Buy 29 deck boards, 13 joist pieces and 3 posts for a 16′ by 12′ deck.
26 before 10% waste · 26 rows × 1 per row
17.84 m²
13 lines × 1 bay, buy 12 ft. Each spans 12′; the IRC table allows 13′ 7″ for 2×10 at this spacing.
16′ per line · 3 posts (3 per line).
Two screws at every joist crossing.
$5.85 per ft² of deck. Materials $1,124. Edit the prices. They are not a quote.
Type the deck's length and width and get the board count, joist lines, beam lines, posts and a cost you can edit. This free deck calculator works for pressure-treated or composite decking. Run it before you order: boards are cheap to over-buy and a nuisance to re-order, because the second lot rarely matches the first.
Quick answer A 12 × 16 ft deck (192 ft²) in 5½ in boards, 16 ft stock, a ⅛ in gap and 10% waste takes 29 deck boards (26 before waste). Underneath: 13 joists at 16 in on centre, plus 3 posts on 1 beam line if it hangs off a ledger.
Deck area is length times width, but boards are bought by the piece, so the calculator counts rows instead. It divides the width the boards cover by the board width plus the gap, rounds up, and multiplies by how many pieces one row needs. Then it adds your waste and rounds up to a whole board. Ten percent covers ordinary cuts. A picture frame border or diagonal boards want 15%.
Joists run across the boards, so the calculator needs to know which way the boards run. Joist lines are the spaces along the run, rounded up, plus one. Then it checks the joist against the span table in IRC R507.6 for the lumber and spacing you picked. If one piece cannot reach, it adds a beam line under the middle and splits the joists into equal bays. This is the check that catches a 2×8 on a 12 ft span before it turns into a bouncy deck. Posts go along each beam line at the spacing you set.
Between them, the board, joist and post counts cover the whole deck: a materials list you can paste into an order. It is a deck material calculator, not a beam design, so footings, beam sizes and the ledger connection still come from your plans and your building department.
A deck needs a way down and a railing. The stair calculator splits the deck height into equal risers, and the baluster spacing calculator spaces the railing along the edge. Thinking about a patio instead? The paver calculator counts pavers, base and sand for the same space.
N = boards to buy, W = distance the rows fill, L = length of one row, b = board width, g = gap, l = board length, w = waste %.
rows = ceil( (across + gap) / (board width + gap) )
pieces per row = ceil( along / board length )
boards to buy = ceil( rows × pieces per row × (1 + waste/100) )
joist lines = ceil( along / joist spacing ) + 1
bays = ceil( joist span / IRC R507.6 span for size, lumber, spacing )
beam lines = bays attached to a ledger
= bays + 1 freestanding
posts = beam lines × ( ceil( beam length / post spacing ) + 1 )Across 12 ft with a 5½ in board and a ⅛ in gap is 26 rows. One 16 ft board covers the length, so the net count is 26. Ten percent waste buys 29. Joists at 16 in along 16 ft make 13 lines. A 2×10 in Douglas fir, hem-fir or SPF may span 13′ 7″ at 16 in (IRC R507.6), which covers the 12 ft width in 1 bay. On an attached deck that is one beam line with 3 posts at 8 ft. Swap to 2×8 and the limit drops to 11′ 1″, so the calculator adds a beam in the middle.
Boards run the 14 ft width in 12 ft stock with a 3/16 in gap and 15% waste for a picture frame cut: 43 rows × 2 pieces = 86 boards net, 99 to buy. Joists span the 20 ft length at 12 in centres. A 2×10 may span 15′ 8″ there, so the planner splits them into 2 bays of 10′, with 3 beam lines because the deck is freestanding.
144 mm boards with a 5 mm gap in 4.8 m lengths running along the 4.8 m side: 25 rows, 25 boards net, 28 with 10% waste. Joists at 400 mm centres make 13 lines. A 2×10 may span 4.14 m at that spacing, so one bay covers the 3.6 m width. UK joists are sized to a different table, so use your decking maker's span chart for the real timber.
Find your deck size and you have a board count before you open the calculator. Every row uses 5½ in boards, a ⅛ in gap and 10% waste, with the boards running the long side in the shortest stock that covers it, joists at 16 in on centre and a deck bolted to a ledger. Your deck will differ a little, so run the real numbers above before you order.
| Deck size | Board length | Boards to buy | Joist lines | Posts |
|---|---|---|---|---|
| 8 × 8 ft | 8 ft | 20 | 7 | 2 |
| 10 × 10 ft | 10 ft | 25 | 9 | 3 |
| 10 × 12 ft | 12 ft | 25 | 10 | 3 |
| 12 × 12 ft | 12 ft | 29 | 10 | 3 |
| 12 × 16 ft | 16 ft | 29 | 13 | 3 |
| 16 × 16 ft | 16 ft | 39 | 13 | 6 |
This is the span limit the calculator holds your joists to, taken from IRC Table R507.6 (40 psf live load, 10 psf dead load, #2 grade, no snow). If one joist cannot cover the deck, the calculator adds a beam line under the middle and splits the joists. The first table compares the three lumber groups the code prints at 16 in on centre. The second shows how closer or wider spacing changes the limit for Douglas fir, hem-fir and SPF. The code prints 12, 16 and 24 in only, so 19.2 in joists use the 24 in column and a spacing you type in between takes the wider column too. That errs on the short side. Heavy snow, a hot tub or a different edition need their own table.
