The deck joist count is bays plus one, and it happens three times
Divide the deck length by the on-center spacing and you get the number of bays, the gaps between joists. That is one short of the joist count. Each bay is closed by a joist at both ends, and neighboring bays share the joist between them, so one is left over at the end of the frame. Fifteen bays means sixteen joists.
Software people have a name for this, the off-by-one error, and they borrowed it from exactly this kind of carpentry. On a deck it bites three times. The joists along the length are the obvious one. The beam lines across the width are the second: a 12 ft width at an 8 ft maximum joist span is two bays, which means three beam lines. The posts along each beam are the third. Get all three wrong and a 20 × 12 deck comes up one joist, one beam and several posts short. On paper that looks like nothing. On a Saturday morning it stops the job.
The last bay is almost never a full one, either. Spacing is a target, and the length won't always divide evenly by it: 240 in at 16 in leaves a final bay of whatever remains, and that remainder is a real measurement you mark on the rim board. The tool reports it instead of rounding it out of sight. A framer who doesn't know the last bay is short finds out with the decking already ripped to width.
The gap between deck boards is a dimension
Deck boards aren't laid tight. The gap between them, typically about 1/8 to 1/4 in (3 to 6 mm), does two jobs. It drains water off the surface so it doesn't sit against the end grain, and it gives the board somewhere to go when it moves.
That gap is part of the width each board takes up. A 5.5 in board with a 3/16 in gap takes up 5.6875 in of deck, so a 12 ft width needs 26 rows. Divide by the bare board width and you get 27. Ignoring the gap overestimates the board count on every deck you price, and that is how a calculator quietly sends you home with lumber you can't return.
Most tools also miss a second detail: N boards have N−1 gaps. The last board meets the edge and has no gap after it. On a wide deck that is a fraction of a board. On a narrow landing it decides whether you finish on a full board or on a two-inch rip. The board count here is the smallest N where N × width + (N−1) × gap covers the deck.
On composite, the gap is a requirement. Wood shrinks as it dries and gains clearance over time. Composite does the opposite: it expands in heat, along its length as well as its width. A composite deck laid tight buckles in its first summer, and the manufacturer will point you to the installation instructions before it points you to the warranty. That is why the composite preset here starts with a wider gap and with joists at 12 in on center instead of 16 in.
Cut waste belongs on its own line
Stock board lengths rarely match the deck dimension. A 16 ft board on a 20 ft deck leaves a 4 ft butt joint that has to land on a joist, and the offcut from the run is often too short to use anywhere. Add miters, stair treads, picture-frame borders and the two or three boards that arrive cupped, and 10% is the normal allowance. Plan closer to 15% on a deck with angles or a diagonal pattern.
Where the waste shows up matters more than the number. This tool applies waste as a percentage on top of the correct quantity and shows it as its own row, in the shopping list and again in the cost breakdown. Waste folded into a total is why two calculators disagree by 12% and nobody can say why. Shown on its own line, you can argue with it. Set it to zero if you are buying exact lengths, push it to 15% for a diagonal deck pattern, and see right away what that decision costs.
Labor is half the deck cost, and it is the half that varies
On a professionally built deck, labor is typically 40 to 60% of the total. That alone would justify splitting it out. The variance makes it essential. Material prices are close to identical between two yards in the same town, and day rates aren't. Two contractors can be 40% apart on the same drawing with neither one being unreasonable.
You can't compare a single bottom-line number to a bid. You can compare a split. Take the material subtotal here, hold it against the material line in the quote, and you learn something specific. If the bid's material is well under the quantities on this page, someone is planning to build with less wood: wider joist spacing, fewer posts, a thinner board, or a paving slab where a poured pier should be. Have that conversation before you sign. After the frame is up, it is too late.
The tool takes labor two ways because contractors quote it two ways: as a rate per square foot of deck, or as a percentage of material. The rate-per-area mode adds railing labor per linear foot and stair labor per step on top. Those are the two parts of a deck where the hours run far out of proportion to the surface area.
A raised deck costs more per square foot
Cost per square foot is the number everyone searches for, and it turns misleading the moment the deck leaves the ground. The surface above scales with area, and the structure below doesn't.
| Height | Post spacing | What else changes |
|---|---|---|
| Under 30 in | ~8 ft | No guard required; labor is kneeling work |
| 30 in to 4 ft | ~8 ft | Guard becomes mandatory, a per-linear-foot cost the low deck never had |
| 4 ft to 8 ft | ~6 ft | More posts and footings; bracing; labor rises with staging |
| Over 8 ft | ~5 ft | Posts, footings and bracing again; two people to lift each piece |
Taller posts are more slender, and slender columns buckle before they crush, so the grid tightens as the deck rises. Every extra post is also an extra footing to dig and pour. The tool tightens the post grid automatically from the height you enter and multiplies the labor rate by a height factor, so its per-square-foot figure moves the way real bids move instead of staying flat.
Watch the 30 in threshold closely, because the cost jumps there all at once. Below it, no guard. Above it, a guard around every open side, and railing is the most expensive linear foot on a deck, often more per foot than the decking is per square foot. Two decks on identical plans, one 28 in high and one 32 in high, will be well more than a few percent apart.
