Triangle Perimeter Calculator
Find the perimeter of a triangle from its sides.
Results update as you type.
How it works
The perimeter of a triangle is the total distance around it, and this calculator finds it by adding the three side lengths. You enter the lengths of all three sides, and it returns the perimeter. The calculation could not be simpler, just the sum of the sides, but this tool adds an important safeguard: it checks the triangle inequality, the rule that any two sides of a triangle must together be longer than the third side. If the lengths you enter violate this, they cannot form a real triangle, so no valid perimeter is returned, alerting you to a mistake or an impossible set of sides. The perimeter applies to any triangle, whether scalene with three different sides, isosceles with two equal sides, or equilateral with all three equal. It is a fundamental geometry calculation used in construction and fencing to measure boundaries, in design and art, in trigonometry problems, and in classrooms. The perimeter is in the same units as the side lengths, so sides in meters give a perimeter in meters. If you know the perimeter and two sides, you can find the third by subtraction. This is distinct from the area, which measures the space inside the triangle. Enter the three side lengths to compute the triangle's perimeter.
Perimeter = a + b + c
The perimeter of a triangle is the sum of the lengths of its three sides. The calculator adds side a, side b, and side c. It also checks the triangle inequality, which requires each side to be shorter than the sum of the other two; if the sides cannot form a real triangle, no valid perimeter is returned.
Source:Euclidean geometry
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter side a, side b and side c.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Perimeter updates instantly as you type — no button to press.
Frequently asked questions
How do you find the perimeter of a triangle?
Add the lengths of all three sides. For a triangle with sides 3, 4, and 5, the perimeter is 12. The calculator sums the three sides you enter and also verifies that they form a valid triangle, returning no perimeter if the sides cannot actually make a triangle.
What is the triangle inequality?
The triangle inequality states that the sum of the lengths of any two sides of a triangle must be greater than the length of the third side. If this fails, the three lengths cannot form a real triangle. The calculator checks this rule, so it flags impossible side combinations rather than giving a meaningless perimeter.
Does this work for all types of triangles?
Yes. The perimeter is the sum of the three sides regardless of the triangle type, so it works for scalene triangles with three different sides, isosceles triangles with two equal sides, and equilateral triangles with all sides equal. Just enter the three side lengths and the calculator adds them.
What is the difference between perimeter and area?
The perimeter is the total length around the triangle, found by adding the sides, and is measured in linear units. The area is the amount of space enclosed inside the triangle, measured in square units, and requires the base and height or other methods to calculate. They measure two different properties of the triangle.
Can I find a missing side from the perimeter?
Yes. If you know the perimeter and two of the sides, subtract those two sides from the perimeter to find the third. For example, a perimeter of 12 with sides 3 and 4 gives a third side of 5. This is a common use when one side length is unknown but the perimeter is given.