The area of a regular decagon is the space inside its ten equal sides. A regular decagon has more sides than most polygons studied in school, and it looks almost like a circle when drawn.

The Formula

A = (10 × t²) / (4 × tan(π/10))

• A — area of the decagon (cm², m², etc.)
• 10 — number of sides
• t — length of one side
• tan(π/10) — tangent of 18°, which equals tan(18°) ≈ 0.32492

This formula comes from splitting the decagon into ten equal triangles from the center and adding up their areas.

Step-by-Step Example