Enter the three side lengths of a triangle to instantly calculate area, perimeter, angles, and triangle type.
Why use an online triangle calculator?
A triangle calculator helps you solve geometry problems faster and with fewer mistakes. Instead of manually applying every formula, you can input side lengths and get instant outputs such as area, perimeter, interior angles, and triangle classification. Whether you are a student, teacher, engineer, designer, or DIY builder, this tool can save time and improve confidence.
What this calculator computes
This online triangle calculator uses the three-side method (SSS). After you enter sides a, b, and c,
it checks if those values form a valid triangle. If valid, it returns:
- Perimeter
- Semi-perimeter
- Area (Heron’s formula)
- All three interior angles (in degrees)
- Triangle type by sides (equilateral, isosceles, or scalene)
- Triangle type by angles (acute, right, or obtuse)
- Inradius and circumradius
- Heights from each side
How to use this online triangle calculator
Step 1: Enter three side lengths
Type positive numbers into fields for side a, side b, and side c. Decimals are supported.
Step 2: Click “Calculate Triangle”
The tool validates your numbers using the triangle inequality theorem. In short, the sum of any two sides must be greater than the third.
Step 3: Read your results
You will see all key geometric outputs in one place. If the triangle is invalid, the tool explains why.
Core formulas used
Perimeter
P = a + b + c
Semi-perimeter
s = (a + b + c) / 2
Area (Heron’s formula)
Area = sqrt(s(s - a)(s - b)(s - c))
Angles (Law of Cosines)
A = arccos((b² + c² - a²) / (2bc))
B = arccos((a² + c² - b²) / (2ac))
C = arccos((a² + b² - c²) / (2ab))
Practical use cases
- Checking dimensions in carpentry and framing
- Classroom geometry assignments and exam prep
- CAD modeling and technical sketching
- Land measurement and basic surveying
- Any situation where quick triangle math is needed
Frequently asked questions
Can I use decimal side lengths?
Yes. The calculator accepts decimal values and displays rounded outputs for readability.
What if my values do not form a triangle?
You will get an error message. Adjust your inputs so that each pair of sides sums to more than the remaining side.
Is this only for right triangles?
No. This tool works for any valid triangle: acute, right, or obtuse.
Final thoughts
A reliable triangle solver should be quick, transparent, and mathematically correct. This online triangle calculator is built to do exactly that: validate your input, apply standard geometry formulas, and present clear results instantly.