How Far Can a Floor Joist Span?
As a rough guide at 16 inches on centre in a residential floor, a 2x6 spans about 9 feet, a 2x8 about 12 to 13, a 2x10 about 15 to 16 and a 2x12 about 18. Those numbers move by a foot or more with species, grade and load, which is why they are a starting point for sizing a job and never a substitute for the span table your building department uses.
Start here
- Floor Joist Span Calculator — the smallest joist that clears your span, and what it costs
- Floor Truss vs I-Joist Cost Calculator — the cost difference between floor trusses, I-joists and sawn lumber
- Framing Lumber Cost Estimator — board feet, piece count and cost for a framing order
- Deck Board & Joist Calculator — board count, joist count and framing for a deck
What span actually means
Span is the clear distance between supports, not the length of the lumber. A joist running 15 feet 6 inches from a sill plate to a centre beam has a span of roughly 15 feet even though it is cut from a 16 foot board, and the bearing at each end is extra. Getting this wrong by a foot is the most common sizing error, and it always errs toward undersizing.
Spans are governed by deflection more often than by strength. A floor built to the limit of its span table is not close to breaking — it is at the limit of how much it may bounce, commonly the span divided by 360 for a living area. That is why a floor can feel springy and still be entirely code-compliant, and why builders routinely size up one step for a floor that will carry tile.
The four things that change the number
Species and grade come first. Southern yellow pine and Douglas fir carry noticeably further than spruce-pine-fir at the same size, and a #1 grade board carries further than a #2. Lumber is stamped with both, and the stamp is what the table is read against.
Spacing is next, and it is the cheapest lever. Moving from 16 to 12 inches on centre buys roughly a foot of span on most sizes at the cost of more joists; moving out to 19.2 or 24 inches loses span and is usually only worth it with engineered members.
Load is the one people forget. Residential floor tables assume 40 psf live and 10 psf dead for living areas and 30 psf live for sleeping rooms. A tile floor with mortar bed, a stone hearth, a full bookcase wall or a hot tub is outside that assumption and needs the span reduced or the framing designed for it.
Species, grade, spacing and load together are what a span calculator is combining — which is why entering the actual four values matters more than remembering any single figure.
When dimensional lumber runs out
Past about 18 feet, solid lumber stops being the answer. An I-joist of the same depth spans further, comes dead straight, and arrives in lengths that dimensional stock does not; an open-web floor truss spans further still and lets ducts and plumbing run through the web instead of through holes you are allowed to drill.
The trade is cost and lead time. Engineered members are ordered rather than picked off a rack, they need a layout drawing, and they cannot be notched or drilled outside the manufacturer rules — the web of an I-joist is not lumber and treating it as such is a structural failure waiting to be found by an inspector.
Holes and notches
Once the joist is sized, most of what goes wrong afterwards is a plumber with a hole saw. In a solid joist, holes stay in the middle third of the depth, no larger than a third of the depth, and at least 2 inches from the edges; notches are permitted only in the outer thirds of the span and never on the underside of the middle third. A hole through the bottom edge at mid-span removes exactly the fibre that is doing the work.
Engineered joists have their own rules printed on the web, and they are not the same rules. Read them before drilling, because a wrongly cut I-joist cannot be repaired with a sister the way solid lumber can.
Common questions
- How far can a 2x8 floor joist span?
- Roughly 12 to 13 feet at 16 inches on centre in a residential floor, depending on species and grade. Southern yellow pine reaches the top of that range; spruce-pine-fir sits near the bottom.
- How far can a 2x10 span?
- Roughly 15 to 16 feet at 16 inches on centre under a standard residential floor load, again depending on species and grade.
- Does joist spacing change the span?
- Yes. Tightening from 16 to 12 inches on centre buys about a foot on most sizes; widening to 24 inches costs more than a foot and usually pushes you into engineered joists.
- Can I drill a hole through a floor joist?
- In solid lumber, yes — within the middle third of the depth, no bigger than a third of the depth, and at least 2 inches from the edges. Never notch the bottom edge in the middle of the span. Engineered joists follow the printed rules on the web instead.
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Updated 09/03/2026