What is a regular pentagon?

A regular pentagon has five sides of equal length and five equal interior angles of 108 degrees each. Its perimeter is P = 5 × s, where s is the side length.

What is an irregular pentagon?

An irregular pentagon has five sides that are not all equal (and/or five angles that are not all equal). Its perimeter is the sum of all five side lengths: P = a + b + c + d + e.

Does the unit matter?

No — enter any consistent unit (cm, m, inches, feet, etc.). The perimeter result will be in the same unit as your input.

Can side lengths be decimal values?

Yes. The calculator accepts any positive decimal number, so measurements like 3.75 or 12.5 work perfectly.

What is the interior angle sum of a pentagon?

The sum of interior angles in any pentagon is (5 - 2) × 180 = 540 degrees. Each angle in a regular pentagon is 540 / 5 = 108 degrees.

How does this differ from area?

Perimeter is the total length around the boundary of the pentagon. Area measures the surface enclosed inside. This tool calculates perimeter only.

Perimeter of a Pentagon Calculator

Find the perimeter of a regular pentagon from one side length, or an irregular pentagon by summing all five side lengths.

Input

All five sides are equal. Enter one side length.

Side length (s)

Result

Enter values and click Calculate.

Summary

A pentagon is a five-sided polygon. For a regular pentagon — where all sides and angles are equal — the perimeter is simply 5 times the side length. For an irregular pentagon, enter each of the five sides individually and the tool adds them together. Both methods show a full step-by-step breakdown so you can verify every calculation.

How it works

Choose Regular or Irregular mode using the tabs.
Regular: enter the single side length and click Calculate.
Irregular: enter all five side lengths (a through e) and click Calculate.
The result panel shows the perimeter and full step-by-step working.
Click Reset to clear the form and start over.

Use cases

Geometry homework and exam practice involving pentagons.
Architecture and design projects with pentagonal floor plans.
Fencing, framing, or tiling work on pentagonal plots.
Verifying manual perimeter calculations quickly.
Teaching the difference between regular and irregular polygons.
Engineering tasks requiring precise five-sided boundary measurements.

Frequently Asked Questions

Last updated: 2026-05-25 · Reviewed by Nham Vu