The direction boards lie.
Ledger to beam. Joists run this way.
Grade to deck surface.
Actual face width, never nominal.
This choice changes waste more than anything else.
Span capacity differs by species.
Advanced options
1/8 dry, 3/16 green PT, 1/4 composite.
On top of real off-cut loss.
This deck calculator works out boards, joists, beams and fasteners — comparing every stock length on real off-cut waste rather than adding a flat percentage, and checking your joist span against the IRC by timber species.
The short answer
Rows = joist span ÷ (board width + gap). Boards per row = run ÷ stock length, rounded up. A 16 × 12 ft deck in 5/4×6 needs 26 rows, and 26 boards if you buy 16 ft stock.
That rounding is where the money goes. The same deck in 12 ft stock needs 52 boards and wastes 50% of what you buy.
On this page
How to use this deck calculator
Enter the deck, pick a board and a species. Three shapes:
- Rectangle — board run, joist span and height
- L-shape — a main section plus a leg
- Known area — area plus the joist span
Two inputs matter more than they look. Board run is the direction the decking lies; joist span is ledger to beam, at right angles to it. Swapping them changes both the board count and the framing.
Leave board length on “compare all and recommend” and the calculator shows every option side by side with its real waste. The header toggle switches to diagonal, which changes more than the waste factor.
The board length trap
Most deck calculators take your area, divide by board coverage, and add 10% for waste. That misses the thing that actually determines how much timber you buy.
Boards come in fixed lengths, and a row needs a whole number of them. If your run is 16 ft and you buy 12 ft boards, each row needs two — and 8 ft of the second one is scrap. Here is the same 16 × 12 ft deck priced across every stock length:
| Stock | Per row | Boards | You buy | Waste | Butt joints |
|---|---|---|---|---|---|
| 8 ft | 2 | 52 | 416 lf | 0% | 26 |
| 10 ft | 2 | 52 | 520 lf | 25% | 26 |
| 12 ft | 2 | 52 | 624 lf | 50% | 26 |
| 16 ft | 1 | 26 | 416 lf | 0% | 0 |
| 20 ft | 1 | 26 | 520 lf | 25% | 0 |
Waste ranges from nothing to half the order, on identical work. Both 8 ft and 16 ft hit zero because 16 divides evenly by both — but only 16 ft gives you an unbroken board with no joints.
That’s the intuition, and it’s wrong. Change the run to 18 ft and the picture inverts:
| Stock | Per row | You buy | Waste |
|---|---|---|---|
| 8 ft | 3 | 624 lf | 33% |
| 10 ft | 2 | 520 lf | 11% |
| 12 ft | 2 | 624 lf | 33% |
| 16 ft | 2 | 832 lf | 78% |
A run 2 ft past a stock length is the worst case: you buy a second full board per row and throw nearly all of it away. On an 18 ft run, 10 ft boards waste 11% while 16 ft boards waste 78%.
The practical move is to design the deck around a board length where you can. A 16 ft run in 16 ft boards is jointless and wasteless. A 17 ft run is neither.
The formula
1. Rows = Joist span (in) ÷ (Board width + gap)
2. Boards per row = Run ÷ Stock length, rounded up
3. Boards = Rows × Boards per row
Rows run across the joist span; each board runs the length of the deck
Use actual board width — a 5/4×6 is 5½ in, not 6
Worked example — 16 × 12 ft deck, 5/4×6 boards, ⅛ in gap, 16 ft stock
Nominal versus actual
| Called | Actually | Coverage with ⅛ in gap |
|---|---|---|
| 5/4×6 | 1 × 5½ in | 5.625 in per row |
| 2×6 | 1½ × 5½ in | 5.625 in per row |
| 2×4 | 1½ × 3½ in | 3.625 in per row |
| 2×8 | 1½ × 7¼ in | 7.375 in per row |
Using nominal widths gives roughly 8% too few boards — the difference between 26 rows and 24 on the example above. Composite boards vary more still: most are 5½ in but some premium lines aren’t, so check the data sheet rather than assuming.
Joist span depends on species, not just size
Search deck joist spans and you’ll be told a 2×8 spans “up to 10 ft” or “up to 12 ft” depending which page you land on. Both are approximations of something the IRC states precisely — and the missing variable is the timber.
| Joist | Southern pine | Douglas fir–larch | Hem-fir / SPF |
|---|---|---|---|
| 2×6 | 9′ 11″ | 9′ 5″ | 8′ 10″ |
| 2×8 | 13′ 1″ | 12′ 5″ | 11′ 8″ |
| 2×10 | 16′ 2″ | 15′ 9″ | 14′ 9″ |
| 2×12 | 18′ 6″ | 18′ 4″ | 17′ 2″ |
IRC Table R507.6, joists at 16 in on centre, 40 psf live load.
