Joist calculator
Work out how many floor or ceiling joists a room needs and the total linear feet of lumber.
Joist calculator
Work out how many floor or ceiling joists a room needs and the total linear feet of lumber.
Enter the room length, width and joist spacing to see the joist count.
Joists needed
Project summary
How it works
This is a material planning estimate, not structural design. Joist size, spacing and doubling requirements depend on span tables, load and local code, so check both before building.
Joist calculator
Interior joists versus deck joists
The layout arithmetic here is the same as a deck: joists run across the room width and are spaced along its length. What differs is the environment. Interior floor and ceiling joists are not exposed to weather the way deck joists are, so lumber grade, moisture treatment and the applicable span table are different for interior framing than for an exterior deck, which is why this is kept separate from the deck joist calculator on this site.
When to double a joist
A joist running parallel and directly under an interior wall is a common candidate for doubling, since that wall transfers its load down through a single joist otherwise. This calculator adds one extra joist when you flag a parallel wall, a common practice rather than a universal code requirement. Whether doubling is actually needed, and whether it needs blocking or a different connection, depends on the load above and your local code.
Frequently asked questions
How far apart should floor joists be spaced?
16 inches on center is the most common spacing for residential floor framing, with 12 or 24 inches also used depending on the joist size, span and expected floor loading. The actual spacing for a given span is set by span tables in your local building code, not by this calculator, which only counts joists at whatever spacing you enter.
Why would I double a joist under a wall?
A wall running parallel to and directly above a joist puts extra load onto that one joist instead of spreading it across the floor, so many framers add a second joist alongside it to help carry that load. This is common practice rather than a universal requirement, and whether it applies to your situation depends on the wall above, the load it carries and local code.
How many joists do I need for a 12 by 12 room?
At 16 inch on centre spacing, you need 10 joists for a 12 foot span. Add one for the ledger connection. The calculator gives you the exact count.
What size joists do I need?
The joist size depends on the span, spacing and load. 2 by 8 joists are common for spans up to 14 feet. 2 by 10 joists handle longer spans. Your local building code span tables specify the minimum.
How far can a joist span?
A 2 by 8 at 16 inch spacing spans about 12 to 14 feet. A 2 by 10 spans 15 to 17 feet. The calculator counts joists, and you select the size that matches your span.
What is the difference between floor and ceiling joists?
Floor joists carry live and dead loads and are typically larger. Ceiling joists support lighter loads. The calculator counts joists based on area and spacing regardless of whether they are floor or ceiling.
Do joists need hangers?
Joist hangers connect joists to beams or headers and provide structural support. They are required in most building codes. The calculator counts joists, and you add hangers for each one.
How much do joists cost?
A 2 by 8 by 12 foot joist costs about 15 to 25 dollars. A 2 by 10 costs about 20 to 35 dollars. The calculator gives you the count so you can price the total.