Joist count is deck width divided by spacing, plus one for the closing joist. Sixteen inches on centre is the residential default, which is a spacing of 1.333 feet.
Spacing and what it is for
Sixteen inches on centre suits most decking. Twelve inches is required for boards laid diagonally, and for some composite products — check the decking manufacturer's instruction, because composite sags between joists in a way wood does not.
Twenty-four inches appears in older decks and is too wide for most modern decking.
What this calculator does not tell you
Span capacity is a structural question. It depends on species, grade, joist depth and spacing together, and the answer lives in a code span table, not in a simple formula.
Undersized joists produce a deck that feels bouncy, and in the worst case they fail under a crowd. Most jurisdictions require a permit for a deck precisely so somebody checks this.
Hangers, and the right nails
Every joist meeting a ledger or beam face needs a hanger, and hangers need the specific structural nails or screws the manufacturer lists.
Do not substitute ordinary deck or drywall screws. Use the exact nails or approved structural connector screws listed for that hanger; diameter, length, coating and installation pattern vary by product and load.
The ledger is where decks fail
Most catastrophic deck collapses start at the ledger — the board bolted to the house. It needs proper through-bolts or structural screws into the rim joist, and flashing above it.
Nailed ledgers and unflashed ledgers are the two things a deck inspector looks at first.
Blocking and why it is in the plan
Blocking is the short pieces installed between joists, and it does two things. It stops the joists twisting as they dry, and it spreads load between them.
Joists that are not blocked will rotate at the ends over time, which shows up as a springy deck and as deck boards that no longer sit flat.
Blocking is usually required at intervals along the span and always at the ends, and it is a real quantity of lumber — count on one piece per bay per row and add it to your order.
Drainage and where rot starts
Almost all deck framing rot starts on the top edge of a joist, where water sits under the gap between deck boards and never quite dries.
Joist tape — a self-adhesive membrane run along the top of each joist — is cheap, fast, and it addresses the exact failure point. On a deck you intend to keep for twenty years it is the best value item on the list.
The other common rot location is where the ledger meets the house. That is a flashing detail, not a lumber one, and it is the single most consequential part of the whole build.