The number of joists you need for a deck depends on several factors, including the joist spacing, the size of the joists, the load requirements, and local building codes. Typically, joists are spaced at either 16 inches on center (16″ o.c.) or 24 inches on center (24″ o.c.). Here’s a general guideline for the number of joists you might need for the mentioned deck sizes with 16″ o.c. spacing:

**For a 10×10 deck: **Using 2×6 joists, you would need approximately 7 joists and using 2×8 joists, you would need approximately 5 joists. **For a 12×12 deck:** Using 2×6 joists, you would need approximately 9 joists and using 2×8 joists, you would need approximately 7 joists. **For an 8×10 deck:** Using 2×6 joists, you would need approximately 5 joists, and using 2×8 joists, you would need approximately 4 joists. **For a 10×12 deck: **Using 2×6 joists, you would need approximately 7 joists and, using 2×8 joists, you would need approximately 6 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.

Standard joist size use for deck is 2×6 (2 by 6). Others typical floor joist size are 2×4 (2-by-4), 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 joists i need**

To figure out number of joists you would need for a deck, 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 joists you needed.

The simple formula to find number of joists are following: joists count = ((subfloor length × 12)/16) + 1. This equation will help you to figure how many joists you needed. For example, if your deck measures 8 by 10 feet, you will need atleast 9 joists which are atleast 8 feet long. i.e (10×12)/ 16) +1 = 7.5 +1 = 8.5, round it as 9.

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

A 12-foot x 12-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 10 pieces of joists. Thus, you will need 10 pieces of beams for your joists, which have to be at least 12 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = ((12 × 12)/16) + 1 = (144/16) + 1 = 9 + 1 = 10. Thus, you would need atleast 10 pieces joists for a 12×12 deck.

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

A 10-foot x 10-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 9 pieces of joists. Thus, you will need 9 pieces of beams for your joists, which have to be at least 10 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = ((10 × 12)/16) + 1 = (120/16) + 1 = 7.5 + 1 = 8.5, round as 9. Thus, you would need atleast 9 pieces of joists for a 10×10 deck.

**How many joists do i need for a 10×12 deck**

A 10-foot x 12-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 10 pieces of joists. Thus, you will need 10 pieces of beams for your joists, which have to be at least 10 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = ((12 × 12)/16) + 1 = (144/16) + 1 = 9 + 1 = 10. Thus, you would need atleast 10 pieces of joists for a 10×12 deck.

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**How many joists do i need for a 8×10 deck**

A 8-foot x 10-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 9 pieces of joists. Thus, you will need 9 pieces of beams for your joists, which have to be at least 8 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = ((10 × 12)/16) + 1 = (120/16) + 1 = 7.5 + 1 = 8.5, round as 9. Thus, you would need atleast 9 pieces of joists for a 8×10 deck.

**How many joists do i need for a 8×12 deck**

A 8-foot x 12-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 10 pieces of joists. Thus, you will need 10 pieces of beams for your joists, which have to be at least 8 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = ((12 × 12)/16) + 1 = (144/16) + 1 = 9 + 1 = 10. Thus, you would need atleast 10 pieces of joists for a 8×12 deck.

**How many joists do i need for a 12×18 deck**

A 12-foot x 18-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 15 pieces of joists. Thus, you will need 15 pieces of beams for your joists, which have to be at least 12 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = ((18 × 12)/16) + 1 = (216/16) + 1 = 13.5 + 1 = 14.5, round as 15. Thus, you would need at least 15 pieces joists for a 12×18 deck.

**How many joists do i need for a 12×16 deck**

A 12-foot x 16-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 13 pieces of joists. Thus, you will need 13 pieces of beams for your joists, which have to be at least 12 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = ((16 × 12)/16) + 1 = (192/16) + 1 = 12 + 1 = 13. Thus, you would need at least 13 pieces joists for a 12×16 deck.

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

A 8-foot x 8-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 7 pieces of joists. Thus, you will need 7 pieces of beams for your joists, which have to be at least 8 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = ((8 × 12)/16) + 1 = (96/16) + 1 = 6 + 1 = 7. Thus, you would need atleast 7 pieces joists for a 8×8 deck.

**How many joists do i need for a 8×16 deck**

A 8-foot x 16-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 13 pieces of joists. Thus, you will need 13 pieces of beams for your joists, which have to be at least 8 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = ((16 × 12)/16) + 1 = (192/16) + 1 = 12 + 1 = 13. Thus, you would need atleast 13 pieces joists for a 8×16 deck.

**How many joists do i need for a 10×16 deck**

A 10-foot x 16-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 13 pieces of joists. Thus, you will need 13 pieces of beams for your joists, which have to be at least 10 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = ((16 × 12)/16) + 1 = (192/16) + 1 = 12 + 1 = 13. Thus, you would need atleast 13 pieces joists for a 10×16 deck.

**How many joists do i need for a 10×20 deck**

A 10-foot x 20-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 16 pieces of joists. Thus, you will need 16 pieces of beams for your joists, which have to be at least 10 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = ((20 × 12)/16) + 1 = (240/16) + 1 = 15 + 1 = 16. Thus, you would need atleast 16 pieces joists for a 10×20 deck.

**How many joists do i need for a 12×14 deck**

A 12-foot x 14-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 12 pieces of joists. Thus, you will need 12 pieces of beams for your joists, which have to be at least 12 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length 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 pieces joists for a 12×14 deck.

**How many joists do i need for a 14×14 deck**

A 14-foot x 14-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 12 pieces of joists. Thus, you will need 12 pieces of beams for your joists, which have to be at least 14 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length 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 pieces joists for a 14×14 deck.

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

A 16-foot x 16-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 13 pieces of joists. Thus, you will need 13 pieces of beams for your joists, which have to be at least 16 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = ((16 × 12)/16) + 1 = (192/16) + 1 = 12 + 1 = 13. Thus, you would need at least 13 pieces joists for a 16×16 deck.

**How many joists do i need for a 16×20 deck**

A 16-foot x 20-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 16 pieces of joists. Thus, you will need 16 pieces of beams for your joists, which have to be at least 16 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = ((20 × 12)/16) + 1 = (240/16) + 1 = 15 + 1 = 16. Thus, you would need at least 16 pieces joists for a 16×20 deck.

**How many joists do i need for a 12×24 deck**

A 12-foot x 24-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 19 pieces of joists. Thus, you will need 19 pieces of beams for your joists, which have to be at least 12 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = (24× 12)/16) + 1 = (288/16) + 1 = 18 + 1 = 19. Thus, you would need at least 19 pieces joists for a 12×24 deck.

**How many joists do i need for a 12×20 deck**

A 12-foot x 20-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 16 pieces of joists. Thus, you will need 16 pieces of beams for your joists, which have to be at least 12 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = (20× 12)/16) + 1 = (240/16) + 1 = 15 + 1 = 16. Thus, you would need at least 16 pieces joists for a 12×20 deck.

**How many joists do i need for a 20×20 deck**

A 20-foot x 20-foot deck of 5/4 x 6 pressure treated wood decking on 16 inches on center spacing of joists will require 16 pieces of joists. Thus, you will need 16 pieces of beams for your joists, which have to be at least 20 feet long.

Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the length of your deck: joists count = (20× 12)/16) + 1 = (240/16) + 1 = 15 + 1 = 16. Thus, you would need at least 16 pieces joists for a 20×20 deck.

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**Conclusions:-**

To figure number of joists you would need for a deck, measure the length of deck in inches. Then divide it by spacing i.e 16″ and add 1 in result to get total number of joists you needed.