Polygon Calculator
Octagon decks, gazebos, fire-pit surrounds, bay windows. Enter the number of sides and any one size — side length, across-the-flats, or center-to-corner — and get the corner miter angle, all the other dimensions, the perimeter for fascia and railing, and the area for decking.
Planning tool — results are estimates for quoting and ordering, not engineering or building-code advice.
Common questions
What's the miter angle for an octagon?
22.5° on each piece — two cuts meet to form the 45° corner (the interior angle is 135°). A hexagon cuts at 30°. The rule is 180 ÷ number of sides.
What does across-the-flats mean?
The width measured between two parallel sides — the way you'd naturally tape an octagon deck. It only exists on even-sided polygons; for odd counts use the side length or center-to-corner.
How much railing does an octagon deck need?
The perimeter — number of sides × side length. This calculator gives it in feet-and-inches from whichever measurement you have.
Related: Circle & arc calculator · Rafter calculator
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This solves one measurement. The Takeoff Builder prices the whole job.
Walk the rooms, enter dimensions, and formula-driven scope turns measurements into quantities, prices, and the estimate itself — from $20/mo, capped, no surprise bills.