About this calculator
The Perimeter Calculator finds the distance around common 2D shapes, including squares, rectangles, triangles, circles, and regular polygons. It is useful for fencing, edging, borders, craft layouts, classroom geometry, and any task where the outside boundary length matters.
perimeter calculator method
The formula depends on the selected shape. Straight-sided shapes add side lengths. Circles use circumference, which is the curved perimeter of the circle.
- Square: P = 4s
- Rectangle: P = 2(l + w)
- Triangle: P = a + b + c
- Circle: C = 2pi r
- Regular polygon: P = n x s
How to use the perimeter calculator
- Choose the shape.
- Enter the relevant side length, radius, or dimensions.
- Check all measurements use the same unit.
- Apply the shape formula.
- Read the perimeter result.
- Round the final answer to the level of precision needed.
Worked examples
Rectangle perimeter
Input: Length 8, width 4
Calculation: P = 2(8 + 4)
Result: 24 units
Circle circumference
Input: Radius 10
Calculation: C = 2pi x 10
Result: About 62.83 units
When perimeter is useful
Perimeter is useful when you are measuring an outside edge: the amount of fencing around a garden, trim around a room, tape around a parcel, or border around a design.
Practical geometry checks
- Use one unit system
- Keep all lengths in the same unit before calculating. Mixing centimetres, metres, inches, or feet in the same formula will produce misleading results.
- Perimeter vs area
- Perimeter is a distance around the outside. Area is the amount of flat space inside a shape. Surface area is the total outside covering of a 3D shape.
- Rounded answers
- Pi, square roots, and trigonometric values are often irrational, so decimal answers are approximations. Keep extra decimal places until the final result.
Edge cases
- Zero or negative dimensions do not describe a real physical shape.
- Regular polygon formulas assume all sides and angles are equal.
- Circle and ellipse formulas use pi, so most decimal answers are rounded.
- A trapezoid/trapezium perimeter needs all four side lengths, not just the parallel sides and height.
Limitations
This calculator is for educational and planning use. For construction, manufacturing, surveying, engineering, or safety-critical work, confirm dimensions, tolerances, and rounding rules with the appropriate professional method or specification.
Frequently asked questions
What units should I enter?
Use any length unit you like, but keep every length in the same unit. If you enter metres, area is in square metres and surface area is also in square metres.
Why are some answers approximate?
Calculations involving pi, square roots, or curved shapes often produce irrational numbers. The calculator rounds the displayed decimal result.
What is the difference between area and surface area?
Area measures a flat 2D region. Surface area measures the total outside area of a 3D object.
Can I use these calculators for homework?
Yes. They are useful for checking answers and understanding formulas, but you should still show the working required by your class or exam.
Do regular polygon formulas work for irregular polygons?
No. Regular polygon formulas assume equal sides and equal angles. Irregular polygons usually need to be split into simpler shapes.
Related calculators
- Rectangle Calculator
- Polygon Calculator
- Circumference Calculator