Deck square footage calculator
A plain rectangle is width times length and needs no tool. This one is for the deck that wraps a corner, steps out into a bay, loses a corner to a diagonal or stops around a hot tub. Build it from pieces, and it gives you the net area and the perimeter — the second number is the one the trim order runs on.
Pre-filled with an L-shaped deck: a main rectangle, a bump-out, one clipped corner and a cut-out around a tree. Change any figure, or add a section.
Area is plane geometry on your measurements. The perimeter is only as right as the shared edges you entered.
What it measures
Most decks that are not rectangles are rectangles that changed their minds. An L is two rectangles. A deck with an angled outside corner is a rectangle with a triangle taken off. A round end is a rectangle with half a circle added. So the tool works the way you would on the back of a takeoff sheet: break the shape into simple pieces, then add or subtract.
Each piece does one of three things. It adds deck. It is clipped off the outline — a cut corner or a notch around a chimney. Or it is a cut-out in the middle, like a hole framed around a tree or a hot tub set down into the deck. Cut-outs cost you area but leave the outside edge alone.
A stair landing is one more rectangle. If it sits apart from the deck, leave its shared edge blank: its whole edge then counts toward the rim and fascia you have to buy.
How to use it
- 01Sketch the deck from above. Draw the outline and split it into rectangles first, then mark any angled corners, round ends and holes.
- 02Enter the biggest rectangle first. Leave its shared edge blank, because nothing is attached to it yet.
- 03Add each other piece. For a bump-out, the shared edge is the seam where it meets the deck. For a clipped corner, it is the two legs of old outline the cut takes away.
- 04Mark the cut-outs. A hot tub or tree hole comes off the area, and its edge goes on its own line for the framing and trim around it.
- 05Fill in the house edge and stair openings. That splits the outline into ledger, rim and fascia, and the railing run.
Measure the finished deck surface, not the frame. If the boards will overhang the rim, the area you buy decking for is a little larger than the area you frame.
Worked example
The calculator opens on this deck: a 16' × 12' main platform on the house, an 8' × 10' bump-out off its outer edge, one outside corner cut back 3' each way, and a 2' × 2' hole framed around a tree.
| Section | Shape | Role | Area, sq ft | Own edge, lf |
|---|---|---|---|---|
| Main deck | Rectangle 16' × 12' | Adds to the deck | 192.0 | 56.0 |
| Bump-out | Rectangle 8' × 10' | Adds to the deck | 80.0 | 36.0 |
| Clipped corner | Right triangle (clipped corner) 3' × 3' | Clipped off the outline | −4.5 | 10.2 |
| Tree | Rectangle 2' × 2' | Cut-out inside the deck | −4.0 | 8.0 |
Net area is 272.0 − 4.5 − 4.0 = 263.5 sq ft. The outline starts at the main deck's 56 lf, gains the bump-out's three free sides, and trades the clipped corner's two legs for its diagonal: 74.2 lf in all. Take off the 16' against the house and 58.2 lf of rim and fascia is left to cover, of which 54.2 lf needs a guard if the deck is high enough to need one.
The formulas
Nothing here is estimated. These are the plane-geometry formulas the calculator runs, so you can check any line by hand.
| Shape | Area | Perimeter |
|---|---|---|
| Rectangle | width × length | 2 × (width + length) |
| Right triangle | ½ × leg × leg | leg + leg + √(leg² + leg²) |
| Half-circle | π × d² ÷ 8 | π × d ÷ 2 + d |
| Circle | π × d² ÷ 4 | π × d |
| Regular octagon | 0.8284 × W² | 8 × W ÷ 2.4142 |
The perimeter is the trim order
Area buys decking. Perimeter buys almost everything else. The exposed edge is your rim joist and the fascia that covers it, and the stretch of picture-frame border if you run one. The edge against the house is the ledger, and it gets the flashing detail in ledger board and flashing. A cut-out's edge is extra framing — headers and trimmers around the hole — and extra fascia or border boards to finish it.
The railing run only matters once the deck is high enough to need a guard. The model code requires one where the walking surface is more than 30" above the grade below (IRC 2021 R312.1). Heights, infill and the rest are in deck railing height code.
Code requirement
Your jurisdiction may amend the model code or adopt a different edition. Confirm with your building department before you cut.
What to do with the number
Boards. Take the net area and the longest run to the deck board calculator — it counts boards and screws, which this tool deliberately does not. An angled corner or a curve wastes more stock than a straight run, so allow for the offcuts there.
Cost. The deck cost calculator prices the area and the railing run with your own quotes. It asks for a width and a length, so for an odd shape enter any pair that multiplies to your net area.
Permit. Area, height and whether the deck touches the house decide whether you need one. Read deck permit requirements and then ask your building department, with this sketch in hand.
Frame. Each rectangle has its own joist run, and the joist span calculator sizes them one at a time.
Sources
- 2021 IRC § R312.1, Guards (UpCodes) read 2026-10-18
Filed under Deck framing & structure — 19 tools, guides and comparisons in this pillar.
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