How many deck boards, how many joists, how many screws. This is a materials count, not a structural design — it tells you what to put in the truck once someone has told you the deck is safe to build.
Nominal sizes lie. A "6 inch" deck board is 5½ inches wide, which is why counting boards off the nominal number leaves you short.
Wet pressure-treated lumber shrinks as it dries, so a tight gap opens up. Composite barely moves across its width but expands along its length — follow the maker's spec, not habit.
Nominal sizes are not real sizes
A 2×6 deck board is 1½ by 5½ inches. A 5/4×6 is about 1 inch by 5½. The nominal number is a historical reference to the rough sawn dimension before drying and planing, and if you count boards using it you will come up short every time. Everything above uses actual widths.
The gap matters for the same reason. Pressure-treated lumber is usually sold wet, and it shrinks across its width as it dries — so a 1/8 inch gap laid today can be 1/4 inch by the end of the summer. Composite does the opposite: it barely moves across the width but expands measurably along its length in heat, which is why the manufacturer specifies end gaps and why ignoring them buckles the deck.
What this calculator deliberately will not tell you
It will not size your joists, beams, posts or footings. Those depend on wood species, grade, span, ground snow load, whether the deck is freestanding or ledger-attached, and local code amendments that vary by jurisdiction. Publishing a single span number for all of that would be a good way to help someone build a deck that fails.
Get those numbers from your building department, or from the American Wood Council's Prescriptive Residential Wood Deck Construction Guide (DCA 6), which is the document most inspectors work from. Then pull the permit. Deck failures overwhelmingly happen at the ledger — the connection to the house — and at the footings, not in the decking you can see.
Ordering notes
Buy the longest boards that suit your deck. A 16-foot deck done in 16-foot boards has no butt joints at all, which looks better and gives water nowhere to sit. If you do have joints, land every one on a joist and stagger them between rows, or you get a visible seam running across the deck.
Add roughly 10% over the calculated count. Some boards will be crowned, split at the end, or badly enough cupped that you would not want them underfoot, and you want the option to reject them at the yard rather than build with them.
Our own attempts, including the parts that went badly: building a deck from scratch, cleaning and sealing it afterwards, and pouring concrete for the footings side of it.
Method
Decking rows = deck depth in inches ÷ (actual board width + gap), rounded up. Joists = deck length in inches ÷ spacing, plus one to close the run. Fasteners = rows × joists × screws per joist; a 5 lb box of deck screws is taken as roughly 350 pieces, which varies by length and brand. Hangers are counted at two per joist, one each end. All figures are material counts on actual, not nominal, dimensions, and exclude stairs, railings, fascia and blocking.