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Triangle Perimeter Calculator & Formula

Add three sides for any triangle perimeter, with notes for scalene, isosceles, and equilateral cases and common homework traps.

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Geometry shapes and measuring tools for perimeter guides

Quick Answer

Triangle perimeter is the sum of all three sides: P = a + b + c.

Formula

  • Equilateral: P = 3s
  • Isosceles: P = 2a + b when two sides match

Introduction

Select Triangle on Perimeter Calculator and type sides a, b, and c. The panel totals them when the three lengths can form a real triangle.

The tool rejects impossible triples that violate the triangle inequality, such as 2, 3, and 10, which cannot close into a shape.

Height and area formulas are separate. Perimeter only needs the three edge lengths that appear on the border of the drawing.

For a full set of numeric walkthroughs across shapes, see perimeter examples and compare triangle rows with rectangle and circle cases.

If you want the measuring-and-checking workflow before you enter numbers, read how to calculate perimeter and return here for triangle-specific labels.

Main Content

Triangle types

Scalene: three different sides; add them directly with no shortcut beyond P = a + b + c.

Isosceles: two equal sides plus a base. You may label the matching sides as legs and the third as base, but perimeter still adds all three numbers.

Equilateral: three equal sides; multiply one side by 3 because s + s + s = 3s.

Right triangles still use perimeter = sum of sides. The Pythagorean theorem helps find a missing side when only two edges and a right angle are given, which is a common follow-up problem.

Formula

  • P = a + b + c
  • Equilateral: P = 3s

Do not use base × height for perimeter. That product belongs to area when height is perpendicular to the base inside the triangle.

Label the diagram before you add so you do not confuse a slanted interior segment with a side on the outline.

Example calculations

Scalene: sides 7, 8, 9 → P = 24.

Equilateral: side 5 → P = 15.

Isosceles: legs 6, base 4 → P = 6 + 6 + 4 = 16.

Right triangle 3-4-5: P = 12, a standard triple that appears in many textbooks.

FAQ

Can I use base and height for perimeter?
Not unless height is actually a side of the triangular outline. Perimeter uses boundary edges only.
What is the triangle inequality?
Each side must be less than the sum of the other two. Otherwise the three lengths cannot close.
Do angles matter for perimeter?
Not when all three sides are known. Angles matter more when you must find a missing side before you add.

Conclusion

Label the diagram first, then add the three sides you see on the border.

Verify with Triangle mode on the home page when you want a quick check.

Calculate triangle perimeter