Joist Span Calculator
Maximum joist spans by species, size and spacing — with which limit actually governs, and the notching and boring rules for what comes after.
Deeper, or doubled up?
Stiffness goes with depth cubed and only linearly with width, so one deeper joist usually beats two shallow ones. But nominal sizes are not evenly spaced, and the gaps close as they get bigger.
What you can cut, and what fits through
IRC R502.8.1 for sawn lumber. Engineered joists follow the manufacturer instead — cutting the flange of an I-joist anywhere ruins it.
Three things the rules of thumb get wrong
Depth is cubed — but the sizes get closer together. Stiffness rises with the cube of depth and only linearly with width, so going one size deeper normally beats adding a second ply. Normally, not always: a 2×8 is 32% deeper than a 2×6 and a 2×10 is 28% deeper than a 2×8, but a 2×12 is only 22% deeper than a 2×10. Doubling up always buys exactly 2×. So deeper wins only while the next size is more than 26% deeper — the cube root of two — and that stops being true after the 2×10. Going deeper is 34–39% cheaper in timber either way.
"Deflection governs, not strength" is only half true. Run the numbers on Douglas Fir-Larch No. 2 at 16″ centres and deflection governs the 2×6 — and bending governs the 2×8, 2×10 and 2×12. The rule of thumb survives because the two limits sit within a few per cent of each other across the whole residential range, which is exactly the kind of nearly-right that hides being wrong. Shear, meanwhile, never comes close: it allows two to three times the governing span.
And on a 2×6, the hole limit isn't the one-third rule. Bored holes may be a third of the depth and must stay 2″ clear of both edges. On a 2×6 that leaves 1½″ of working height — less than the 1.83″ a third of the depth would allow. The two rules are usually quoted separately and the tighter one is easy to miss. It has a blunt consequence: pipe is named by its bore, not its outside diameter, so 1½″ waste is really 1.900″ and will not go through a 2×6 at all. A 3″ drain needs a 2×12 — a 2×10 won't take it.
- L/360 is not a comfort standard. On a 15 ft span it permits half an inch of live-load sag. A floor can pass inspection and still feel lively — L/480 is what people use when it has to feel solid.
- Deflection goes with the fourth power of span. Double the span and you get sixteen times the sag. Shortening the span with a mid-span beam does more than anything you can do to the joists themselves.
- Doubling a joist loses the repetitive-member credit. Three or more members at 24″ centres or less share load through the decking and get a 15% bump on bending. A built-up beam is one member and doesn't — so two joists are worth less than twice one.
- Deck framing spans less than the same timber indoors. The wet-service factor takes 15% off bending and 10% off stiffness before anything else is considered.
- Southern Pine works differently. Its published values are already depth-specific, so no size factor is applied on top — whereas every other species here gets one, ranging from 1.3 on a 2×6 down to 1.0 on a 2×12.
How to use
- Pick the size, species and spacing you are working with.
- Choose the use — bedrooms and attics carry lighter loads than living areas.
- Read which of the three limits governs, since that tells you what to change.
- Check the notching and boring table before anyone runs pipe through it.
Frequently asked questions
Is it better to double a joist or go one size deeper?
Usually deeper, but not always, and the crossover is exact. Stiffness rises with the cube of depth and only linearly with width, so a second ply always buys precisely twice the stiffness while going deeper buys the depth ratio cubed. Deeper therefore wins only while the next size is more than 26 per cent deeper — the cube root of two. A 2x8 is 32 per cent deeper than a 2x6 and a 2x10 is 28 per cent deeper than a 2x8, so deeper wins both times. A 2x12 is only 22 per cent deeper than a 2x10, so doubling wins there.
Does deflection or strength govern a floor joist?
It depends on the size, which is not what the rule of thumb says. For Douglas Fir-Larch No. 2 at 16 inch centres carrying a normal floor load, deflection governs the 2x6 and bending governs the 2x8, 2x10 and 2x12. The rule of thumb survives because the two limits sit within a few per cent of each other across the whole residential range — nearly right in exactly the way that hides being wrong.
Does shear ever limit a joist?
Effectively never in residential floor framing. Shear typically allows two to three times the governing span, so it is not worth spending attention on. It starts to matter for short beams carrying heavy loads, where the span is too short for bending to build up but the reaction at the support is still large.
What does L/360 actually allow?
The span divided by 360, measured under live load only — so half an inch of sag on a 15 foot span. It is a limit derived from what plaster and finishes tolerate rather than from what feels solid underfoot, and it says nothing about how the floor responds to someone walking across it. A floor can pass inspection and still feel lively, which is why L/480 is common where the floor has to feel right rather than merely comply.
Why does my floor bounce even though it passes?
Because bounce is a dynamic response and the deflection limit is a static one. They are related but not the same, and nothing in the span check addresses the frequency at which the floor wants to vibrate. Deeper joists help most, then a shorter span, then blocking and a stiffer subfloor. Adding a second ply of the same depth helps least of the options, which is unfortunately the one people usually reach for first.
How much difference does joist spacing make?
Going from 24 to 16 inch centres adds roughly a foot to a typical span, and 12 inch centres adds a little more again. It is a smaller lever than depth, because halving the spacing halves the load per joist while doubling the depth multiplies stiffness by eight. Spacing is usually decided by the sheathing rather than the joists in any case.
Can I bore a hole through a joist for plumbing?
Within limits, and there are two of them that interact. The hole may be up to a third of the joist depth, and it must also stay two inches clear of both the top and bottom edges. On a 2x6 the edge clearance leaves only an inch and a half to work in, which is less than the 1.83 inches a third of the depth would allow — so the tighter rule is the one that applies, and it is easy to miss because the two are usually quoted separately.
What size pipe fits through a joist?
Less than the name suggests, because pipe is sized by its bore while a hole has to clear the outside diameter. A "1½ inch" waste line is actually 1.900 inches across, so it will not go through a 2x6 at all. A 2x8 takes up to 2 inch waste at 2.375 inches. A 3 inch drain is 3.500 inches across, which needs a 2x12 — a 2x10 will not take it, since its limit is 3.08 inches.
How deep can I notch a joist?
A sixth of the depth, no longer than a third of the depth, and never in the middle third of the span where the bending stress is highest. At the very ends, where the bending stress is nearly zero, a quarter of the depth is permitted. None of this applies to engineered joists: an I-joist or an LVL is cut only where the manufacturer allows, and cutting the flange of an I-joist anywhere at all ruins it.
Why does a doubled joist span less than twice as far?
Two reasons, and the second surprises people. Span rises with the square root of strength rather than proportionally, so twice the section is not twice the span. And doubling loses the repetitive-member credit: three or more joists at 24 inch centres or less share load through the decking and get a 15 per cent bump on bending, while a built-up beam counts as one member and gets nothing.
Do deck joists span as far as floor joists?
No. The wet-service factor takes 15 per cent off the bending value and 10 per cent off stiffness before anything else is considered, so the same timber spans noticeably less outdoors. Deck framing also has to deal with a lateral load at the ledger and with connections that live in the weather, neither of which is a span question at all.
Can I build from these numbers?
Verify them against the span table your jurisdiction has adopted first. These are calculated by the NDS method the published tables are derived from, and they land within a couple of per cent — but they are sometimes marginally longer, and where the two differ the adopted table governs. Bearing length, hangers, blocking, and anything carrying a point load or a wall above are separate questions that a span figure does not answer.
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