Composite vs. wood decking is a ten-year question
Pressure-treated pine is the cheapest way to get a deck. Cedar and redwood cost more and look better. Composite and PVC cost two to three times the material of pressure-treated per square foot and want tighter joist spacing on top of that. Most side-by-side comparisons stop there and declare wood the winner.
That comparison stops one line too early. Pressure-treated and cedar decks want cleaning and refinishing roughly every two years. That is five treatments over a decade, each one product plus a weekend or a contractor's day across the whole deck area. Composite wants a wash. Enter your treatment cost in the tool and it works out the ten-year total for both. The crossover typically lands between year six and year nine, earlier on a large deck than a small one, because maintenance scales with area and keeps coming back, while the price gap, though also per area, is paid once.
None of that makes composite automatically right. It gets hot underfoot in direct sun, you can't sand it back when it scratches, and a hardwood deck that has been oiled really does look better at year fifteen. Those are arguments about looks and comfort. Make them openly, on their own terms, and keep them out of a price comparison that stopped at the checkout.
Two things this deck calculator can't do for you
The first is the ledger board. On an attached deck, the ledger is the board bolted to the house, and it carries half the deck's load. It is also, by a wide margin, the most common structural failure on a home deck. The wood rarely breaks. The ledger was nailed instead of bolted, or fastened through a rim joist with no blocking, or put up without flashing so the wall behind it rotted quietly for eight years. This tool counts your material. It doesn't design that connection, so get it inspected.
The second is spans and sizes. The calculator counts how many joists you need at the spacing you enter. It doesn't tell you whether a 2×8 is deep enough for that span in that species and grade, or how big the footings need to be for your soil and frost depth. Those come from a span table or an engineer. Get the sizes first, then come back and count.
Both of these are what the permit is for. In most jurisdictions an attached deck, or one above roughly 30 in, needs one, and the inspection exists to catch the ledger and the footings. Check with your building department before ordering, because a rejected detail changes the framing plan and every quantity on this page with it.
Privacy
Every figure stays in your browser. The arithmetic runs on your device, nothing is uploaded or logged, and there is no account. That matters more here than on most calculators, because what you type is your supplier pricing and, if you are the one bidding, your labor rate.
Frequently asked questions
How many joists does a 20 ft deck need at 16 in on center?
Sixteen. 20 ft is 240 in, and 240 ÷ 16 is 15, but 15 is the number of bays, the spaces between joists. Every bay is closed by a joist at each end and neighboring bays share the joist between them, so one joist is left over at the end of the frame: 15 bays, 16 joists. Divide and stop there and you drop exactly one joist, always the one at the outer edge, which is where the decking has nothing under it and flexes when you walk on it.
Does the gap between deck boards change the number of boards?
Yes, and if you ignore it you order too many. A 5.5 in board with a 3/16 in gap takes up 5.6875 in of deck width, so a 12 ft deck takes 26 rows, one fewer than the 27 you get from 144 ÷ 5.5. The fine point is that N boards have N−1 gaps. The last board meets the edge and has no gap after it. On a wide deck the difference is a board or two. On a narrow one it decides whether you end on a clean edge or a ripped strip.
What percentage of deck cost is labor?
Usually 40 to 60% on a professionally built deck, and higher on a raised one. That is why this calculator shows material and labor as two separate subtotals, with the labor share as a percentage. You can't compare a single bottom-line figure against a contractor's bid, and you can compare a split. If a bid's material column comes in far under the quantities here, the contractor is planning to build with less wood than the deck needs: wider joist spacing, fewer posts, or a thinner board.
Is composite decking cheaper than pressure-treated wood?
On build day, no. By year ten, usually yes. Composite runs roughly two to three times the material cost per square foot and wants joists at 12 in on center instead of 16 in, which adds framing on top of the board price. Against that, it needs no sanding, no stain and no oil. A pressure-treated or cedar deck wants refinishing about every two years, which over a decade is five treatments in product and labor across the whole deck area. Put your treatment cost in the tool and see which column wins at year ten. The crossover usually falls somewhere between years six and nine.
Why does a raised deck cost more per square foot than a ground-level one?
Because the structure underneath doesn't scale with the surface above. Taller posts are more slender and buckle sooner, so the post grid tightens: 8 ft spacing at ground level, closer to 6 ft above about 4 ft of height and 5 ft above 8 ft. That multiplies posts and footings. Labor per square foot rises too, because height means ladders, staging and two people to lift each piece. And above 30 in a guard is mandatory, a cost charged per linear foot that a ground-level deck never carries.
Do I need a permit to build a deck?
In most US jurisdictions, yes, once the deck is attached to the house or raised above a threshold height, commonly 30 in. Freestanding low platforms under a certain area are often exempt. The permit earns its fee: the inspection catches the two failures that actually hurt people, the ledger connection to the house and the footing depth. Check with your building department before you order material, because a rejected ledger detail can change the framing plan and with it the quantities on this page.