Triangle Perimeter Calculator
Add three positive side lengths to find a triangle’s perimeter. The tool also checks that the sides can form a non-degenerate triangle.
How to use Triangle Perimeter Calculator
1. Choose Measurement Unit, then enter Side A, Side B, Side C without a unit suffix. 2. Use the selected ordinary, square or cubic unit for each measurement as explained below; changing the selector relabels inputs and does not convert them. 3. Select Calculate, or press Enter, and read triangle perimeter. 4. Select Select Copy result to copy the answer, or Reset to clear the form.
Why use this tool?
Estimate the boundary length of a triangular design when all three sides are measured, including triangles without a right angle.
How the calculation works
Perimeter = side A + side B + side C. The two shorter sides must total more than the longest side; equality would form a straight line. Results use up to six decimal places. Tiny nonzero results below 0.000001 use scientific notation with six significant digits. Browser floating-point arithmetic can introduce small rounding differences. Inputs and calculated magnitudes must not exceed 9,007,199,254,740,991. Negative physical measurements and arithmetic underflow are rejected. Zero dimensions are treated as a degenerate limit where the formula allows them; divisors and sides of an actual triangle must be positive. Formula reference: https://openstax.org/books/prealgebra-2e/pages/c-geometric-formulas
Example
Side A = 5 cm, Side B = 6 cm and Side C = 7 cm give 18 cm. Sides 1, 2 and 4 are rejected because they cannot form a triangle.
Practical tip
Keep all sides in one unit and check the triangle inequality before using the total for a physical boundary.
Frequently asked questions
Why are 1, 2 and 3 rejected?
The two shorter sides total exactly the longest side, producing a degenerate straight line rather than a triangle. A strict greater-than condition is required.
Does the order of the three sides matter?
No. Their sum and triangle validity are unchanged when the three values are rearranged.
Does changing Measurement Unit convert my numbers?
No. The selector states the unit of the numbers you enter and labels the answer. Convert measurements first when changing units. Area inputs use square units and volume inputs use cubic units; result precision follows the rounding method above.