Joist Calculator
Count floor or deck joists and the hangers and blocking they need.
What you'll need to order
- dimensional joist lumber
- joist hangers
- hanger nails
- blocking
- rim board
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
joists = (run × 12 ÷ spacing) + 1
The "+1" covers the joist at each end of the run. Blocking or bridging is required at mid-span on runs over 8 ft to stop the joists twisting under load.
Worked example
With length along the beam 20 ft, joist spacing 16 in, joist span 12 ft, this calculator returns 16 .
Frequently asked questions
How far can a 2x10 joist span?
Roughly 15 to 16 ft at 16 inch centres for a residential floor in Douglas fir, and less for a deck carrying live load. Span tables vary by species, grade and load — check the table your code adopts.
What spacing should floor joists be?
Sixteen inches on centre for most floors. Drop to 12 inches under tile or stone, which cannot tolerate deflection. Twenty-four inches works only with engineered I-joists or LVL.
Do joists need blocking?
Yes on any run over 8 ft. A row of solid blocking or cross-bridging at mid-span stops the joists rolling and stiffens the whole floor noticeably. It is one of the cheapest ways to reduce bounce.
Can I notch or drill a joist?
Within limits. Holes must sit in the middle third of the depth and be no larger than a third of it. Notches are permitted only in the outer thirds of the span and never on the bottom edge in mid-span.