The number of joists needed for your deck depends on various factors, including the size (length and width) of the deck, the spacing between joists, and the load-bearing capacity of the wood material used.
The standard spacing between joists can be either 12″ or 16 inches on center (OC) depending on the length the joists need to span. The size of a deck can vary based on your choice and available space. A common is around 12 feet by 12 feet or 10’×10′ for a small deck, but larger decks are often 16 feet by 20 feet or more.
The size of joists needed for your deck framing depends on beam spacing, species of wood, load-bearing capacity, and size of a deck. However, in general, a deck spanning from 6 to 9 feet would require 2×6 joists, while a deck spanning from 10 to 13 feet would need 2×8 joists. In addition, a deck spanning from 14 to 16 feet would need 2×10 joists.
How many joists do i need for my deck
In general, at 16-inch joist spacing, a 10×10 deck would require 9 2×8 joists, while a 10×12 and 12×12 deck would require 10 2×8 joists. Likewise, at same spacing, a 12×16 deck would require 13 2×10 joists, while a 12×20 and 16×20 deck would require 16 2×10 joists.
| SL.no. | Deck size | 16″ spacing | 12″ spacing | 19.2″ spacing | 24″spacing |
| 1 | 12×20 | 16 joists | 21 joists | 14 joists | 11 joists |
| 2 | 12×16 | 13 joists | 17 joists | 11 joists | 9 joists |
| 3 | 10×10 | 9 joists | 11 joists | 8 joists | 6 joists |
| 4 | 12×12 | 10 joists | 13 joists | 9 joists | 7 joists |
| 5 | 16×20 | 16 joists | 21 joists | 14 joists | 11 joists |
| 6 | 10×12 | 10 joists | 13 joists | 9 joists | 7 joists |
| 8 | 8×8 | 7 joists | 9 joists | 6 joists | 5 joists |
How do I calculate how many joists I need for deck?
To determine the number of joists needed for your deck, you’ll want to consider the spacing between them. Typically, joists are spaced 16 or 12 inches apart. Measure the length of your deck in inches, then divide it by 12 or 16 and in results add 1 to determine the approximate number of joists required for your deck.
For example, if your deck span is 16 feet long and you’re using 16-inch joist spacing: Number of joists = Deck length (in inches)/ Joist spacing + 1. So, (16×12)/16 + 1 = 13 joists needed. Round up to the nearest whole number if necessary.
How many joists do i need for a 20 foot deck
A deck that measures 20 feet or 240 inches in length should have 16 2×10 joists with 16-inch joist spacing, 21 2×10 joists with 12-inch joist spacing, 14 2×10 joists with 19.2-inch joist spacing, or 11 2×10 joists with 24-inch joist spacing.
How many joists do i need for a 16 foot deck
A deck that measures 16 feet or 192 inches in length should have 13 2×10 joists with 16-inch joist spacing, 17 2×10 joists with 12-inch joist spacing, 11 2×10 joists with 19.2-inch joist spacing, or 9 2×10 joists with 24-inch joist spacing.
How many joists do i need for a 12 foot deck
A deck that measures 12 feet or 144 inches in length should have 10 2×8 joists with 16-inch joist spacing, 13 2×8 joists with 12-inch joist spacing, 9 2×8 joists with 19.2-inch joist spacing, or 7 2×8 joists with 24-inch joist spacing.
How many joists do i need for a 10 foot deck
A deck that measures 10 feet or 120 inches in length should have 9 2×8 joists with 16-inch joist spacing, 11 2×8 joists with 12-inch joist spacing, 8 2×8 joists with 19.2-inch joist spacing, or 6 2×8 joists with 24-inch joist spacing.
How many joists do i need for an 8 foot deck
A deck that measures 8 feet or 96 inches in length should have 7 2×6 joists with 16-inch joist spacing, 9 2×6 joists with 12-inch joist spacing, 6 2×6 joists with 19.2-inch joist spacing, or 5 2×6 joists with 24-inch joist spacing.
| SL.no. | Deck length | 16″ spacing | 12″ spacing | 19.2″ spacing | 24″ spacing |
| 1 | 20 feet | 16 joists | 21 joists | 14 joists | 11 joists |
| 2 | 16 feet | 13 joists | 17 joists | 11 joists | 9 joists |
| 3 | 12 feet | 10 joists | 13 joists | 9 joists | 7 joists |
| 4 | 10 feet | 9 joists | 11 joists | 8 joists | 6 joists |
| 5 | 8 feet | 7 joists | 9 joists | 6 joists | 5 joists |
Conclusions:
The number of joists needed for your deck depends on various factors. However, with 16-inch joist spacing, you’ll need approximately 13 joists for a 12×16 deck, 9 joists for a 10×10 deck, 10 joists for a 12×12 deck, 16 joists for a 16×20 deck, 10 joists for a 10×12 deck, and 16 joists for a 12×20 deck.