The number of floor joists needed for a 10′, 12′, 14′, 16′, 18′, 20′, & 24 foot deck depends on various factors, including the spacing between the joists and the size of the joists themselves. Typically, for residential decks, floor joists are spaced either 16 inches or 24 inches on center. Here’s a rough estimate of the number of joists you would need for different deck lengths with 16-inch on-center spacing:

For a 10-foot deck size you may need approximately 9 joists. For a 12-foot deck you may need approximately 10 joists. For a 14-foot deck you may need approximately 12 joists. For a 16-foot deck you may need approximately 13 joists. For a 18-foot deck you may need approximately 15 joists. For a 20-foot deck you may need approximately 16 joists. For a 24-foot deck: you may need approximately 19 joists.

There are different depth of floor joist are used in floor construction such as 2×6, 2×8, 2×10 and 2×12, made of different species of wood (southern yellow pine & Douglas fir) and different grade such as grade – 1, grade – 2, grade – 3. Southern yellow pine is strongest species of wood that can span more distance compared to other species of wood. Grade 1 lumber have higher strength than other grade Lumber due to fewer knots.

Floor joists are typically a horizontal structural member that runs across an open space of building on supporting wall or studs. Their function is similar as beams, except floor joists are supported by wall, foundations, or beams framing.

Floor joists usually run between walls or rafters to support a ceiling or floor. They transfer the dead load of the subfloor and floor finishes and live load of people, furniture and furnishings to beams, walls, headers, or foundation walls. It is like the skeleton/ framing of a building that supports the load which the floor is built to bear. Joists are technically beams that typically spans relatively short distances compared to beams and are made of wood, steel, or engineered wood or lumber.

Standard floor size is 2×8 (2 by 8). Others typical floor joist size are 2×6 (2-by-6), 2×8 (2-by-8), 2×10 (2-by-10), 2×12 (2-by-12) and even greater. Most standard, floor joist spacing is 16 inches apart on center, but this can vary depending on building codes and the requirements of the structure. So, 12 inches, 19.2 inches and 24 inches apart on centre are also used.

**How do i figure out how many floor joists i need**

To figure out how many floor joists you need, measure the length of subfloor in feet. Multiply the subfloor length in feet by 12 to convert in inches. Then divide it by spacing i.e 24″ or 16″, 19.2″, or 12″ and add 1 in result to get total number of floor joists you needed.

The simple formula to find number of floor joists are following: floor joists count = ((subfloor length × 12)/16) + 1. This equation will help you to figure how many floor joists you needed. For example, if your subfloor is 10 feet long, you will need at least 9 floor joists. i.e (10×12)/ 16) +1 = 7.5 +1 = 8.5, round it as 9.

**How many joists for 10 foot deck**

A 10 foot long deck with joists 16″ spaced on centre will require 9 joists. Use the formula for joists count = ((subfloor length × 12)/16) + 1, and put the span of your deck: joists count = ((10 × 12)/16) + 1 = (120/16) + 1 = 7.5 + 1 = 8.5, round up as 9. Thus, you would need at least 9 joists for a 10 foot deck.

**How many joists for 12 foot deck**

A 12 foot long deck with joists 16″ spaced on centre will require 10 joists. Use the formula for joists count = ((subfloor length × 12)/16) + 1, and put the span of your deck: joists count = ((12 × 12)/16) + 1 = (144/16) + 1 = 9 + 1 = 10. Thus, you would need at least 10 joists for a 12 foot deck.

**How many joists for 14 foot deck**

A 14 foot long deck with joists 16″ spaced on centre will require 12 joists. Use the formula for joists count = ((subfloor length × 12)/16) + 1, and put the span of your deck: joists count = ((14 × 12)/16) + 1 = (168/16) + 1 = 10.5 + 1 = 11.5, round as 12. Thus, you would need atleast 12 joists for a 14 foot deck.

**How many joists for 16 foot deck**

A 16 foot long deck with joists 16″ spaced on centre will require 13 joists. Use the formula for joists count = ((subfloor length × 12)/16) + 1, and put the span of your deck: joists count = ((16 × 12)/16) + 1 = (192/16) + 1 = 12 + 1 = 13. Thus, you would need at least 13 joists for a 16 foot deck.

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**How many joists for 18 foot deck**

A 18 foot long deck with joists 16″ spaced on centre will require 15 joists. Use the formula for joists count = ((subfloor length × 12)/16) + 1, and put the span of your deck: joists count = ((18 × 12)/16) + 1 = (216/16) + 1 = 13.5 + 1 = 14.5, round as 15. Thus, you would need atleast 15 joists for a 18 foot deck.

**How many joists for 20 foot deck**

A 20 foot long deck with joists 16″ spaced on centre will require 16 joists. Use the formula for joists count = ((subfloor length × 12)/16) + 1, and put the span of your deck: joists count = ((20 × 12)/16) + 1 = (240/16) + 1 = 15 + 1 = 16. Thus, you would need atleast 16 joists for a 20 foot deck.

**How many joists for 24 foot deck**

A 24 foot long deck with joists 16″ spaced on centre will require 19 joists. Use the formula for joists count = ((subfloor length × 12)/16) + 1, and put the span of your deck: joists count = ((24 × 12)/16) + 1 = (288/16) + 1 = 18 + 1 = 19. Thus, you would need at least 19 joists for a 24 foot deck.

**How many joists do i need for a 10×10 deck**

If your deck is 10×10 foot with joists which spaced 16 inches on center will require 9 joists. Use the formula for joists count = ((subfloor length × 12)/16) + 1, and put the span of your deck: joists count = ((10 × 12)/16) + 1 = (120/16) + 1 = 7.5 + 1 = 8.5, round up as 9. Thus, you would need atleast 9 joists for a 10×10 deck.

**How many joists do i need for a 12×12 deck**

If your deck is 12×12 foot with joists which spaced 16 inches on center will require 10 joists. Use the formula for joists count = ((subfloor length × 12)/16) + 1, and put the span of your deck: joists count = ((12 × 12)/16) + 1 = (144/16) + 1 = 9 + 1 = 10. Thus, you would need atleast 10 joists for a 12×12 deck.

**How many joists do i need for a 8×8 deck**

If your deck is 8×8 foot with joists which spaced 16 inches on center will require 7 joists. Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the span of your deck: joists count = ((8 × 12)/16) + 1 = (96/16) + 1 = 6 + 1 = 7. Thus, you would need atleast 7 floor joists for a 8×8 deck.

**How many joists do i need for a 16×16 deck**

If your deck is 16×16 foot with joists which spaced 16 inches on center will require 13 joists. Use the formula for joists count = ((subfloor length × 12)/16) + 1, and put the span of your deck: joists count = ((16 × 12)/16) + 1 = (192/16) + 1 = 12 + 1 = 13. Thus, you would need at least 13 joists for a 16×16 deck.

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**Conclusions:**

A 10 foot long deck with joists 16″ spaced on centre will require 9 joists. To figure out number of floor joists you would need, use the formula for joists count = ((subfloor length × 12)/16) + 1.