Deck Calculator
A deck is ordered two ways at once: square feet for the framing and the budget, linear feet for the boards that actually go down. Enter the deck's dimensions below and the calculator returns the area, how many rows of decking it takes, the linear footage, the board count, and the joists underneath.
Deck square footage
For a rectangular deck it is the usual formula:
Area = length × width
A 16 by 12 foot deck is 192 square feet. That number prices the job, sizes the footings and tells you whether the deck needs a permit, which in most jurisdictions kicks in above a certain area or height.
For an L-shaped or wrapped deck, split it into rectangles, work out each one, and add them. The irregular shape calculator walks through that decomposition, and the how to calculate square footage guide covers the same method for any awkward outline.
Worth saying plainly: deck square footage is not house square footage. A deck is outdoor space and never counts toward a home's gross living area, which do porches and decks count as square footage explains in full. Keep the two numbers in separate buckets.
The board coverage rule
This is the part most estimates get wrong, and it comes down to the gap between nominal and actual.
A "6 inch" deck board, whether it is 5/4x6 or 2x6, is actually 5.5 inches wide. You then leave a gap between boards so water drains and the timber can move, conventionally 1/4 inch. So each row of decking occupies:
5.5 + 0.25 = 5.75 inches, or 0.479 feet
Which gives the rule worth remembering:
Linear feet of decking ≈ square feet × 2.087
A 192 square foot deck therefore needs about 401 linear feet of board. Order off 5.5 inches instead of 5.75 and you will be about 4 percent over; order off a nominal 6 inches and you will be 9 percent short, which is a whole afternoon's problem.
Counting rows instead
For an actual order, count rows rather than dividing area, because you cannot buy part of a row.
Rows = deck width ÷ 0.479 ft, rounded up
On that 16 by 12 deck with boards running the 16 foot direction, the rows cross the 12 foot width: 12 ÷ 0.479 = 25.04, so 26 rows. Each row is 16 feet, giving 416 linear feet. That is slightly more than the 401 the area formula predicts, and the difference is the rounding on the last row. Use the row count to order.
| Board | Actual width | Row coverage with 1/4 in gap | Linear ft per sq ft |
|---|---|---|---|
| Nominal 6 in (5/4x6, 2x6) | 5.5 in | 5.75 in | 2.09 |
| Nominal 4 in (2x4) | 3.5 in | 3.75 in | 3.20 |
Narrow boards look good and cost more in both material and labour, because you are laying and fastening nearly half as much again.
Turning linear feet into boards
Decking is sold in fixed lengths, usually 8, 10, 12, 16 and 20 feet. Divide your linear footage by the board length, then add waste.
For the 16 by 12 deck at 416 linear feet in 16 foot boards: 416 ÷ 16 = 26 boards exactly, plus 10 percent for cuts, mistakes and the occasional bad board, so order 29.
Buy board lengths that match your deck where you can. A 16 foot deck run in 16 foot boards has no butt joints at all. The same deck in 12 foot boards means a seam in every row, which needs blocking underneath and looks busier.
Joists and framing
Decking is only half the order. The frame underneath is usually 2x8 or 2x10 joists at 16 inches on centre, which is the standard spacing for most decking laid square to the joists.
Number of joists = (deck length ÷ 1.33 ft) + 1
On the 16 by 12 deck, that is 16 ÷ 1.33 = 12, plus one for the end, so 13 joists each spanning 12 feet.
Two things change that. Laying boards diagonally means the span between supports grows by about 40 percent, so the joists move to 12 inches on centre. And many composite boards require 12 inch spacing regardless, because they sag more than timber. Check the decking manufacturer's span table before you frame, not after.
Standard deck sizes
There is no code-mandated deck size, but a few dimensions come up again and again because they suit both furniture and timber lengths.
| Deck size | Square feet | What it suits |
|---|---|---|
| 10 x 10 ft | 100 | A small seating area, two chairs and a side table |
| 12 x 12 ft | 144 | A table for four, the most common small deck |
| 12 x 16 ft | 192 | Table plus a grill, the workhorse size |
| 14 x 20 ft | 280 | Dining and lounging zones separated |
| 16 x 20 ft | 320 | A large entertaining deck |
| 20 x 20 ft | 400 | Multiple zones, often multi-level |
The usual guidance is to keep a deck in proportion to the house rather than maximising it. A deck much larger than the room it opens off tends to feel empty, and a 20 by 20 attached to a small bungalow looks like a stage.
