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.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- ICC International Residential Code 2021, Tables R502.3.1(1) and R502.3.1(2) — allowable floor joist spans by species, grade and spacing
The edition matters as much as the number. Jurisdictions adopt code revisions on their own schedule, so verify which one applies where you are building.
Worked example
With length along the beam 20 ft, joist spacing 16 in, joist span 12 ft, this calculator returns 16 .
Joists needed for floors of common size
Open any row to load it into the calculator above — the link carries the numbers with it, so it can go straight to a supplier or a crew.
| Job | Size | Joists needed |
|---|---|---|
| Small bedroom floor | 12 ft × 16 in × 10 ft | 10.00 |
| Typical room | 20 ft × 16 in × 12 ft | 16.00 |
| Whole-floor run | 40 ft × 16 in × 14 ft | 31.00 |
| Deck framing at 16 in | 16 ft × 16 in × 12 ft | 13.00 |
| Same deck at 12 in for tile | 16 ft × 12 in × 12 ft | 17.00 |
Blocking at mid-span is required on runs over 8 ft to stop the joists twisting.
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.