Triangle Perimeter Calculator

Calculate a valid triangle perimeter from three side lengths. The tool checks triangle inequality and explains P = a + b + c with an example.

triangle-perimeter-calculator

Perimeter

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Formula: P = a + b + c

Enter three positive side lengths in centimeters. The sum of any two sides must be greater than the third side.

The Triangle Perimeter Calculator adds three side lengths after checking that they can form a real triangle. Enter positive measurements in centimeters, and the tool returns the total distance around the triangle.

This check matters because not every set of three positive numbers is a possible triangle. For example, sides of 1 cm, 2 cm, and 10 cm cannot meet to form a closed shape, so the calculator reports an error instead of presenting a misleading perimeter.

How to Use the Triangle Perimeter Calculator

Enter Side a, Side b, and Side c as positive numbers.
Use centimeters for all three measurements.
Select Calculate.
Read the perimeter, or correct a highlighted side if the measurements do not form a triangle.

Triangle Perimeter Formula

P = a + b + c

Here, P is the perimeter and a, b, and c are the three side lengths. For a triangle with sides 3 cm, 4 cm, and 5 cm:

P = 3 + 4 + 5 = 12 cm

Triangle Inequality Check

A valid triangle must satisfy all three conditions:

a + b > c
a + c > b
b + c > a

The inequalities are strict. If two sides add up to exactly the third side, the shape is degenerate and has no enclosed area, so this calculator rejects it.

When This Calculator Is Useful

Use the perimeter when estimating fencing, edging, trim, or any material that follows the boundary of a triangular space. Students can also use the displayed formula and validation message to check geometry exercises.

If you need the enclosed space rather than the boundary length, use the Triangle Area Calculator. If you know all three sides and need area, use Heron's Formula Calculator.

Frequently asked questions

How do I calculate the perimeter of a triangle?

Add the three side lengths: P = a + b + c. For sides of 3 cm, 4 cm, and 5 cm, the perimeter is 12 cm.

How does the calculator check whether the sides form a triangle?

It applies the triangle inequality: the sum of any two sides must be greater than the third side. Equality is not enough because it creates a degenerate shape.

What if I only know two sides?

Two side lengths are not enough to determine one perimeter. You need the third side or additional information that uniquely determines it.

Can I use different measurement units?

The fields are labelled in centimeters. Convert every side to centimeters before calculating so all three values use the same unit.

Does this work for scalene, isosceles, and equilateral triangles?

Yes, provided all three side lengths are positive and satisfy the triangle inequality.

Is this calculator free to use?

Yes. It is free to use online and does not require an account.

Is triangle perimeter the same as triangle area?

No. Perimeter is the distance around the triangle, measured in linear units. Area is the space inside it, measured in square units.