Southern pine 2×8 carries 13′ 1″ — a 12 ft span is comfortable. Hem-fir 2×8 carries 11′ 8″, so the same deck fails and needs 2×10.
That’s a 12% difference in capacity between two things stamped “2×8”, and it’s exactly the gap the rules of thumb fall into. Check the grade stamp on the timber you’re actually buying — hem-fir and SPF are common in the north and west, southern pine in the south and east.
Diagonal decking costs you twice
Laying boards at 45° is a common upgrade, and calculators handle it by bumping the waste factor from 10% to 15%. That covers one of the two costs.
Cost one
+41% decking
A board crossing at 45° travels √2 times further than one going straight across. That’s the part everyone counts.
Cost two
+31% framing
The unsupported span between joists grows by the same √2. At 16 in centres, a diagonal board actually spans 22.6 in.
Why it matters
Warranty
22.6 in exceeds what most decking is rated for, so manufacturers require 12 in centres for diagonal work. It’s a condition, not advice.
So a diagonal deck needs about 41% more decking and a third more joists, hangers and framing labour. It’s a real design decision rather than a 5% waste adjustment, and the calculator tightens the spacing automatically when you switch to diagonal.
Board gaps
| Decking | Gap | Why |
|---|---|---|
| Kiln-dried or dry PT | ⅛ in | Little shrinkage left; this is the finished gap |
| Green pressure-treated | 3/16 in | Wet from the yard. Shrinks to about ⅛ in over the first season |
| Composite | ¼ in | Manufacturer requirement — thermal movement, and more at butt joints |
| Hardwood (ipe) | 3/16 in | Dense and stable, but moves seasonally |
Gap tight on green pressure-treated and you’ll get gaps anyway as it dries — just uneven ones, wherever each board happens to shrink most. The gap also does real work: water and debris need somewhere to go, and a deck that holds both rots from the top.
Beams, posts and footings
- Beam span between posts is typically 6–8 ft for a doubled 2×8, more for a tripled or larger beam. The calculator defaults to 8 ft and you can change it.
- An attached deck needs one beam line — the house carries the other end through the ledger. A freestanding deck needs two.
- Posts are 4×4 up to about 8 ft of height, 6×6 above — and 6×6 is worth using anyway for the larger bearing surface at the beam.
- Footings must reach below the frost line and are sized from tributary area. Use the deck footing calculator, which handles the IRC R507.3 sizing properly.
Fasteners
Count crossings, not square feet. A board crosses a joist every 16 inches, so a 16 ft board crosses 13 of them.
| Method | Per crossing | Notes |
|---|---|---|
| Face screws | 2 | Visible, cheapest, fastest. Pre-drill within 2 in of board ends |
| Hidden clips | 1 | No visible heads, but needs grooved boards and roughly doubles the labour |
The 16 × 12 example comes to 676 screws — about 3.5 per square foot, which is where the “4 per square foot” rule of thumb comes from. Use hot-dipped galvanised or stainless with pressure-treated timber; the copper in modern treatment corrodes plain steel quickly.
The ledger is the safety-critical part
Deck failures are overwhelmingly ledger failures, and the ledger almost always fails at the attachment rather than the timber. The classic mistake is lag screws driven into sheathing with nothing solid behind them.
IRC R507.9 requires through-bolts or approved structural screws into the house rim joist, at a specified pattern. Flash the top edge so water runs out over the ledger rather than sitting behind it — trapped water rots the rim joist you’re bolted to, and that failure is invisible until it isn’t.
Decks more than 30 in above grade need guards, and balusters must be spaced so a 4 in sphere can’t pass. Elevated and multi-storey decks are permit-and-inspection work in most jurisdictions.
Deck estimating mistakes to avoid
- Using nominal board width. A 5/4×6 is 5½ in — nominal gives ~8% too few boards.
- Adding 10% waste and calling it done. Real off-cut waste runs 0–78% depending on stock length.
- Assuming longer boards waste less. On an 18 ft run, 16 ft stock is the worst option available.
- Trusting a single joist span figure. Species changes it by 12%.
- Treating diagonal as a waste adjustment. It also forces 12 in joist centres.
- Butting green PT boards tight. They shrink and gap unevenly.
- Landing butt joints on a single joist. Each board gets ¾ in of bearing. Double the joist.
- Plain steel fasteners in treated timber. Modern treatment corrodes them.
Frequently asked questions
How many deck boards do I need?
