Perimeter Calculator
Find the perimeter or circumference of eight common shapes (with the formula and the substituted numbers shown).
Details
Perimeter
16
in the same length unit as the inputs
This works out the perimeter of a shape: the total distance around its outside edge.
Pick the shape, enter its measurements, and it returns the perimeter in ordinary length units.
What perimeter measures
Perimeter is the distance all the way around the outside of a shape. Walk the boundary of a field and the distance you cover is its perimeter.
Because it is a length rather than a covering, the answer is in ordinary units — feet, metres, inches — not square units. That is the quickest way to tell it apart from area.
It is what you need for anything that goes along an edge: fencing, skirting board, picture framing, edging a flower bed, or trim around a room.
For a rectangle you only measure two sides, because opposite sides match. Adding them and doubling is quicker than measuring all four.
What to enter
- Shape
- Rectangle, square, triangle, circle, parallelogram or trapezoid. The fields change to match.
- Side lengths
- For most shapes you need every side. A rectangle only needs two, since the opposite sides are equal.
- Radius (for a circle)
- A circle's perimeter is called its circumference, and it needs the radius or diameter rather than sides.
The formulas
- Rectangle
- 2 × (length + width)
- Square
- 4 × side
- Triangle
- a + b + c. Just add the three sides.
- Circle
- 2πr, called the circumference. See the circle calculator.
- Any irregular shape
- Add every side. There is no shortcut, and none is needed.
What this assumes
All measurements use the same unit.
The shape is closed. An open boundary, like fencing three sides of a garden, needs the open side left out.
How to calculate the perimeter of a shape
Add up every side. The formulas are only shortcuts for shapes where sides repeat.
- rectangle
- 2(length + width), since opposite sides match
- circle
- 2πr, the one shape with no sides to add
Measure every side. Or use a shortcut where sides repeat: a rectangle only needs two measurements.
Check the units match. Convert everything to one unit before adding.
Add them up. The answer stays in ordinary length units, not square units.
Leave out any open sides. Fencing three sides of a garden against a wall means adding only those three.
See a worked example: why the same fence encloses different gardens
- Option A
- 12 ft × 8 ft rectangle
- Option B
- 10 ft × 10 ft square
Option A perimeter: 2 × (12 + 8) = 40 ft. Area: 12 × 8 = 96 sq ft.
Option B perimeter: 4 × 10 = 40 ft. Area: 10 × 10 = 100 sq ft.
Identical fencing, but the square encloses 4 more square feet.
For a fixed perimeter, the closer a rectangle gets to a square, the more it encloses. A circle would do better still.
Same 40 ft of fence, 96 vs 100 sq ft
Frequently asked questions
Perimeter is the distance around the edge, in feet or metres. [Area](/math/area-calculator) is the space inside, in square feet or square metres.
Fencing and trim are perimeter jobs. Flooring, turf and paint are area jobs. The units tell you which one you have calculated.
Yes, and the difference can be large. A 12 × 8 rectangle and a 10 × 10 square both have a perimeter of 40, but enclose 96 and 100 square feet.
Push it further and a 19 × 1 rectangle still has a perimeter of 40 while enclosing only 19 square feet. For a fixed perimeter, the more square-like the shape, the more it holds.
The circumference. It is the same idea, just with its own name because a circle has no sides to add.
It works out as 2πr, so a circle of radius 5 has a circumference of about 31.42.
Start with the perimeter, then subtract any side you are not fencing, such as one against a house or an existing wall.
Add a little for gates, corner posts and waste. Fencing is sold in fixed panel lengths, so round up to whole panels rather than buying the exact figure.
Measure every side and add them. There is no formula, and none is needed.
For a curved boundary, lay a flexible tape or a length of string along it, then measure the string.
Problems people actually run into
Buying fencing for all four sides when one is already walled
The perimeter is the full boundary, but the job often is not. A garden with a house along one edge needs three sides, not four.
On a 12 × 8 plot backing onto a wall, that is 32 feet rather than 40 — a fifth of the order, wasted if nobody checks.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Last updated: September 4, 2026