What size floor joist need to span 10′, 12′, 15′, 16′, 18′, 20′, 24′ & 25 feet depends on various factors, including the type of wood or material used, the intended load, and local building codes.

To provide a rough idea, we can suggest some common lumber sizes for residential construction using Douglas fir-larch, which is a common choice:

1. 10 feet span: 2×6

2. 12 feet span: 2×6

3. 15 feet span: 2×8

4. 16 feet span: 2×8

5. 18 feet span: 2×10

6. 20 feet span: 2×10

7. 24 feet span: 2×12

A #2-DFL 2×6 floor joists spaced 16 inches on center (OC) can typically span from 6 to 12 feet for a normal load weight condition.

A #2-DFL 2×8 floor joists spaced 16 inches on center (OC) can typically span from 12 to 16 feet for a normal load weight condition.

A #2-DFL 2×10 floor joists spaced 16 inches on center (OC) can typically span from 16 to 20 feet for a normal load weight condition.

A #2-DFL 2×12 floor joists spaced 16 inches on center (OC) can typically span from 20 to 24 feet for a normal load weight condition.

A #2-DFL 2×14 floor joists spaced 16 inches on center (OC) can typically span from 24 to 28 feet for a normal load weight condition.

A #2-DFL 2×16 floor joists spaced 16 inches on center (OC) can typically span from 28 to 32 feet for a normal load weight condition.

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** can be define as several small beams. 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.

Not all the type of species of wood created equally strong and type of tree used to make joist affects the span allowed. 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.

Proper floor joist sizing is specified by the International Residential Code – the IRC. It outlines max span lengths for wood joists of all sizes – from 2×6 to 2×12 and greater. The code also identifies joist sizing and spans for various joist spacing – from 12” up to 24” apart.

**Floor joist size**

The size of floor joists depends on factors like span, load, and local building codes. Typically, 2×8 or 2×10 joists spaced 16 or 24 inches apart are common for residential construction and other typical floor joist sizes include 2×6 (2-by-6), 2×8 (2-by-8), 2×10 (2-by-10), 2×12 (2-by-12) and even greater.

Spacing between floor joist for different sizes are kept at 12 inches, 16 inches and 24 inches apart. The joist which are kept at lower spacing can allow longer distance to span as compared to joist spaced further apart. Standard spacing between floors joist is 16 inches.

**Floor joist spacing**

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, and 24 inches apart on centre are also used. Floor joists help distribute the load of a structure. The joist which are kept at lower spacing can allow longer distance to span as compared to joist spaced further apart.

**How far apart are floor joists**

Most often, floor joist spacing is 16 inches apart on center used in old houses, on a deck, for a shed and in a mobile home.

The spacing between floor joists can vary depending on the building code and the size and type of the joists being used. Typically, for residential construction in the United States, floor joists are spaced 16 inches on center (16″ OC) or 24 inches on center (24″ OC).

**What size floor joist to span 10′, 12′, 15′, 16′, 18′, 20′, 24′ & 25 feet**

The size of floor joists needed to span from 6 to 12 feet have to be 2×6 inch lumber, while the size of floor joists needed to span from 12 to 16 feet need to be 2×8 inch lumber. Moreover, the size of floor joists needed to span from 16 to 20 feet ought to be 2×10 inch lumber, while the size of floor joists needed to span from 20 to 24 feet ought to be 2×12 inch lumber.

**What size floor joist to span 10 feet**

The size of floor joists needed to span 10 feet depends on various factors, but for a basic residential application with a standard live load, 2×6 joists spaced 16 inches on center are commonly utilized.

A #2-SYP size of floor joist can span to 10 feet for a live load of 30 pounds per square foot and dead load of 10 pounds per square foot when spaced 16 inches apart.

**What size floor joist to span 12 feet**

The size of floor joists needed to span 12 feet depends on various factors, but a common recommendation for residential construction might be 2×6 joists at 16 inches on center for a 12-foot span.

**What size floor joist to span 15 feet**

The size of floor joists needed to span 15 feet depends on various factors, but for a basic residential application with a standard live load, 2×8 joists spaced 16 inches on center are commonly utilized.

**What size floor joist to span 16 feet**

The size of floor joists needed to span 16 feet depends on various factors, but a common recommendation for residential construction might be 2×8 joists at 16 inches on center for a 16-foot span.

**What size floor joist to span 18 feet**

The size of floor joists needed to span 18 feet depends on various factors, but for a basic residential application with a standard live load, 2×10 joists spaced 16 inches on center are commonly utilized.

**What size floor joist to span 20 feet**

The size of floor joists needed to span 20 feet depends on various factors, but a common recommendation for residential construction might be 2×10 joists at 16 inches on center for a 20-foot span.

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**What size floor joist to span 24 feet**

The size of floor joists needed to span 24 feet depends on various factors, but for a basic residential application with a standard live load, 2×12 joists spaced 16 inches on center are commonly utilized.

**What size floor joist to span 25 feet**

The size of floor joists needed to span 25 feet depends on various factors, but a common recommendation for residential construction might be 2×14 joists at 16 inches on center for a 25-foot span.

**What size floor joist to span 6′, 7′ & 8 feet**

The size of floor joists needed to span from a 6 to 8 foot depends on various factors, but a common recommendation for residential construction might be 2×6 joists at 16 inches on center for a 6 to 8-foot span.

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

To figure out number of floor joists you would 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 atleast 9 floor joists. i.e (10×12)/ 16) +1 = 7.5 +1 = 8.5, round it as 9.

The number of floor joists needed for a deck depends on the spacing between the joists and the size of the joists and local building code. Typically, joists are spaced 16 or 24 inches apart.

Typically, at 16 inches oc, you would need around 9 floor joists for a 10×10 deck, while you would need around 7 floor joists for a 8×8 deck. Moreover, at same oc spacing, you would need around 10 floor joists for a 12×12 deck, while you would need around 13 floor joists for a 16×16 deck.

**How many floor joists do I need for a 10×10 deck**

Typically, joists are spaced 16 or 24 inches apart. For example, if you use 16-inch spacing, you would need around 9 floor joists for a 10×10 deck.

Use the formula for floor 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 it as 9.

**How many floor joists do I need for a 8×8 deck**

Typically, for a standard deck, joists are spaced either 16 inches or 24 inches apart. If you choose 16-inch spacing, you would need 7 floor joists, and if you choose 24-inch spacing, you would need 5 joists for an 8×8 deck.

**How many floor joists do I need for a 12×12 deck**

Typically, a common spacing is 16 inches, 24 inches on center. For a 12-foot span at 16″ spacing, you might need around 10 floor joists and if you choose 24-inch spacing, you would need 7 joists for a 12×12 deck.

Use the formula for floor 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 joists.

**How many floor joists do I need for a 16×16 deck**

Typically for a 16-foot span at 16″ spacing, you might need around 13 floor joists and if you choose 24-inch spacing, you would need 10 joists for a 16×16 deck.

Use the formula for floor 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 atleast 13 floor joists for a 16×16 deck.

**Conclusions:-**

In general, at 16 inches oc, a 2×6 size of floor joist can span from 6 to 12 feet, while a 2×8 size of floor joist can span from 12 to 16 feet. Moreover, at same oc, a 2×10 size of floor joist can span from 16 to 20 feet, while a 2×12 size of floor joist can span from 20 to 24 feet.