Heron's formula calculator
Heron's formula gets the area from the sides alone, which suits triangular plots and site measurements where the height is hard to measure.
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How to use it
- Enter side a, side b and side c.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
s = (a + b + c) / 2, then Area = √(s(s - a)(s - b)(s - c)). The tool uses an equivalent reordered form that limits rounding error in thin triangles.
Example
Inputs: Side a = 13, Side b = 14, Side c = 15. Result: Area: 84; Semi-perimeter s: 21; Perimeter: 42.
Limits and caveats
- Use one length unit; the area is in that unit squared, such as square meters.
- Assumes flat ground. Very large parcels on the Earth need geodetic survey methods.
Questions about this tool
When is Heron's formula useful?
When you know all three sides but not the height, which is common when measuring land with a tape.
How do I find the area of a four-sided plot?
Split it into two triangles along a diagonal, measure the diagonal, work out each triangle with Heron and add them.
Why do I sometimes get an error?
If two sides together are not longer than the third, no triangle exists and there is no area.