Perimeter Calculator
Perimeter is the total distance around the outside of a flat shape. Choose a shape, enter its sides in any mix of units, and get the length of fence, trim, edging or border you need.
- Formula used
- Perimeter = 2 × (length + width)
Saved setups
Save a set of inputs you reuse — your usual rate, your loan, your room sizes — and load it back in one tap.
Your recent calculations
Results you calculate here are kept on this device so you can come back to them.
Formula
How to use it
- Choose the shape.
- Enter the side lengths, each in its own unit.
- Pick the unit for the answer.
- For materials, add a little for waste and subtract any openings such as gates or doorways.
Worked examples
A 12 ft by 10 ft rectangle: 44 ft around, which is 13.41 m
- Perimeter
- 44 ft
A circle with a 3 m radius
- Perimeter
- 18.85 m
An ellipse with semi-axes of 5 m and 3 m (Ramanujan’s approximation)
- Perimeter
- 25.53 m
Why the ellipse is approximate
Unlike a circle, an ellipse has no simple exact formula for its perimeter. This calculator uses Ramanujan’s approximation, which is accurate to a tiny fraction of a percent for all but extremely flattened ovals.
Triangles that cannot exist
Three lengths only make a triangle if each one is shorter than the other two added together. Sides of 2, 3 and 10 cannot close, so the result is left blank.
Questions people ask
What is the perimeter of a 12 ft by 10 ft room?
2 × (12 + 10) = 44 feet.
How much fencing do I need for a 50 ft by 100 ft lot?
2 × (50 + 100) = 300 feet, before subtracting the width of any gates.
What is the difference between perimeter and area?
Perimeter is a length — the distance around the edge. Area is the surface inside, measured in square units. A 10 ft square has a perimeter of 40 ft and an area of 100 ft².