Triangle Perimeter Calculator
Add up the three sides of a triangle — with a check that they can actually form one, and the area, angles and heights alongside.
The perimeter is simply a + b + c, the distance all the way round.
How the triangle perimeter calculator works
The perimeter is simply a + b + c, the distance all the way round. The only thing worth checking is that the three lengths can close: any two of them must add up to more than the third, otherwise there is no triangle to have a perimeter. The other properties are calculated from the same three sides.
Formula: a² = b² + c² − 2bc·cos A; a / sin A = b / sin B = c / sin C; area = ½ab·sin C
Worked examples
| Inputs | Perimeter | Note |
|---|---|---|
| 3, 4, 5 | 12 | perimeter 12 |
| A fence around a triangular plot | 72 | perimeter 72 metres of fencing |
| An equilateral triangle of side 9 | 27 | perimeter 27 |
FAQFrequently asked questions
How do I find the perimeter of a triangle?
Add the three side lengths. If you only know some of them, solve the triangle first — the calculator does both.
What if I only know two sides and an angle?
Use the SAS or SSA page to find the missing side, and the perimeter is shown there too.
Do the units matter?
Only that they match. Enter all three sides in the same unit and the perimeter comes out in that unit.
Why does it say my sides are not a triangle?
Because two of them do not add up to more than the third, so they cannot meet to close the shape.
Is the perimeter the same as the semi-perimeter?
No — the semi-perimeter used in Heron's formula is exactly half the perimeter.
Where these figures come from
- NIST Digital Library of Mathematical Functions — reference definitions for elementary and special functions
- Wolfram MathWorld — definitions and formulas for every topic on this page
- NIST/SEMATECH e-Handbook of Statistical Methods — the statistical formulas (mean, variance, z, confidence intervals, sample size)
- Australian Curriculum (ACARA) — Mathematics — the terms and methods taught in Australian schools
Last checked: September 2026. Formulas are fixed by mathematics and do not change with tax years or regulations.