| Joist size | Southern pine | Douglas fir, hem-fir, SPF | Redwood, western cedar |
|---|---|---|---|
| 2×6 | 9′ | 8′ 4″ | 8′ |
| 2×8 | 11′ 10″ | 11′ 1″ | 10′ 7″ |
| 2×10 | 14′ | 13′ 7″ | 13′ |
| 2×12 | 16′ 6″ | 15′ 9″ | 15′ 1″ |
| Joist size | 12 in on centre | 16 in on centre | 24 in on centre |
|---|---|---|---|
| 2×6 | 9′ 6″ | 8′ 4″ | 6′ 10″ |
| 2×8 | 12′ 6″ | 11′ 1″ | 9′ 1″ |
| 2×10 | 15′ 8″ | 13′ 7″ | 11′ 1″ |
| 2×12 | 18′ | 15′ 9″ | 12′ 10″ |
Here is what the deck cost calculator says materials come to at its sample prices: $22 a board, $26 a joist piece, $28 a post and $4 a foot of beam. That is the lumber-yard part of what a deck costs. Think of it as a deck estimator for a first budget, not a quote, because permits, footings, railings and stairs all come on top. Want the installed number? Type an installer's rate per square foot into the Labor field and the calculator adds it, so deck installation cost and labor cost per square foot come from real quotes, not a guessed average.
| Deck size | Materials | Per square foot |
|---|---|---|
| 8 × 8 ft | $710 | $11.09 |
| 10 × 10 ft | $908 | $9.08 |
| 10 × 12 ft | $942 | $7.85 |
| 12 × 12 ft | $1,030 | $7.15 |
| 12 × 16 ft | $1,124 | $5.85 |
| 16 × 16 ft | $1,830 | $7.15 |
Deck joists, beams, posts and ledger attachment live in IRC Section R507. The joist limits here are Table R507.6, which assumes 40 psf live load, 10 psf dead load, #2 grade lumber and no snow load, so a heavy-snow area, a hot tub or a different edition needs its own table. The American Wood Council explains the same rules in its DCA 6 deck construction guide, and the IRC text (R507) is in IRC 2021 Chapter 5, Floors.
The span is measured across the whole deck, with no cantilever. A beam set back from the edge so the joists overhang it (up to a quarter of the joist span in the code) can let a smaller joist reach in one span. This calculator does not model that, so it errs toward adding a beam line. It also does not size beams, footings or the ledger connection.
In the UK and much of Europe, joist centres are usually 400 mm or 600 mm and the decking maker, not the IRC, sets the span. Use the 400 mm and 600 mm chips, then check the manufacturer's table.
Section numbers refer to IRC 2021 section numbers; England Approved Document K. Local amendments are common, so confirm with your building authority.
Hover a question to read the answer. Click to keep it open.Tap a question to open it.
29, with 5½ inch boards, 16 foot stock, a ⅛ inch gap, the boards running the 16 foot way and 10% waste. The count before waste is 26. Load the worked example to see the joists and posts too.
16 inches is the usual for 5/4 wood and most composite. Use 12 inches for thin composite or diagonal boards. 24 inches only works where the decking maker allows it. Wider spacing also shortens the span a joist is allowed, so it can add beams. A 2×8 in Douglas fir spans 11 ft 1 in at 16 inches but only 9 ft 1 in at 24. The deck joist spacing chart on this page has every size.
Yes. Composite changes the count only if the real cover width or the gap changes. A 5½ inch composite board and a 5½ inch pine board with the same gap give the same count. Hidden fasteners often want a ⅛ to 3/16 inch gap, so enter the gap on the maker's box instead of guessing.
The calculator checks your joist against IRC Table R507.6 and, when one piece cannot span, splits the deck into equal bays. An attached deck counts the house ledger as one support, so beam lines equal the number of bays. A freestanding deck needs one more. Posts are spaced along each beam at the distance you set, 8 feet by default. It is a quantity estimate, not a beam design.
10% covers ordinary cuts on a rectangle. Picture-frame borders, breaker boards and diagonal layouts usually want 15%. The buy count is the net count times one plus waste, rounded up.
Across the joists, always. Running them the long way uses fewer butt joints when your stock is as long as the deck. The toggle swaps which side the boards fill and which side the joists span.
Lumber first. At the calculator's sample prices the 12 × 16 ft deck needs about $1,124 of boards, joists, posts and beam, roughly $5.85 per square foot. The calculator multiplies each price you type by its count. Labor, permits, footings, railings and stairs are extra, and prices swing a lot by region, so use today's tag prices and written quotes, not a rule of thumb.
Type an installer's quoted rate per square foot into the Labor field. The calculator multiplies it by the deck area, adds the materials and shows the installed total and the cost per square foot. The labor cost to build a deck per square foot depends on your region, the deck height and the decking, so use a written quote and not an average.