Also known as "inscribed circle", it is the largest circle that will fit inside the triangle.
Each of the triangle's three sides is a tangent to the circle.

Try this Drag the orange dots on each vertex
to reshape the triangle. Note how the incircle adjusts to always be the largest circle that will fit inside the triangle.

The center of the incircle, called the incenter, is the intersection of the angle bisectors. The bisectors are shown as dashed lines in the figure above.

Incenter | The location of the center of the incircle. The point where the angle bisectors meet. |

Inradius | The radius of the incircle. The radius is given by the formula:
where: a is the area of the triangle. In the example above, we know all three sides, so Heron's formula is used. p is the perimeter of the triangle, the sum of its sides. |

For the special case of an equilateral triangle the inradius is also given by the formula where S is the side length.

- Triangle definition
- Hypotenuse
- Triangle interior angles
- Triangle exterior angles
- Triangle exterior angle theorem
- Pythagorean Theorem
- Proof of the Pythagorean Theorem
- Pythagorean triples
- Triangle circumcircle
- Triangle incircle
- Triangle medians
- Triangle altitudes
- Midsegment of a triangle
- Triangle inequality
- Side / angle relationship

- Perimeter of a triangle
- Area of a triangle
- Heron's formula
- Area of an equilateral triangle
- Area by the "side angle side" method
- Area of a triangle with fixed perimeter

- Right triangle
- Isosceles triangle
- Scalene triangle
- Equilateral triangle
- Equiangular triangle
- Obtuse triangle
- Acute triangle
- 3-4-5 triangle
- 30-60-90 triangle
- 45-45-90 triangle

- Incenter of a triangle
- Circumcenter of a triangle
- Centroid of a triangle
- Orthocenter of a triangle
- Euler line

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