A regular decagon has 10 interior angles and 10 exterior angles. All interior angles are equal, and all exterior angles are equal.
Each interior angle of a regular decagon measures 144°. The sum of all 10 interior angles is:
Each exterior angle of a regular decagon measures 36°. The sum of all 10 exterior angles is:
144° + 36° = 180° — an interior angle and its matching exterior angle always add up to 180°.
The sum of the interior angles of any polygon is found with (n − 2) × 180°, where n is the number of sides. For a decagon: (10 − 2) × 180° = 1,440°.
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