Dimensions that land on even board lengths save money. A 16 foot run uses 16 foot boards with no offcut; a 17 foot run means buying 20 foot boards and throwing away three feet of every one.
Worked example: a 16 by 12 deck
- Area: 16 × 12 = 192 sq ft.
- Rows: boards run the 16 foot direction, so rows cross the 12 foot width. 12 ÷ 0.479 = 25.04, round up to 26 rows.
- Linear feet: 26 × 16 = 416 linear feet.
- Boards: 416 ÷ 16 = 26, plus 10 percent waste = 29 boards of 16 foot 5/4x6.
- Joists: (16 ÷ 1.33) + 1 = 13 joists at 12 feet, 16 inches on centre.
- Cost: at $3.50 a linear foot for pressure-treated 5/4x6, the decking alone is about $1,456 before framing, fasteners and footings. The price per square foot calculator runs that either direction.
Common mistakes
Using the nominal board width. A "6 inch" board is 5.5 inches, and with the gap each row covers 5.75. Dividing by 6 leaves you nearly 9 percent short.
Forgetting the gap. A quarter inch per row does not sound like much until you realise it is a whole extra row on a 12 foot width.
Ordering by area instead of rows. Area divided by coverage gives a linear footage, but you buy whole rows, so round up the row count first.
Skipping the waste factor. Ten percent is the floor. Go higher on a deck with angles, a picture frame border, or diagonal boards.
Framing at 16 inches for composite or diagonal boards. Both usually need 12 inch spacing. Read the manufacturer's span table before the joists go in.
Counting the deck in the home's square footage. It is outdoor space. It adds value as an amenity, not as living area.
Frequently asked questions
How many deck boards do I need?
Divide the deck's width by the row coverage, which is 0.479 feet for a nominal 6 inch board with a quarter inch gap, and round up to get the number of rows. Multiply by the deck length for linear feet, divide by your board length, then add 10 percent. A 16 by 12 deck needs 26 rows, 416 linear feet, and about 29 boards of 16 foot 5/4x6.
How do I calculate deck square footage?
Multiply length by width for a rectangular deck. A 16 by 12 deck is 192 square feet. For an L-shaped deck, split it into rectangles, calculate each and add them. This is the deck's own area, not part of the house's living area.
How many linear feet of decking per square foot?
About 2.09 for a nominal 6 inch board, because each row covers 5.75 inches once you allow a quarter inch gap. Narrow 2x4 decking needs about 3.20 linear feet per square foot, which is why it costs noticeably more to lay.
How far apart should deck joists be?
Sixteen inches on centre is standard for timber decking laid square to the joists. Drop to 12 inches for diagonal boards, because the span between supports grows by roughly 40 percent, and for most composite decking, which sags more than timber. Always check the decking manufacturer's span table.
What is the most common deck size?
Twelve by sixteen, at 192 square feet, is the workhorse: enough for a table and a grill without dominating a typical back garden. Ten by ten and twelve by twelve are the common smaller sizes, and sixteen by twenty is a large entertaining deck.
Does a deck count as square footage?
No. A deck has no walls, no roof and no heat, so it is outdoor space rather than gross living area and never counts toward a home's square footage. It still adds real value, but as an amenity valued on its own line. The porches and decks guide covers the appraisal rule.
How much waste should I add on decking?
Ten percent for a plain rectangular deck. Push it to 15 percent for angled corners, a picture frame border, or diagonal boards, all of which produce offcuts that cannot go back down.
Why is a 6 inch deck board only 5.5 inches?
Because the nominal size refers to the rough sawn dimension before the timber is dried and planed. The finished board is smaller, and every decking calculation has to use the actual 5.5 inches. The same nominal-versus-actual gap shows up in framing timber and in concrete block, where an 8x8x16 unit actually measures 7 5/8 inches.
For flooring inside the house, the flooring calculator handles the same linear-versus-square problem, and the linear foot calculator converts between the two for any board width.
Related tools
Porches and decks guide | Flooring calculator | Linear foot calculator | Price per square foot