Divide the joist span in inches by the board width plus gap for the row count, then divide the run by your stock length and round up for boards per row. A 16 × 12 ft deck in 5/4×6 needs 26 rows — and 26 boards in 16 ft stock, or 52 in 12 ft stock.
What board length should I buy?
One that divides evenly into your run. A 16 ft run in 16 ft boards wastes nothing and has no joints; in 12 ft boards it wastes 50%. Longer isn’t automatically better — on an 18 ft run, 10 ft boards waste 11% while 16 ft boards waste 78%.
How much waste should I add for decking?
A flat percentage isn’t the right tool, because real off-cut waste depends entirely on how your stock length divides into the run — anywhere from 0% to 78%. Work out the actual off-cut first, then add about 5% on top for damaged boards and bad cuts.
How far can a 2×8 deck joist span?
At 16 in on centre: 13′ 1″ in southern pine, 12′ 5″ in Douglas fir–larch and 11′ 8″ in hem-fir or SPF. That’s a 12% spread, so a 12 ft span passes in southern pine and fails in hem-fir. Check the grade stamp on the timber you’re buying.
What joist spacing do I need?
16 in on centre is standard for residential decks. Tighten to 12 in for diagonal boards, thin composite products, or heavy loads like hot tubs. 24 in is only for dense hardwood or premium composite explicitly rated for it — using it otherwise causes bounce and sag.
How much more does a diagonal deck cost?
About 41% more decking, because a board at 45° travels √2 further across the frame. Plus roughly a third more framing, because the unsupported span also grows by √2 — 16 in becomes 22.6 in, beyond most board ratings, so manufacturers require 12 in joist centres.
What gap should I leave between deck boards?
⅛ in for dry or kiln-dried timber, 3/16 in for green pressure-treated because it shrinks to about ⅛ in over the first season, and ¼ in for composite per manufacturer requirement. The gap drains water and debris — a deck that holds both rots from the top.
How many screws for a deck?
Two per joist crossing with face screws. A 16 ft board at 16 in centres crosses 13 joists, so 26 screws per board. A 16 × 12 ft deck comes to about 676 screws, roughly 3.5 per square foot. Hidden clips use one per crossing.
Is a 5/4×6 board really 6 inches wide?
No — it’s 5½ inches, and 1 inch thick. Using the nominal 6 inches gives you roughly 8% too few boards. The same applies to 2×6 (5½ in) and 2×8 (7¼ in). Composite boards are usually 5½ in but some premium lines differ, so check the data sheet.
How far apart should deck posts be?
Typically 6–8 ft along the beam for a doubled 2×8, further for larger beams. Use 4×4 posts up to about 8 ft of height and 6×6 above, though 6×6 is worth using regardless for the bearing surface it gives the beam.
Do I need a permit for a deck?
Usually, especially above 30 in from grade, which also triggers guard requirements. Elevated and multi-storey decks are permit-and-inspection work almost everywhere. Ground-level platforms are sometimes exempt, but check before building rather than after.
Can I use this deck calculator in metric?
Dimensions are in feet and inches, since US lumber and the IRC span tables are dimensioned that way. Multiply feet by 0.3048 for metres and inches by 25.4 for millimetres.
Standards and sources
- IRC Table R507.6 — deck joist spans for three species groups at 12, 16 and 24 in on centre, 40 psf live load
- IRC R507.9 — ledger attachment to the primary structure
- IRC R507.3 — footing sizing, covered by the deck footing calculator
- Nominal versus actual lumber dimensions per American Softwood Lumber Standard PS 20
- Board gap and diagonal joist spacing requirements from composite decking manufacturer installation instructions, which are warranty conditions
Related calculators
How this deck calculator works out its numbers
Rows come from the joist span divided by the board’s actual face width plus your gap, rounded up. Boards per row are the run divided by stock length, rounded up — and that rounding is where the waste comes from, so the calculator reports the linear feet you buy against the linear feet you lay rather than applying a percentage.
All five stock lengths are evaluated on that basis. When set to recommend, it picks the one with the least off-cut waste, and reports the butt joints each option creates so you can weigh waste against having an unbroken board.
Joists are the run divided by spacing, plus one. The span check reads IRC Table R507.6 for your species group at your spacing, and selects the smallest joist that carries the span — reporting the actual limit so you can see the margin. Diagonal decking applies √2 to both the board length and the row count, and tightens spacing to 12 in on centre, which is the manufacturer requirement rather than an option.
This produces quantities, not a structural design. The span table covers common prescriptive cases at 40 psf; hot tubs, planters, roof loads, cantilevers beyond the permitted ratio and unusual geometry all fall outside it. Ledger attachment is the safety-critical detail and is governed by R507.9. Decks over 30 in above grade need guards, and most jurisdictions require a permit and inspection.