Joist count by deck dimension
| Dimension crossed | 12 in O.C. | 16 in O.C. | 24 in O.C. |
|---|---|---|---|
| 8 ft | 9 joists | 7 joists | 5 joists |
| 10 ft | 11 joists | 9 joists | 6 joists |
| 12 ft | 13 joists | 10 joists | 7 joists |
| 14 ft | 15 joists | 12 joists | 8 joists |
| 16 ft | 17 joists | 13 joists | 9 joists |
| 18 ft | 19 joists | 15 joists | 10 joists |
| 20 ft | 21 joists | 16 joists | 11 joists |
| 24 ft | 25 joists | 19 joists | 13 joists |
What does on-center mean?
On-center spacing is the distance from the centerline of one joist to the centerline of the next. A nominal 16-inch O.C. layout therefore repeats joist centerlines every 16 inches rather than leaving a 16-inch clear gap between boards.
Why spacing cannot be chosen from a chart alone
Allowable joist spacing and span depend on the decking product, joist species and grade, joist size, span, loads, connections and applicable requirements. Manufacturer installation instructions can also impose maximum joist spacing for decking products, particularly when boards are installed diagonally.
Calculate your deck layout
Enter your project dimensions and selected on-center spacing to visualize joists and estimate decking, fasteners and preliminary footings.
Open Deck Materials Calculator →See the Deck Planning Hub for the dedicated joist, screw and footing calculators.