Triangle perimeter calculator
Calculate a triangle’s perimeter from three sides and check whether the lengths can form a valid triangle.
Enter three positive side lengths in the same unit. Calculate their sum with automatic triangle-inequality validation.
Result
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How to use this calculator
Enter three positive side lengths in the same unit. Calculate their sum with automatic triangle-inequality validation.
Formula and method
P=a+b+c. The longest side must be strictly shorter than the sum of the other two sides.
Worked example
Sides 3, 4 and 5 give a perimeter of 12 units. Sides 1, 2 and 3 lie on a line and do not form a valid triangle.
Limits and interpretation
No height or angles are needed. Convert mixed units first. The result is a length, not an area.
Frequently asked questions
How should I interpret the result?
Sides 3, 4 and 5 give a perimeter of 12 units. Sides 1, 2 and 3 lie on a line and do not form a valid triangle.
What inputs and assumptions are required?
No height or angles are needed. Convert mixed units first. The result is a length, not an area.