About the Joist Span Calculator
This joist span calculator looks up the maximum distance a 2x6, 2x8, 2x10 or 2x12 joist may span between supports, and checks the span you are planning against it. Choose a deck or an interior floor, the lumber species group and the on-center spacing; for decks it also gives the allowable cantilever (overhang) beyond the beam.
Deck spans come from Table 2 of the American Wood Council’s DCA 6 Prescriptive Residential Wood Deck Construction Guide, the same values used in IRC Table R507.6: No. 2 grade, 40 psf live load plus 10 psf dead load, wet service and an L/360 deflection limit. Floor spans come from Table R502.3.1(2) of the 2012 IRC (the edition Public.Resource.Org publishes in full; check the edition your jurisdiction has adopted) for residential living areas: No. 2 grade, 40 psf live load, 10 psf dead load and L/360.
Use it to choose joist size and spacing when planning a deck, a floor framing layout or an addition. Spans are clear spans between the faces of supports. Heavy loads such as hot tubs, tile over large areas, or snow loads above 40 psf on decks require engineered design, and your local building department may have adopted a different code edition or amendments.
How to use the joist span calculator
- 1Choose whether the joists support a deck or an interior floor.
- 2Pick the lumber species group printed on the grade stamp.
- 3Select the joist size and on-center spacing you plan to use.
- 4Enter the clear span between supports to check it.
- 5If it is over-spanned, try a deeper joist, tighter spacing or an extra beam.
Formula and method
Joist spans are table look-ups rather than a single formula. The tables were calculated from National Design Specification bending strength and stiffness values for each species and grade, so that the joist neither overstresses in bending nor deflects more than span/360 under the design load. Deck tables also assume wet service (lower design values) and, for Douglas fir-larch, hem-fir and SPF, incising for preservative treatment.
For decks, the allowable cantilever beyond the beam is the smaller of the table overhang (based on a 220 lb point load at the tip and span/180 deflection) and one-quarter of the actual back span. This calculator uses your planned span for the L/4 limit when it is within the allowable span. Spans assume No. 2 grade lumber, uniformly loaded, simply supported joists.
- L
- Joist span between the faces of supports
- LO
- Table maximum overhang (cantilever) for the size and spacing
Worked examples
Southern pine 2x8 deck joists at 16 in, 11 ft span
DCA 6 Table 2 allows Southern pine 2x8s at 16 in on center to span 11′-10″. An 11 ft span leaves 10 in to spare. The overhang limit is the lesser of 2′-0″ and 11 ÷ 4 = 2′-9″, so 2′-0″.
Cedar 2x10 deck joists at 24 in, 12 ft span
Redwood and western cedars are less stiff, so 2x10s at 24 in may only span 10′-7″. A 12 ft span is about 1′-5″ too long; moving to 16 in spacing (13′-0″) would work.
Douglas fir 2x10 floor joists at 16 in
IRC Table R502.3.1(2) lists 15′-5″ for Douglas fir-larch No. 2 2x10s at 16 in on center under a 40 psf live and 10 psf dead load, so a 14′-6″ span passes with 11 in to spare.
Hem-fir 2x12 floor joists at 24 in
At 24 in on center, hem-fir No. 2 2x12s may span 14′-4″, so a 15 ft span is 8 in too long. Dropping to 19.2 in spacing (16′-1″) solves it.
Frequently asked questions
How far can a 2x8 deck joist span?+
Under DCA 6 and the IRC, a No. 2 Southern pine 2x8 at 16 in on center can span 11′-10″; Douglas fir-larch, hem-fir or SPF can span 11′-1″; and redwood or western cedar 10′-7″, assuming 40 psf live load.
How far can a 2x10 floor joist span?+
For a living-area floor (40 psf live, 10 psf dead, L/360) a No. 2 Douglas fir-larch 2x10 at 16 in on center may span 15′-5″, hem-fir 15′-2″ and spruce-pine-fir 15′-5″ per IRC Table R502.3.1(2).
Why are deck joist spans shorter than floor joist spans?+
Deck lumber is assumed to be wet in service, which lowers its design strength and stiffness, and preservative-treated Douglas fir, hem-fir and SPF are incised, which reduces their values further.
How far can deck joists cantilever past the beam?+
DCA 6 limits the overhang to the smaller of the table value (for example 2′-0″ for Southern pine 2x8s at 16 in) and one-quarter of the actual joist span. Longer cantilevers need engineering.
Is joist span measured center to center?+
In DCA 6 and the IRC prescriptive tables, span is the clear distance between the faces of the supports, not including any overhang. Bearing length at each end is in addition to the span.
Does a hot tub change the joist span?+
Yes. The tables assume 40 psf live load; a filled hot tub with people in it typically puts several times that load on its footprint, so its support must be designed by an engineer or placed on a separate slab.