Inscribed Circle Radius Calculator

Work out inscribed circle radius instantly with clear inputs, formula shown and shareable results.

About Inscribed Circle Radius Calculator

The incircle touches all three sides. Splitting the triangle into three triangles with apex at the incentre gives Area = r·s, so the inradius is simply area divided by the semiperimeter.

Formula

Inradius

r = Area / s, s = (a + b + c) / 2

Worked example

With the default values below, Inscribed Circle Radius Calculator returns Inradius r of 1.0000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Side aminimum 0: 3
Side bminimum 0: 4
Side cminimum 0: 5

Results

Inradius r: 1.0000
Triangle area: 6.0000
Semiperimeter s: 6.0000
Area of the incircle: 3.1416

Step by step

1Values used
2Inradius
3Inradius r= 1.0000
4Triangle area= 6.0000
5Semiperimeter s= 6.0000
6Area of the incircle= 3.1416

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

For the 3-4-5 triangle?

Area = 6 and s = 6, so r = 1 exactly.

Is there a shortcut for right triangles?

Yes: r = (a + b − c)/2 where c is the hypotenuse, which also gives 1 for the 3-4-5 triangle.