Deck square footage calculator
Deck area looks like the simplest number on the drawing and it is the one most often wrong on a material list. Two reasons: an L-shaped deck is not one rectangle, and the perimeter is not the railing run — the side against the house gets no guard.
Deck dimensions
How this is calculated
- Rectangle: area = length × width. Perimeter = 2 × (length + width).
- L-shape: area = main + wing. Perimeter = 2 × (main length + main width) + 2 × (wing length + wing width) − 2 × shared edge. The shared edge is counted twice, so it is subtracted twice.
- Railing run = perimeter − the length attached to the house. On an L, the house side is the main length plus any segment of the wing that also touches the wall.
- Boards = ⌈(width in inches) ÷ (5.5 + 0.125)⌉ rows, using actual face width, not nominal.
Worked example
A 16 × 12 ft deck against the house, with an 8 × 6 ft wing:
- Main area = 16 × 12 = 192 sq ft; wing = 8 × 6 = 48 sq ft; total 240 sq ft
- Perimeter = 2(16 + 12) + 2(8 + 6) − 2(6) = 56 + 28 − 12 = 72 ft
- House side = 16 + 8 = 24 ft, so railing run = 72 − 24 = 48 ft
- Boards across the 12 ft width = ⌈144 ÷ 5.625⌉ = 26 rows, not 24 — the 5.5″ face is what covers ground
The 26 versus 24 gap is the whole reason material lists come up short. A 2×6 is 5.5″ wide, so a 12 ft span holds 26 boards, and 26 × 7.75″ of nominal width is 16.8 ft of lumber that does not exist on the plan.
Which number you actually need
Three different quantities get called "deck size" and they are used for different things:
- Decking area drives board count, stain quantity and the cost estimate.
- Footprint area (including the framing overhang) drives permit fees, which are often charged per square foot of footprint.
- Railing run drives post count, baluster count and the railing line on the estimate — it is typically 60–70% of the perimeter, not all of it.
For a deck with stairs, subtract the stair opening from the decking area but add the stair footprint back into the permit area if your jurisdiction measures the whole structure. That is a local rule, not arithmetic, so it is left out here.
What square footage hides
- It does not handle curved or angled decks. A 45° corner or a radius needs the shape broken into triangles and segments first; a two-rectangle model will understate the area.
- It does not deduct the rim or fascia. Area is measured to the outside face of the frame, which is where decking usually stops.
- It does not size anything. Joist spacing, beam size and footing depth are span-table and code questions, not area questions.
- It ignores board direction. Turning the boards 90° changes the row count and the waste; use the board calculator for that.
Questions people actually ask
How do I calculate square footage of an L-shaped deck?
Split it into two rectangles, multiply each, and add. For the perimeter, count every outside edge once: 2 × (main length + main width) + 2 × (wing length + wing width), then subtract twice the shared edge, because a shared edge has no outside face.
Is deck square footage measured to the frame or the boards?
Measure to the outside face of the frame. Decking normally stops flush with the rim, so the difference is a few square feet. If you are buying stain, add the fascia board area separately — it is often 10–15% of the total coated surface.
How many square feet is a 12x16 deck?
192 sq ft. A 12 × 16 deck is the most common size in the US because it holds a table, four chairs and a grill without triggering a larger beam. It is also close to the size at which many jurisdictions start requiring engineered plans.
Does a deck under 200 sq ft need a permit?
Sometimes. Some jurisdictions exempt decks below 200 sq ft that are under 30″ high and not attached to the house. The exemption is local and narrow — check before assuming, because an unpermitted deck can block a sale.