Perimeter Calculator
Calculate perimeter and circumference of shapes
What is it?
This calculator finds the perimeter (or circumference for a circle) of common shapes — rectangle, square, circle, and triangle — using the standard formula for each.
Formula
- Rectangle: Perimeter = 2 × (Length + Width)
- Square: Perimeter = 4 × Side
- Circle: Circumference = 2 × π × Radius
- Triangle: Perimeter = Side A + Side B + Side C
Formula Explanation
Perimeter is simply the total distance around the outside edge of a shape — for straight-sided shapes, that means adding up every side's length; for a circle, the circumference formula (2πr) captures the same idea for a curved boundary.
Example Calculation
A rectangle with length 8 and width 5 has a perimeter of 2 × (8 + 5) = 26 units.
How to Use
- Select the shape you want to calculate.
- Enter the required measurements for that shape.
- Click Calculate to see the perimeter and formula used.
Benefits
- Covers four common shapes in a single calculator.
- Automatically labels the result "Circumference" for circles.
- Shows the formula used alongside the result.
Use Cases
- Estimating fencing, trim, or border material needed for a space.
- Homework and coursework involving basic geometry.
- Calculating the boundary length of a garden, room, or plot of land.
What Your Result Means
The result is the total length around the outside of the shape, expressed in the same unit as your input measurements (e.g. meters if you entered meters).
Tips
- Keep all measurements in the same unit before calculating.
- For a circle, make sure you enter the radius, not the diameter.
- For a triangle, check that your three sides can actually form a valid triangle.
Common Mistakes
- Mixing different units (like meters and centimeters) within the same calculation.
- Entering the diameter instead of the radius for a circle.
- Entering triangle sides that don't satisfy the triangle inequality (an impossible triangle).
FAQs
What is the difference between perimeter and circumference?
They're the same concept — the total distance around a shape's boundary. "Circumference" is simply the term used specifically for circles.
Do the triangle sides need to form a valid triangle?
Yes, for the sides to form a real triangle, the sum of any two sides must be greater than the third side (the triangle inequality).
Do I use radius or diameter for a circle's circumference?
This calculator expects the radius — if you only know the diameter, divide it by 2 first, or double your result if you accidentally entered the diameter.
Why is a square's perimeter just 4 × side?
Because all four sides of a square are equal in length, adding them together is the same as multiplying one side by 4.
Can perimeter and area use the same measurements?
Yes, the same length and width (or radius, or sides) you use for area also apply to perimeter — they simply measure different properties of the shape.
Results are in the same unit as your input measurements.
Last updated: July 25, 2026