Triangle calculator

Heron’s formula finds area from three sides. With semiperimeter s, area = √(s(s − a)(s − b)(s − c)).

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First side

Second side

More inputs and assumptions

Third side

Answer

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Heron’s formula finds area from three sides. With semiperimeter s, area = √(s(s − a)(s − b)(s − c)).

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Precision and calculation record

Decimal places in numeric results and text downloads

Rounding changes displayed numbers, not the calculation. Text, fractions, matrices and CSV schedules keep their own formatting. Automatic keeps up to 12 significant digits, with money shown to cents where applicable.

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Formula and assumptions

Sides must be positive and form a nondegenerate triangle. A stable reordered form reduces cancellation for thin triangles, but floating-point precision is still finite.

Automatic precision shows numbers to 12 significant digits. Thousands separators such as 1,000 are accepted; use a dot for decimals.

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Updated 2026-09-26

Using Triangle calculator

Heron’s formula finds area from three sides. With semiperimeter s, area = √(s(s − a)(s − b)(s − c)).

Sides must be positive and form a nondegenerate triangle. A stable reordered form reduces cancellation for thin triangles, but floating-point precision is still finite.

Next: Rectangle calculator

A worked example

With first side = 3, second side = 4, third side = 5, the area is 6.

Next: Triangle area calculator

Before you use the result

Sides must be positive and form a nondegenerate triangle. A stable reordered form reduces cancellation for thin triangles, but floating-point precision is still finite.

Example results for first side

These examples use Second side: 4; Third side: 5. They are reference calculations, not recommended settings.

First side	Area
1.5	2.48039185412
3	6
6	9.92156741649

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