Regular octagon calculator
Tell the tool what you know — the side length, the long diagonal, the circumradius, the inradius (apothem) or the area — and it derives every other property of a regular octagon: area, perimeter, long diagonal, flat-to-flat width, circumradius and inradius.
The area of a regular octagon is 2(1 + √2) × side² ≈ 4.8284 × side² and its flat-to-flat width is (1 + √2) × side. All calculations stay in your browser.