Quick answer
The perimeter of a triangle is the sum of its sides: P = a + b + c. Sides 3, 4 and 5 give 12.
Enter the three sides. The calculator returns the perimeter.
Related tools: Triangle Calculator: Area, perimeter, angles and type of a triangle from its three sides. Perimeter Calculator: Perimeter of common shapes. 45-45-90 Triangle Calculator: Sides of a 45°-45°-90° triangle from one leg.
How to use the Triangle Perimeter Calculator
- Enter side a (above 0).
- Enter side b (above 0).
- Enter side c (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Side a | A number | > 0 |
| Side b | A number | > 0 |
| Side c | A number | > 0 |
Formula and method
Formula
P = a + b + c
IEEE-754 double precision; shown to at most 6 decimal places.
Sum of sides; the triangle inequality is enforced.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Side a = 3; Side b = 4; Side c = 5.
| Result | Value |
|---|---|
| Perimeter | 12 |
Example 2
Inputs: Side a = 2.5; Side b = 2.5; Side c = 4.
| Result | Value |
|---|---|
| Perimeter | 9 |
The first line is the headline result (perimeter). IEEE-754 double precision; shown to at most 6 decimal places.
Triangle inequality
The sides must satisfy the triangle inequality: the sum of any two must exceed the third. Sides 1, 2 and 3 are degenerate (1 + 2 = 3) and are rejected.
Uses
Fencing a triangular plot, framing or trim. Together with the area, the perimeter characterises a triangle in the Triangle calculator.
How the result changes with side a
Holding every other input at the first worked example, this table changes only side a. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Side a | Perimeter |
|---|---|
| 1.5 | 10.5 |
| 3 | 12 |
| 6 | 15 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Side c: entering "5" shows "These sides cannot form a triangle." beside that field and no result.
Choosing between the Triangle Perimeter Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Triangle Calculator](/math/triangle-calculator/) | side a, side b and side c | Area |
| [Perimeter Calculator](/math/perimeter/) | shape and a | Perimeter |
| [45-45-90 Triangle Calculator](/math/45-45-90-triangle-calculator/) | leg length | Leg |
| [30-60-90 Triangle Calculator](/math/30-60-90-triangle-calculator/) | short leg length | Short leg |
| [Right Triangle Calculator](/math/right-triangle-calculator/) | leg a and leg b | Hypotenuse c |
Background: geometry measurements
Geometry results carry units that depend on the quantity: a perimeter or length is in linear units such as cm, an area or surface area is in square units (cm²) and a volume is in cubic units (cm³). Use one length unit for every input, because the calculator multiplies the numbers as given.
Flat (2D) shapes have an area and a perimeter; solids (3D) have a surface area and a volume. Two familiar checks: a circle of radius r has area πr² and circumference 2πr, and a sphere has volume (4/3)πr³ and surface area 4πr².
Assumptions and limitations
- Degenerate triangles rejected
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for side c to confirm this.
Frequently asked questions
What is the perimeter of an equilateral triangle with side 5?
An equilateral triangle with side 5 has perimeter 15.
Do sides need the same unit?
Yes. Use the same unit for all three sides so the sum is meaningful.
Can sides 2, 3, 6 form a triangle?
No. 2 + 3 = 5 is less than 6, so these lengths cannot close into a triangle.
What does the Triangle Perimeter Calculator calculate?
Perimeter of a triangle from three sides. It returns Perimeter.
What formula does the Triangle Perimeter Calculator use?
P = a + b + c. Sum of sides; the triangle inequality is enforced.
What do I need to enter in the triangle Perimeter Calculator?
You enter Side a, Side b and Side c.
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Degenerate triangles rejected.
What is an example of a triangle Perimeter result?
With side a 3, side b 4 and side c 5, the perimeter is 12.
Related calculators
- Triangle Calculator — Area, perimeter, angles and type of a triangle from its three sides.
- Perimeter Calculator — Perimeter of common shapes.
- 45-45-90 Triangle Calculator — Sides of a 45°-45°-90° triangle from one leg.
- 30-60-90 Triangle Calculator — Sides of a 30°-60°-90° triangle from the short leg.
- Right Triangle Calculator — Solve a right triangle from its two legs: hypotenuse, angles, area and perimeter.
- Triangle Area Calculator — Area of a triangle from base and height.
Sources
- OpenStax, Prealgebra 2e (Rice University): perimeter, area, volume, circumference and the Pythagorean theorem
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.