The Perimeter Formula, Explained with Examples
By ToolNimba Editorial Team June 20, 2026 4 min read
Perimeter is the total distance around the outside of a two-dimensional shape. If you walked along every edge of a shape and back to where you started, the length you covered is the perimeter. Because it is a length, perimeter is always measured in single units such as centimetres, metres, inches or feet, never square units.
Quick answer
Perimeter is the total distance around a shape. For a rectangle, perimeter = 2 x (length + width). For a square, perimeter = 4 x side. For any other polygon, just add the lengths of all the sides.
What perimeter actually measures
The word perimeter comes from the Greek for "measure around". For any polygon, the rule never changes: add up the lengths of all the sides. The named formulas below are just shortcuts that take advantage of sides that are equal, so you do less adding. Perimeter answers questions like how much fencing a yard needs, how much trim borders a picture frame, or how long the edge of a running track is.
This is different from area, which measures the space inside a shape and is given in square units. A garden can have a small area but a long perimeter, or a large area with a short perimeter, so the two are not interchangeable. If you need the space inside instead, see the area of a circle guide.
Perimeter formulas for common shapes
Here are the formulas you will use most often. In every case the result is a single length, so keep your units consistent before you add anything.
Perimeter formulas by shape
| Shape | Formula | What the letters mean |
|---|---|---|
| Rectangle | 2 x (length + width) | length and width are the two side lengths |
| Square | 4 x side | side is the length of one edge |
| Triangle | a + b + c | a, b and c are the three side lengths |
| Regular polygon | number of sides x side length | all sides are equal |
| Circle | 2 x pi x radius | this is called the circumference |
A circle has no straight sides, so its "perimeter" gets a special name: the circumference. The idea is identical, the distance all the way around, but the formula uses pi (about 3.14159). For more on that case, read the circumference formula guide.
How to find the perimeter of a rectangle
A rectangle has two pairs of equal sides: two lengths and two widths. You could add length + width + length + width, but it is faster to add one length and one width and then double the result. That is exactly what the formula 2 x (length + width) does.
Example: a rectangle is 8 metres long and 5 metres wide.
- Add one length and one width: 8 + 5 = 13.
- Multiply by 2: 2 x 13 = 26.
- The perimeter is 26 metres.
A quick check: adding all four sides directly gives 8 + 5 + 8 + 5 = 26, which matches. Both methods always agree, so use whichever you find easier.
How to find the perimeter of a square
A square is the simplest case because all four sides are the same length. Instead of adding the same number four times, just multiply that side length by 4.
Example: a square tile has sides of 6 inches.
- Take the side length: 6 inches.
- Multiply by 4: 4 x 6 = 24.
- The perimeter is 24 inches.
Because a square is a rectangle with equal sides, the rectangle formula gives the same answer: 2 x (6 + 6) = 2 x 12 = 24. The 4 x side shortcut is simply the faster route.
Triangles and other polygons
For a triangle there is no shortcut to memorise, you just add the three sides: a + b + c. A triangle with sides 3, 4 and 5 has a perimeter of 3 + 4 + 5 = 12 units. The same logic covers any irregular shape: measure every side and add them.
For a regular polygon, where every side is equal, multiply the side length by the number of sides. A regular hexagon (6 sides) with 10 cm sides has a perimeter of 6 x 10 = 60 cm. A regular pentagon (5 sides) of 7 cm has a perimeter of 5 x 7 = 35 cm.
Common mistakes to avoid
- Mixing up perimeter and area. Perimeter is a length in single units; area is space in square units. Doubling every side doubles the perimeter but quadruples the area.
- Using mismatched units. Convert everything to the same unit before adding. You cannot add metres to centimetres directly.
- Forgetting a side. With irregular shapes it is easy to skip an edge, so count your sides against the figure.
- Counting only two sides of a rectangle. Remember there are two lengths and two widths, which is why the formula doubles the sum.
If your measurements come from a floor plan or a plot of land, you may also want the area for flooring or turf. The square footage guide walks through that calculation.
Calculate area and perimeter instantly
Enter the side lengths below and the calculator returns the perimeter and area together, with the steps shown.
📐 Try the free tool Area Calculator Free area calculator for rectangles, squares, triangles, circles, trapezoids and parallelograms. Enter dimensions for instant area, the formula used, and full working.It handles rectangles, squares, triangles and circles, so you do not have to remember which formula to use.
Frequently asked questions
What is the perimeter formula?
Perimeter is the total distance around a shape, found by adding the lengths of all its sides. For a rectangle it is 2 x (length + width); for a square it is 4 x side; for a triangle it is a + b + c.
How do you find the perimeter of a rectangle?
Add one length and one width, then multiply by 2. For a rectangle 8 m by 5 m: 8 + 5 = 13, and 2 x 13 = 26 metres. This works because a rectangle has two equal lengths and two equal widths.
What is the perimeter of a square?
Multiply the side length by 4, because all four sides are equal. A square with 6 inch sides has a perimeter of 4 x 6 = 24 inches.
Is perimeter the same as area?
No. Perimeter is the distance around a shape, measured in single units like metres. Area is the space inside the shape, measured in square units like square metres. They answer different questions.
What units is perimeter measured in?
Perimeter is a length, so it uses single units such as centimetres, metres, inches or feet. It is never given in square units, which are reserved for area.
How do you find the perimeter of a circle?
A circle has no sides, so its perimeter is called the circumference. It equals 2 x pi x radius, where pi is about 3.14159. A circle with a radius of 5 cm has a circumference of about 31.4 cm.