📐 Perimeter of a Triangle Calculator
Calculate the perimeter of a triangle from side lengths
How to Use This Calculator
Enter Three Side Lengths
Input the lengths of all three sides of the triangle (a, b, c). Make sure all values are positive numbers.
Calculate
Click "Calculate Perimeter" to compute the perimeter and other triangle properties.
Review Results
See the calculated perimeter, semiperimeter, area, side lengths, and angles.
Formula
Perimeter = a + b + c
Semiperimeter: s = (a + b + c) / 2
Basic Formula:
P = a + b + c
Simply add all three side lengths together.
Semiperimeter:
s = P / 2 = (a + b + c) / 2
The semiperimeter is half the perimeter and is used in many triangle formulas, including Heron's formula for area.
Special Cases:
- Equilateral: P = 3a (all sides equal)
- Isosceles: P = 2a + b (two sides equal)
- Right Triangle: P = a + b + √(a² + b²)
Triangle Inequality:
For a valid triangle, the sum of any two sides must be greater than the third:
- a + b > c
- a + c > b
- b + c > a
About Perimeter of a Triangle Calculator
The Perimeter of a Triangle Calculator finds the total length around a triangle by adding all three side lengths. The perimeter is the distance you would travel if you walked along all three sides of the triangle.
When to Use This Calculator
- Geometry: Calculate perimeter in triangle problems
- Construction: Determine fencing or material needed for triangular areas
- Education: Learn how to calculate triangle perimeter
- Land Surveying: Calculate perimeter of triangular plots
- Engineering: Design calculations requiring perimeter measurements
- Art & Design: Calculate frame lengths for triangular designs
Why Use Our Calculator?
- ✅ Simple Calculation: Instantly find perimeter from three sides
- ✅ Validates Input: Checks triangle inequality theorem
- ✅ Complete Information: Shows semiperimeter, area, and angles
- ✅ Accurate: Precise mathematical calculations
- ✅ Educational: Helps understand triangle properties
- ✅ Free: No registration required
Key Concepts
- Perimeter: Total distance around the triangle (sum of all sides)
- Semiperimeter: Half the perimeter, used in many triangle formulas
- Triangle Inequality: Must be satisfied for valid triangle
- Units: Perimeter has the same units as the side lengths
Examples
- Example 1: Sides 5, 6, 7 → Perimeter = 5 + 6 + 7 = 18
- Example 2: Equilateral with side 10 → Perimeter = 3 × 10 = 30
- Example 3: Isosceles with equal sides 8 and base 6 → Perimeter = 2 × 8 + 6 = 22
- Example 4: Right triangle with legs 3 and 4 → Perimeter = 3 + 4 + 5 = 12
Frequently Asked Questions
What is the perimeter of a triangle?
The perimeter is the total distance around the triangle, calculated by adding all three side lengths: P = a + b + c.
How do you find the perimeter if you only know two sides?
You need all three sides to calculate the perimeter directly. If you only know two sides, you'd need additional information (like angles or the third side) to find the perimeter.
What is the semiperimeter?
The semiperimeter (s) is half the perimeter: s = (a + b + c) / 2. It's used in Heron's formula for area and other triangle formulas.
Can any three numbers make a triangle?
No! They must satisfy the triangle inequality: the sum of any two sides must be greater than the third side. For example, 1, 2, 4 cannot form a triangle because 1 + 2 = 3 < 4.
What's the perimeter of an equilateral triangle?
For an equilateral triangle with side length a, the perimeter is 3a. Simply multiply the side length by 3.
Do I need the angles to find the perimeter?
No! The perimeter only depends on the side lengths. You don't need to know the angles to calculate it. However, angles are useful if you need to find a missing side first.
What units are used for perimeter?
Perimeter has the same units as the side lengths. If sides are in meters, perimeter is in meters. If sides are in inches, perimeter is in inches.