; the factored form is just quicker to compute.
How to use this calculator
Enter the length and width in the same unit. The perimeter is the total distance around the shape, returned in linear units. For four-sided shapes with parallel bases see the trapezoid area calculator.
Keep perimeter and area straight: perimeter is a distance measured in metres or inches, while area is a surface measured in square units. Mixing units between the two inputs is the usual source of error.
Worked example
To fence a garden 10 m by 4 m, the perimeter is 2 × (10 + 4) = 28 metres of fencing. The enclosed area is 40 square metres.
Real jobs usually need a little more than the bare perimeter. Subtract the width of any gate, and add an allowance for overlap or waste at corners. If posts are set at fixed intervals, divide the perimeter by the spacing to estimate how many you need, rounding up and adding one for each corner. The perimeter of a circle has its own name and its own formula, covered by the circle circumference calculator.
Frequently asked questions
- What is the formula for the perimeter of a rectangle?
- P = 2 × (length + width). Add the two sides and double the result.
- What is the perimeter of a 10 by 4 rectangle?
- 28 units (2 × 14).
- Does this work for a square?
- Yes — enter the same number for length and width, and the perimeter is 4 × side.
- What units is the perimeter in?
- The same linear unit you used for the sides — inches, cm, metres, etc.
- Can two rectangles have the same perimeter but different areas?
- Yes, and the difference can be large. A 10 by 4 rectangle and a 13 by 1 rectangle both have a perimeter of 28, but their areas are 40 and 13 square units. For a fixed perimeter, the area is greatest when the shape is closest to a square.