Triangle Perimeter Calculator
Calculate the perimeter of any triangle from its three side lengths instantly with this free geometry calculator in your browser.
Enter Values
Shape
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the three sides.
- Press Calculate.
How it works
Perimeter = the sum of the three sides.
Worked example
3, 4, 5 m. = 12 m.
Common mistakes
- Adding only two sides — a triangle has three.
- Mixing units between sides; convert them to match before adding.
Frequently asked questions
How do I find a triangle's perimeter?
Add the lengths of all three sides. For a right triangle, you can find a missing side with the Pythagorean theorem first.
Can I find the perimeter from area?
Not directly — area doesn't fix the side lengths. You need the three sides, or enough angles and one side, to get the perimeter.
Can any three lengths make a triangle?
Only if each side is shorter than the sum of the other two — the triangle inequality. Otherwise the sides can't close up.
Related tools
- Triangle Area Calculator
- Trapezoid Perimeter Calculator
- Rectangle Perimeter Calculator
- Equilateral Triangle Area Calculator
- Pythagorean Theorem Calculator
- Regular Hexagon Area Calculator
Explore more in Geometry, Shape, Area & Volume.
Cite this tool
Zerdly. (2026). Triangle Perimeter Calculator. Retrieved from https://www.zerdly.com/tools/triangle-perimeter-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.