How to use the joist spacing calculator
- Enter the width of the area in feet, measured across the joists. The joists run at right angles to this measurement.
- Enter the spacing from center to center in inches. Common choices are 12, 16, 19.2 and 24, but use the spacing from your plans or local code.
- Optionally enter the joist length and the nominal size to get total lumber and board feet, and tick the box if you want two rim joists across the ends.
- Read the number of joists, the spacing of the last gap and the hanger count.
Formula
Spaces = width (in) / spacing (in), rounded up
Joists = spaces + 1
Last space = width - (joists - 2) x spacing
Board feet = joists x length (ft) x nominal thickness x nominal width / 12
Board feet use the nominal size, so a 2x8 is counted as 2 by 8 inches, which is how lumber is priced.
A 12 foot wide floor at 16 inch centers
The width is 12 x 12 = 144 inches, and 144 / 16 = 9 spaces, so there are 9 + 1 = 10 joists. With a joist length of 10 feet that is 100 feet of lumber, which is 133.3 board feet of nominal 2x8.
A 12.5 foot width is 150 inches, and 150 / 16 = 9.375, which rounds up to 10 spaces and 11 joists. The last space is 150 - 9 x 16 = 6 inches. Adding two 12.5 foot rim joists gives 135 feet of lumber and 22 hangers if every joist is hung at both ends.
The extra joist at the end
Counting joists is a fence-post problem: n spaces need n + 1 joists. When the width is not an exact multiple of the spacing, the final space is smaller than the rest, but there still has to be a joist at the end to carry the edge of the floor or deck. That is why the tool rounds the spaces up before adding one.
If your plans call for a different layout, such as a doubled joist under a wall or at an opening, add those separately.
Counting is not sizing
This tool does not decide whether a joist size and spacing are strong enough for a span and a load. That depends on the species and grade of lumber, the loads and the building code in your area. Check span tables in your code, or ask an engineer or building inspector, before you order.
For material and layout estimates, see the deck calculator, the board foot calculator, the stud calculator and the rough opening calculator.
Assumptions and limits
- The first joist sits at the start of the width and the last one at the far end, with equal spacing in between except possibly the last space.
- Spacing is measured center to center and is the value you enter.
- Hangers are counted as two per joist and only apply if the joists are hung at both ends.
- Blocking, bridging, doubled joists and the size needed for the span are not included.
Frequently asked questions
How many joists do I need for a 12 foot width?
At 16 inch centers you need 10 joists, since 144 inches divided by 16 is 9 spaces and one more joist closes the last space. At 24 inch centers you need 7.
Why add one to the number of spaces?
Every space needs a joist on each side, and joists are shared by neighboring spaces, so the count is always one more than the number of spaces.
What does on center mean?
It means the distance from the center of one joist to the center of the next. It is measured to the middle of the lumber, not to its edge.
Can this tell me what size joist I need?
No. The size depends on the span, the load, the wood and local code. Use the span tables in your building code or ask an engineer or inspector.