Triangle Perimeter Calculator
Enter the three side lengths below. The calculator checks whether the values can form a triangle and then computes the perimeter.
What Is the Perimeter of a Triangle?
The perimeter of a triangle is the total distance around it. In plain language, you add all three sides together. If a triangle has side lengths a, b, and c, then:
Perimeter (P) = a + b + c
This works for every type of triangle: scalene, isosceles, and equilateral. As long as you know all three side lengths, finding perimeter is straightforward.
How to Use This Calculator
- Enter side a, side b, and side c.
- Optionally add a unit such as cm, m, in, or ft.
- Click Calculate Perimeter.
- The tool validates your values and shows the perimeter and semiperimeter.
Important Rule: Triangle Inequality
Not every set of three positive numbers can form a triangle. The triangle inequality must be true:
- a + b > c
- a + c > b
- b + c > a
If one of these fails, the side lengths do not make a valid triangle. This calculator checks that automatically.
Worked Examples
Example 1: Scalene Triangle
Suppose sides are 6, 7, and 9 meters. The perimeter is:
P = 6 + 7 + 9 = 22 m
Example 2: Isosceles Triangle
Suppose sides are 5, 5, and 8 centimeters.
P = 5 + 5 + 8 = 18 cm
Example 3: Equilateral Triangle
If every side is 4 inches, then:
P = 4 + 4 + 4 = 12 in
Why Perimeter Matters
Perimeter is used in practical situations like fencing, framing, trimming materials, architecture sketches, and land measurement. Anytime you need boundary length, perimeter is the quantity you want.
Common Mistakes to Avoid
- Mixing units (for example, using cm for one side and m for another without conversion).
- Forgetting to include one side in the sum.
- Using values that fail the triangle inequality.
- Confusing perimeter with area (they are different measurements).
Quick FAQ
Can a triangle have a side length of 0?
No. Triangle sides must all be greater than zero.
What is semiperimeter?
Semiperimeter is half of the perimeter: s = P / 2. It is often used in area formulas like Heron’s formula.
Do I need a special triangle type to use this?
No. The formula P = a + b + c applies to all triangles.