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Square footage and perimeter of a rectangular, L-shaped, clipped-corner, octagon or round deck.
With these inputs:
| Result | Value |
|---|---|
| Deck area | 192 sq ft |
| Square metres | 17.84 m² |
| Perimeter (railing, fascia, skirting) | 56 ft |
| Decking to order with 10% waste | 212 sq ft |
| Shape | Rectangle: length × width |
Railing length is the perimeter less the side against the house and any stair openings. The 10% waste is a common allowance for straight-laid boards; diagonal and picture-frame layouts use more. 1 sq ft = 0.09290304 m² (NIST SP 811).
Almost every deck price, material list and permit form starts from the square footage. This works out the area of the common deck shapes — a rectangle, an L-shaped deck with a second section, a rectangle with 45° clipped corners, a regular octagon and a round deck — and the perimeter you need for railing, fascia and skirting.
Multiply the length by the width in feet. A 16 by 12 ft deck is 192 sq ft. For an L-shaped deck, split it into two rectangles and add them: 16 × 12 plus a 4 × 6 extension is 192 + 24 = 216 sq ft.
Measure the width across two opposite flat sides and use area = 0.8284 × width². A 12 ft octagon is 0.8284 × 144 = 119.3 sq ft. Each side is 0.4142 × width, so the 12 ft octagon has sides of just under 5 ft.
Half the square of the leg. A 45° clip that runs 3 ft along each side removes 3 × 3 ÷ 2 = 4.5 sq ft, and replaces 6 ft of edge with a 4.24 ft diagonal, which shortens the railing.
π × diameter² ÷ 4. A 14 ft round deck is 3.1416 × 196 ÷ 4 = 153.9 sq ft, and its edge is π × 14 = 44 ft.
No. Buy about 10% more for straight boards to cover cuts and defects, and more for diagonal or picture-frame patterns. The deck board calculator turns the area into boards and lengths.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.