Math
Triangle Calculator with Sides, Angles, Area, and Perimeter
Solve triangle dimensions from three sides, then calculate area, perimeter, internal angles, and triangle type.
Search intent guide
Use this page when you need a fast answer to triangle calculator
Use this triangle calculator when you need sides, angles, area, perimeter, or triangle type from a valid set of known measurements. It is most helpful when you want one tool to solve geometry questions without jumping between several formulas.
Editorial review
Best for
- Solving triangle dimensions from side and angle data
- Checking area and perimeter together
- Classifying triangle type after solving the shape
Before you start
- Use one consistent unit for all side lengths
- Make sure the given values form a valid triangle setup
- Check whether the problem gives enough information to solve uniquely
Inputs
Provide all three sides to compute area and perimeter.
Results
If the sides do not form a valid triangle, results are hidden.
Area
6
Perimeter
12
Type
Scalene
Angle A
36.87
Angle B
53.13
Angle C
90
Semiperimeter
6
Inradius
1
Circumradius
2.5
Altitudes
| Side | Altitude |
|---|---|
| A | 4 |
| B | 3 |
| C | 2.4 |
Area from base and height
Use this when you know the base length and height.
Area
30
Sponsored
Ad placement reserved.
How this Triangle Calculator works
Computes area, perimeter, angles, and triangle type from three sides using Heron's formula and the law of cosines. Validates the triangle inequality before returning results.
Quick guide
Jump to the section you need, then return to the triangle calculator.
Best use cases
- Solve side-only triangles when you need area, perimeter, and angles from three known lengths.
- Validate whether three measurements can actually form a triangle before using them in design or homework.
- Check geometric scaling effects by comparing similar triangles and their changing areas.
Inputs
- Side A length
- Side B length
- Side C length
- Angle unit (degrees or radians)
Outputs
- Triangle area
- Triangle perimeter
- Triangle type (equilateral, isosceles, scalene)
- All three angles
Assumptions
- Sides must satisfy the triangle inequality
- Units are consistent across all sides
Common mistakes
- Ignoring the triangle inequality and expecting valid output from side lengths that cannot form a triangle.
- Mixing units across sides, such as centimeters for one side and inches for another.
- Assuming the triangle is right-angled without verifying the side relationship or returned angles.
Tips
- If results show invalid, check side lengths
- Keep units consistent (cm, in, or m)
Triangle Calculator formula guide
Use these triangle calculator formulas to audit the output or explain it to someone else.
3 formulas
Triangle Calculator examples
Review a ready-made triangle calculator scenario, copy it, then tweak inputs to match your case.
Example
3-4-5 triangle
Inputs
- Side A 3, Side B 4, Side C 5
- All sides in the same unit
Outputs
- Area equals 6
- Perimeter equals 12
This classic right triangle is useful for checks.
Example
Isosceles example
Inputs
- Side A 5, Side B 5, Side C 6
- All sides in the same unit
Outputs
- Area about 12
- Perimeter equals 16
Area is approximate due to square root rounding.
Triangle validity check
| Sides | Valid | Note |
|---|---|---|
| 3, 4, 5 | Yes | Satisfies inequality |
| 2, 2, 5 | No | 2 + 2 is not greater than 5 |
| 5, 5, 9 | No | 5 + 5 is not greater than 9 |
| 5, 5, 6 | Yes | Valid isosceles triangle |
A + B must be greater than C for all side pairs.
Area scaling (3-4-5 family)
Focus point
3-4-5
6
Position
#1 of 3
Original order
Share of total
7.14%
Total: 84
Scaling sides by 2 multiplies area by 4.
References
- Heron formula for triangle area
Learn more
Guides connected to the triangle calculator
Use these short guides when you want the decision framework behind the numbers, not just the raw output.
Category guide
Math Calculator Guide
Pick the right math calculator by problem type: arithmetic, ratios, coordinates, geometry, or measurement.
Directly related
Articles that mention this calculator
FAQ
Triangle Calculator FAQ
What does the Triangle Calculator do?
Computes area, perimeter, angles, and triangle type from three sides using Heron's formula and the law of cosines. Validates the triangle inequality before returning results. It is part of our math toolkit.
When should I use this calculator?
Use it when you need to Solve side-only triangles when you need area, perimeter, and angles from three known lengths. Validate whether three measurements can actually form a triangle before using them in design or homework.
What inputs do I need?
Typical inputs include Side A length, Side B length, Side C length.
How are the results calculated?
We follow the formulas and assumptions outlined in the "How this calculator works" section. You will see outputs like Triangle area, Triangle perimeter, Triangle type (equilateral, isosceles, scalene).
What mistakes should I avoid?
Common mistakes include Ignoring the triangle inequality and expecting valid output from side lengths that cannot form a triangle. Mixing units across sides, such as centimeters for one side and inches for another.
Can I share or download the results?
Use the Copy link or Print buttons to share your results. If a table or chart appears, you can download the data as CSV.
Is my data stored?
No. Calculations run in your browser and we do not store your inputs.
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