Deck Square Footage Calculator
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.
Square footage by deck shape
| Shape | Formula | Example | Area |
|---|---|---|---|
| Rectangle | length × width | 16 × 12 ft | 192 sq ft |
| L-shaped | main + extension | 16 × 12 + 4 × 6 ft | 216 sq ft |
| Two 45° clipped corners | rectangle − ½ leg² each | 16 × 12, 4 ft legs | 176 sq ft |
| Octagon | 0.8284 × width² | 12 ft across flats | 119.3 sq ft |
| Round | π × diameter² ÷ 4 | 14 ft across | 153.9 sq ft |
Common deck sizes
| Deck | Square feet | Square metres | Perimeter |
|---|---|---|---|
| 10 × 10 | 100 | 9.29 | 40 ft |
| 12 × 12 | 144 | 13.38 | 48 ft |
| 12 × 16 | 192 | 17.84 | 56 ft |
| 16 × 16 | 256 | 23.78 | 64 ft |
| 16 × 20 | 320 | 29.73 | 72 ft |
| 20 × 20 | 400 | 37.16 | 80 ft |
Measuring an existing deck
- Measure the decking surface, outside edge of the rim to the house, not the footprint of the posts below.
- Split odd shapes into rectangles and triangles; a triangle is half its base times its height.
- Count stairs and landings separately. A landing is its own rectangle; stair treads are priced per step.
- Railing follows the perimeter less the side against the house and the stair opening.
With the area in hand, the deck cost calculator prices the build, the deck board calculator counts the boards, and the deck railing calculator turns the perimeter into posts and rail sections. Sizes for specific decks are in the 16 x 16 deck cost and material list and its neighbours.
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Frequently asked questions
- How do I calculate the square footage of a deck?
- 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.
- How do you figure square footage of an octagon deck?
- 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.
- How much does a clipped corner take off?
- 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.
- What is the square footage of a round deck?
- 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.
- Is deck square footage the same as decking to buy?
- 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.
