Triangle inequality
The sum of any two sides must be greater than the third side. The calculator rejects side lengths that cannot form a triangle.
Example
For side lengths 5, 6 and 7, the perimeter is 18. The semiperimeter is 9, which can be used with Heron's formula to estimate area.
How to use the result
Use perimeter for edging, fencing, trim or total boundary length. Use triangle area when you need surface coverage.
Common mistakes
Side lengths 2, 3 and 8 cannot form a triangle because 2 + 3 is not greater than 8. Also keep all three side measurements in the same unit.
Last reviewed: 2026-05-17