perimeter calculator

What Is Perimeter?

Perimeter is the total distance around the outside of a two-dimensional shape. If you walk around the edge of a rectangle, triangle, or any closed figure, the distance you cover is the perimeter. It is measured in linear units such as centimeters, meters, feet, or inches.

This matters in practical situations like fencing a yard, framing a picture, adding trim around a room, or calculating how much border material you need for a project.

Common Perimeter Formulas

Square

A square has four equal sides.

• Formula: P = 4 × side

Rectangle

A rectangle has opposite sides equal.

• Formula: P = 2 × (length + width)

Triangle

The perimeter is the sum of all three sides.

• Formula: P = a + b + c

Circle (Circumference)

The perimeter of a circle is called the circumference.

• Formula: P = 2 × π × radius

Regular Polygon

A regular polygon has sides of equal length.

• Formula: P = number of sides × side length

How to Use This Perimeter Calculator

• Select your shape from the dropdown menu.
• Enter the required dimensions for that shape.
• Optionally enter a unit like cm or ft.
• Click Calculate Perimeter to see the result and formula.

Quick Examples

Example 1: Rectangle

If length = 12 m and width = 5 m, then P = 2 × (12 + 5) = 34 m.

Example 2: Triangle

If sides are 7 cm, 8 cm, and 9 cm, then P = 7 + 8 + 9 = 24 cm.

Example 3: Circle

If radius = 10 in, then P = 2 × π × 10 ≈ 62.83 in.

Why Units Matter

Always keep units consistent. Don’t mix meters and centimeters unless you convert first. Perimeter is a one-dimensional measure, so your final unit should be linear (like m, cm, ft), not square units.

Common Mistakes to Avoid

• Using area formulas when you need perimeter formulas.
• Forgetting one side of a shape in manual calculations.
• Using diameter instead of radius in the circle formula without adjustment.
• Entering impossible triangle side lengths (this tool checks for that).

Where Perimeter Is Used in Real Life

• Construction and architecture planning
• Landscaping and fencing projects
• Interior design (molding, trim, borders)
• Manufacturing and material estimation

Final Thoughts

A perimeter calculator saves time and reduces errors, especially when switching between different shapes. Use the tool above whenever you need a fast and accurate edge-length total for geometry, schoolwork, or real-world planning.