# Perimeter Calculator

Calculate the perimeter of rectangles, triangles, circles, and regular polygons. Also shows area. Enter dimensions and get instant results.

## What this calculates

Calculate the perimeter (or circumference) of common shapes. Select a shape, enter the dimensions, and get the perimeter and area instantly.

## Inputs

- **Shape** — options: Rectangle, Triangle, Circle, Regular Polygon — Select the shape to calculate the perimeter for.
- **Length / Side a** — min 0 — Length for rectangle, side a for triangle, or side length for polygon.
- **Width / Side b** — min 0 — Width for rectangle, or side b for triangle.
- **Side c** — min 0 — Side c for triangle.
- **Radius** — min 0 — Radius of the circle.
- **Number of Sides** — min 3, max 100 — Number of sides for regular polygon (3-100).

## Outputs

- **Perimeter** — The perimeter (or circumference for a circle).
- **Area** — The area of the shape.
- **Shape Info** — formatted as text — A description of the shape and formula used.

## Details

The perimeter is the total distance around the outside of a two-dimensional shape. For simple shapes, the perimeter formulas are well-known: rectangles use P = 2(l + w), triangles use P = a + b + c, circles use C = 2πr (circumference), and regular polygons use P = n × s where n is the number of sides and s is the side length.

Perimeter calculations are essential in everyday applications like fencing a yard, framing a picture, measuring running tracks, and cutting fabric or ribbon. For construction, knowing the perimeter helps determine the amount of baseboard, fencing, or edging material needed.

This calculator also computes the area as a bonus output. For triangles with all three sides known, it uses Heron's formula: A = √(s(s-a)(s-b)(s-c)) where s is the semi-perimeter (a+b+c)/2. Understanding the relationship between perimeter and area is important. Shapes with the same perimeter can have very different areas, and among all shapes with a given perimeter, the circle encloses the maximum area.

## Frequently Asked Questions

**Q: What is perimeter?**

A: Perimeter is the total length of the boundary around a 2D shape. For a rectangle it is 2(length + width), for a triangle it is the sum of all three sides, and for a circle it is called circumference: 2πr.

**Q: What is the difference between perimeter and area?**

A: Perimeter is the distance around a shape (measured in linear units like meters). Area is the space enclosed by the shape (measured in square units like square meters). Two shapes can have the same perimeter but very different areas.

**Q: Is circumference the same as perimeter?**

A: Yes, circumference is simply the perimeter of a circle. It is the total distance around the circle, calculated as C = 2πr or C = πd, where r is the radius and d is the diameter.

**Q: How do I calculate the perimeter of an irregular shape?**

A: For an irregular shape, measure and add up the lengths of all individual sides. If the shape has curved sections, you may need to approximate them with straight segments or use calculus (arc length formula) for exact values.

---

Source: https://vastcalc.com/calculators/math/perimeter
Category: Math
Last updated: 2026-04-21