Deck Calculator
Buy 29 deck boards and 13 joists.
| Material | Quantity |
|---|---|
| Deck area | 192 ft² |
| Board rows (5.5 in face + 0.125 in gap) | 26 |
| Decking, exact | 416 linear ft |
| Decking, +10% waste | 458 linear ft |
| 16 ft boards to buy | 29 |
| Joists at 16 in o.c. | 13 (156 linear ft) |
| Deck screws 2 per board per joist | 676 |
Board rows are counted on the covered width — the board face plus one gap — because that is what actually consumes deck width. Joist count includes the closing joist at the far end, which is the one an off-by-one drops. This estimates the deck surface and its joists only: ledger board, beams, posts, footings, joist hangers, railings, and stairs are separate and depend on span and local code. Check a span table before settling on joist size and spacing — 16 in on centre is standard, but 5/4 decking laid diagonally needs 12 in.
Enter your deck size and board width to find how many deck boards, joists, and screws you need — with the board gap and a waste factor built in.
How to use this calculator
- Enter the deck length (parallel to the house) and width (out from the house). Decking runs the length; joists run the width.
- Pick your decking board. The list shows face widths, not nominal names — a 5/4×6 measures 5.5 inches across.
- Set the board gap. Use 1/8 inch for dry or composite decking; go near zero for wet pressure-treated lumber, which will shrink and open its own gap.
- Choose joist spacing and the board length you can actually buy — matching stock length to deck length avoids butt joints entirely.
- Add a price per board for a decking cost.
The formula
Covered width = Board face + Gap · Rows = Deck width ÷ Covered width · Linear feet = Rows × Deck length
Boards = (Linear feet × Waste) ÷ Stock length · Joists = (Deck length ÷ Spacing) + 1 · Screws = Rows × Joists × 2
Worked example — a 16 × 12 ft deck
5/4 × 6 pressure-treated decking, 1/8 in gap, joists at 16 in on centre, buying 16 ft boards, 10% waste.
- Deck area: 16 × 12 = 192 ft²
- Covered width: 5.5 + 0.125 = 5.625 in
- Board rows: 144 in ÷ 5.625 = 25.6 → 26 rows
- Linear feet: 26 × 16 = 416 ft
- With 10% waste: 458 ft
- Boards to buy: 458 ÷ 16 = 28.6 → 29 boards
- Joists: (16 ft ÷ 16 in) + 1 = 13, each 12 ft long
- Screws: 26 × 13 × 2 = 676
Buying 16 ft boards for a 16 ft deck means every board runs the full length with no butt joints — worth paying a little more for. A deck 17 ft long is a different job entirely, because every row needs a joint and those joints must land on a joist.
Why the gap changes the board count
A 5.5 inch board does not cover 5.5 inches of deck. With a 1/8 inch gap it covers 5.625, and over a 12 foot width that difference is most of a row. Ignoring the gap gives you 25 rows where you need 26 — which is a return trip to the yard, in a colour and batch that may not match.
The gap itself is not cosmetic. Decking needs drainage and air movement underneath or it stays wet and rots from below. What the gap should be depends entirely on the moisture content of the lumber when you lay it:
| Decking | Gap when laying | Why |
|---|---|---|
| Wet pressure-treated | Butt tight to 1/16 in | Shrinks as it dries and opens its own gap |
| Kiln-dried treated (KDAT) | 1/8 in | Already dry; will not shrink much |
| Cedar or redwood | 1/8 in | Usually sold at a stable moisture content |
| Composite | 3/16 in end to end, per maker | Expands with heat rather than shrinking |
Joist spacing and why it is not just about strength
16 inches on centre is the residential default and works with 5/4 and 2-inch decking laid perpendicular. The two reasons to go tighter are both worth knowing before you frame:
- Diagonal decking. Laying boards at 45 degrees increases the span between bearing points by about 40%. Joists have to come to 12 inches on centre to compensate.
- Composite decking. Most brands require 16 inches for perpendicular and 12 inches for diagonal, and some require 12 inches throughout for stair treads. Check the manufacturer's instructions — exceeding their spacing voids the warranty.
Going the other way, 24 inches on centre is only acceptable with true 2-inch decking, and even then the deck feels springy underfoot. It is a false economy: you save a handful of joists and get a deck nobody likes walking on.
What this calculator does not cover
This estimates the deck surface and its joists. A complete deck also needs:
- Ledger board and flashing where the deck attaches to the house. Improper ledger attachment and missing flashing cause more deck collapses than undersized lumber does — this is the detail to get right or hire out.
