Triangle Calculator
Free triangle calculator: enter three sides to instantly find the area, perimeter and all angles, and classify right,...
What Triangle Calculator does
A triangle calculator that accepts three side lengths and instantly returns the area, perimeter, all three angles, and the triangle's classification (right, isosceles, or equilateral). It validates the input using the triangle inequality before computing results, so users cannot enter impossible side combinations. The page includes a built-in example using the 3-4-5 right triangle, showing how Heron's formula and the law of cosines produce the area, perimeter, and angles without requiring manual trigonometry. It serves students checking homework, builders measuring rafters, and anyone needing quick geometric metrics from three measurements.
How to use the SmartCalc Triangle Calculator
- 1
Open the Triangle Calculator on SmartCalc.top
- 2
Enter the lengths of side a, side b, and side c in the labeled fields
- 3
Click calculate to validate the triangle and display area, perimeter, and all three angles
- 4
Review the classified triangle type and derived metrics shown below the inputs
Best for
Students, DIYers, and builders who need fast, accurate triangle measurements from three side lengths without manual formula work.
Limitations
- Requires three valid side lengths that satisfy the triangle inequality
- Only computes properties based on side inputs; no angle-first or coordinate-based calculations
- Results are estimates rounded to one decimal place, not exact irrational values
Triangle Calculator FAQ
- What if I enter side lengths that cannot form a triangle?
- The calculator will reject the input and display an error, requiring the longest side to be shorter than the sum of the other two.
- How are the angles calculated from the three sides?
- The tool uses the law of cosines to find each angle, then lists all three angles and classifies the triangle as right, isosceles, or equilateral based on the side lengths.
- Can I use this to check if a corner is square?
- Yes — entering a 3-4-5 triangle instantly confirms a right angle, since 9 + 16 = 25 satisfies the Pythagorean theorem.
- Does the calculator show the formula steps?
- It displays the formula logic for the example triangle and provides the Heron's formula calculation, but does not walk through every step for custom inputs.
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