Triangle Perimeter Calculator
Enter the three sides — get the perimeter and semi-perimeter, with a validity check.
How It Works
The perimeter is simply the sum of the three sides. The calculator also checks the triangle inequality — each side must be shorter than the other two combined — and reports the semi-perimeter, which is what Heron's area formula uses.
Frequently Asked Questions
What is the semi-perimeter for?
Half the perimeter (s) is used in Heron’s formula for area and in incircle radius calculations.
Why do I get an error for 1, 2, 10?
Because 1 + 2 < 10 â no triangle can close with those lengths.
Isosceles or equilateral shortcuts?
Equilateral: P = 3a. Isosceles: P = 2a + b. The general form works for all of them.