Online triangle inradius calculator. Enter the area and semiperimeter to find the inradius r=area/semiperimeter, with the formula—ideal for triangle geometry.
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r = area S / semiperimeter p
Triangle area
Half the perimeter
📖 Tutorial | Incenter
1. Definition
Angle bisectors meet at the incenter, equidistant from the sides — the incircle center.
2. Formula & Notation
S
Area
p
Semiperimeter
r
Inradius, S/p
3. Properties & Laws
Equidistant from the sides;
r = area / semiperimeter;
Always inside;
S = p·r.
4. Steps
Enter area S and semiperimeter p;
Click "Find r";
Or click "Load Example".
5. Worked Examples
Example: area 24, semiperimeter 12. Solution: r = 24/12 = 2.
6. Common Pitfalls
p is the semiperimeter; Incenter is inside; Not the circumcenter.