Triangle Calculator

Enter three triangle sides and get the area, perimeter, semiperimeter, angles, and triangle type with the formula steps shown.

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Smoke mascot beside a labeled triangle, side-length inputs, angle arcs, and a small area grid. — The tool art matches the SSS workflow: enter three side lengths, then check area, perimeter, angles, and triangle type. View in the smoke-kawaii gallery

3-side check Heron formula steps Angle estimates Tab-only history

Side aSide bSide cUnit label

scalene acute triangle84 cm^2

Perimeter: 42 cm
Semiperimeter: 21 cm
Angles: 53.1301023542 degrees, 59.4897625939 degrees, 67.380135052 degrees

Steps

Check the triangle inequality: each pair of sides must add to more than the third side.
Find semiperimeter: (13 + 14 + 15) / 2 = 21.
Use Heron's formula: area = sqrt(s(s-a)(s-b)(s-c)).
The area is 84 cm^2. Angles are found with the law of cosines.

How to use the Triangle Calculator

Enter the three side lengths from the same triangle, such as 13, 14, and 15.
Use one unit for all three sides and add an optional label such as cm, m, or in.
Press Calculate triangle to check the triangle inequality, area, perimeter, angles, and type.
If you only know base and height, use the Area Calculator instead of this three-side mode.

What people use it for

Find the area of a triangle when you know all three side lengths.

Check whether three side lengths can close into a real triangle.

Estimate triangle angles with the law of cosines.

Tell whether the triangle is scalene, isosceles, equilateral, acute, right, or obtuse.

Quick examples

Classic Heron example

13, 14, 15 cm

Area = 84 cm^2, perimeter = 42 cm

Right triangle check

3, 4, 5 m

Area = 6 m^2, scalene right triangle

Isosceles triangle

8, 8, 10 in

Area is about 31.22 in^2

Need the guide or a nearby tool?

Need a slower walkthrough, a related calculator, or the full library? These links keep you close to the task you started.

How to use the Triangle Calculator See slower examples, input meanings, formula notes, result limits, and mistakes to avoid.
Calculators Browse related calculators and utilities in the same category.
Calculator and utility guides Find more guides that use real tool inputs and example numbers.
All free tools Search every live tool, converter, calculator, AI helper, and browser utility.

Frequently asked questions

Quick answers about formulas, units, valid measurements, examples, copying, and private in-browser history.

What can I use the Triangle Calculator for?

Use it when you know all three side lengths and want area, perimeter, semiperimeter, angles, and triangle type in one check.

What formula does the Triangle Calculator use?

It uses Heron's formula for area: s = (a + b + c) / 2, then area = sqrt(s(s-a)(s-b)(s-c)). Angles are estimated with the law of cosines.

What do the main Triangle Calculator inputs mean?

The main inputs are the numbers, operation, mode, or known values the calculator needs. Keep units consistent, enter percentages the way the page label shows, and use the examples as a quick check before trusting the answer.

How should I read the Triangle Calculator answer?

Read the headline answer, then check the supporting lines and examples to understand how the calculator got there. If one input changes, rerun the tool and compare the new answer instead of guessing.

What should I double-check before trusting the Triangle Calculator?

Check units, signs, rounding, and the selected mode before copying the answer. If the number feels weird, rerun one of the examples first, then put your own values back in slowly.

Can any three side lengths make a triangle?

No. The sides must pass the triangle inequality. Each pair of sides must add to more than the third side, or the shape cannot close.

Do I need the triangle height?

No. This page is for the three-side case. If you know base and height instead, use the Area Calculator triangle mode.

Does this replace a right triangle calculator?

Use this page for any triangle from three sides. Use the Right Triangle Calculator or Pythagorean Theorem Calculator when the problem is only about a 90-degree triangle.

Why are the angles rounded?

The angles come from the law of cosines and are rounded for reading. Tiny decimal differences are normal, but the three angles should add to about 180 degrees.

Does the side order matter?

The area, perimeter, and triangle type stay the same if you swap the side order. The angle labels follow side a, side b, and side c, so keep the order clear if you are matching a drawing.

Can I use decimal side lengths?

Yes. Use positive decimal side lengths when your measurements are not whole numbers, and keep every side in the same unit.

How should I enter units?

Enter the same length unit for every side. The calculator reports perimeter in that unit and area in square units.

Is my triangle history private?

Yes. Recent triangle answers stay only in the current browser tab while you use the page. They are not sent to a server.