Area formulas
- Rectangle: length × width
- Triangle: base × height ÷ 2 — useful for angled corners.
- Circle: π × (diameter ÷ 2)²
- Half circle: half of the circle — for rounded deck ends.
Example: a deck with an angled corner and a tree
A 16 × 12 ft main deck (192 sq ft) with an 8 × 6 ft triangular corner extension (24 sq ft), built around a tree with a 3 ft round cut-out (about 7.1 sq ft), comes to 208.9 sq ft.
Measure to the outside edge of the decking, not the framing, if you're estimating boards or stain.
Frequently asked questions
How do I calculate the square footage of a deck?
For a rectangle, multiply length by width: a 16 × 12 ft deck is 192 sq ft. For other shapes, split the deck into rectangles, triangles and circles, work out each one and add them up. Subtract anything that isn’t decked, like a hot tub cut-out.
Should I include stairs in deck square footage?
Usually not in the deck area itself, but you need the stair surface for stain and tread boards. The deck stair calculator counts tread boards, and the stain calculator adds stair surfaces.
How do I convert square feet to square metres?
Divide square feet by 10.764. The calculator shows both — switch units at the top.
How many deck boards per square foot?
With 5-1/2" boards and a 3/16" gap, each linear foot of board covers about 0.47 sq ft, so you need roughly 2.1 linear feet per square foot before waste. For an exact count with board lengths and a cut list, use the deck board calculator.