Math · Geometry

Triangle Calculator

Give any three side lengths and instantly get the area, every angle, heights, radii and the triangle type — drawn live as you type.

Side a opposite A

Length a

Length b

Length c

All lengths must be positive and satisfy the triangle inequality: a + b > c, a + c > b, b + c > a.

Triangle area

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Angle A

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opposite a

Angle B

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opposite b

Angle C

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opposite c

Perimeter

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Circumradius R

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Inradius r

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Height h_a

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Height h_b

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Height h_c

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Enter three sides to see Heron’s formula worked out.

Heron’s formula

Area = √[ s(s−a)(s−b)(s−c) ] , s = (a+b+c)/2

Angles come from the cosine rule: cos A = (b² + c² − a²)/(2bc), and then B and C follow the same pattern. The circumradius is R = abc/(4Δ) and the inradius is r = Δ/s, where Δ is the area.

📏 Live drawing

The triangle is redrawn on every keystroke, scaled to fit and labelled with all three sides.

📎 Triangle type

Detects scalene, isosceles, equilateral plus acute, right and obtuse automatically.

🪁 Radius & heights

Circumradius, inradius and all three altitudes from one set of inputs.

✓ Validation

Rejects impossible inputs with a clear message instead of showing wrong numbers.

Geometry doesn’t stop at triangles

Check the Pythagorean theorem tool for right triangles or the slope tool for lines.

Pythagorean Theorem