Deck Joist Calculator
Enter your deck width and joist spacing to get a field joist line count. This estimates quantity only — it does not check allowable span.
Perpendicular to the joists.
On-center.
Extra support, e.g. under a hot tub.
Not included by default. There's no general formula for this — count your own perimeter framing (it differs for a ledger-attached vs. freestanding deck) and enter it here.
Field joist lines
13
- Base joist lines
- 13
- Rim/band joists
- 0
- Total joists
- 13
›Calculation details
- Base joist lines
- 13
- Additional joists
- 0
- Doubled joists
- 0
- Rim/band joists
- 0
- Total joists
- 13
This assumes a joist line at both outer edges. The final joist bay may be narrower than the selected on-center spacing.
How it works
The base count assumes a joist line at both outer edges of the deck, with the rest spaced at your chosen on-center distance in between — so the very last bay is often slightly narrower than the spacing you entered, which is normal. Additional and doubled joists (for a hot tub, a seam, or other extra support) are added on top of that base count.
FAQ
Does this tell me if my joists are strong enough?
No — allowable span depends on lumber species, grade, size, spacing, and load, all set by your local building code. This calculator only counts joist lines; it doesn't evaluate whether your spacing is structurally adequate.
Why isn't there a rim/band joist formula?
How many you need depends on whether the deck is attached to the house with a ledger board or freestanding, which changes the perimeter framing. Rather than guess, this calculator asks you to enter that count directly if you want it included.