- Beams, posts, and footings, sized from your joist span and local frost depth.
- Joist hangers and structural fasteners. Use hardware rated for contact with treated lumber; ordinary galvanised hardware corrodes fast against modern treatment chemicals.
- Railing, required by code on any deck more than 30 inches above grade.
- Stairs, which have their own rise and run rules.
Common mistakes to avoid
- Forgetting the gap in the row count. Costs you a row, and the return trip may not match.
- Gapping wet treated lumber. It shrinks. Butt it tight and let it open naturally.
- Buying board lengths that do not match the deck. A 12 ft deck built from 16 ft boards wastes a quarter of every board. Match the stock to the run where you can.
- Dropping the closing joist. A 16 ft deck at 16 in on centre needs 13 joists — the one at the far end is easy to miss when you divide.
- Using the wrong fasteners. Modern treated lumber is corrosive to standard galvanised screws and hangers. Use hot-dip galvanised or stainless rated for it.
How we calculate this
Board rows come from the deck width divided by the covered width of one board — its face plus one gap — because that is what actually consumes deck width. Rows × deck length gives linear feet, the waste factor is applied to that, and the result is divided by your stock board length and rounded up. Joists are counted at your spacing along the deck length, including the closing joist at the far end, so a 16 ft deck at 16 in on centre gets 13 joists rather than 12. Screws assume two per board at every joist crossing. All arithmetic runs in metric internally and converts once for display.
Sources
Frequently asked questions
How many deck boards do I need for a 16x12 deck?
A 16 by 12 foot deck is 192 square feet. Using 5.5-inch boards with a 1/8-inch gap, each row covers 5.625 inches, so 12 feet of width needs 26 rows. At 16 feet per row that is 416 linear feet, or 458 feet with a 10% waste allowance — 29 boards of 16 feet. You will also need 13 joists at 16 inches on centre and about 676 deck screws.
How do I calculate deck boards?
Divide the deck width by the covered width of one board, which is the board face plus one gap. A 5.5-inch board with a 1/8-inch gap covers 5.625 inches, not 5.5 — forgetting the gap is the most common mistake and it under-counts rows on any deck of size. Multiply the resulting row count by the deck length to get linear feet, add waste, then divide by the stock length you are buying.
How much gap should I leave between deck boards?
It depends on how wet the lumber is. Kiln-dried or composite decking gets a 1/8 to 3/16 inch gap because it will not shrink much. Wet pressure-treated lumber should be butted nearly tight — it shrinks as it dries and will open its own gap, often to 1/4 inch. Gapping wet lumber at 1/8 inch is how people end up with gaps wide enough to lose a phone through.
What joist spacing should I use for decking?
16 inches on centre is the standard for 5/4 and 2-inch decking laid perpendicular to the joists. Drop to 12 inches if you are laying boards diagonally, since the diagonal increases the effective span, or if your composite decking specifies it — many brands require 12 inches for a 45-degree layout. 24 inches is only acceptable with full 2-inch thick decking, and it feels bouncy.
How many screws do I need for a deck?
Two screws per board at every joist crossing. This calculator works that out from your board rows and joist count — for a 16 by 12 foot deck at 16 inches on centre that is about 676 screws, so buy a 5-pound box. Hidden fastener systems use clips instead and have their own coverage rate, usually quoted in square feet per box.
Should deck boards run parallel or perpendicular to the house?
Perpendicular to the joists, which normally means parallel to the house. The joists run out from the ledger board, so decking crosses them. If you want boards running the other way — out from the house — the framing has to change: you either add blocking or run the joists parallel to the house instead, which changes the beam and post layout entirely. Decide this before ordering.
Does this include the framing, footings, and railing?
No. It covers the deck surface and its joists. Ledger board, beams, posts, footings, joist hangers, railings, and stairs are separate, and they depend on your span, height above grade, and local code. A deck more than 30 inches off the ground needs a guard rail, and any deck attached to a house needs a properly flashed ledger — that detail causes more deck failures than undersized lumber does.
How much does deck lumber cost?
Pressure-treated 5/4 by 6 decking usually runs $1.50 to $3 per linear foot, cedar $3 to $6, and composite $4 to $12. Enter your price per board to see the decking cost here — but note that decking is typically only 40 to 50% of a deck's material cost once framing, hardware, footings, and railing are added, and materials are usually about half of a contracted build.