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The unsupported distance the joist crosses between beam and ledger.
Measure face of support to face of support, not centre to centre — the bearing at each end is not part of the span. Where a joist runs over an intermediate beam it is two spans, not one, and the longer of the two governs.
Centre-to-centre spacing of the joists.
Spacing sets the strip of deck each joist carries, so closing it up is often cheaper than going deeper. Decking material may impose its own maximum: composite boards and diagonally laid timber frequently need closer support than the joists alone would require.
The total design load on the deck surface, live plus dead.
The residential deck live load in the adopted code plus the self-weight of decking, joists and any finish. Decks carrying a hot tub, a planter or a snow accumulation are governed by that concentrated case instead, and this uniform check does not cover it.
The bending design value for your species and grade, with adjustments applied.
Take the reference value from the NDS Supplement for the species, grade and size, then apply the adjustment factors that belong to a deck: wet service, load duration, repetitive member, and the size factor for the depth you end up with. Preservative-treated lumber often carries an incising factor as well.
The stiffness of the species and grade, from the same published table.
Deflection is what people feel underfoot, and it depends on stiffness rather than strength — two species of identical bending value can differ by a third in springiness. Use the reference modulus with the wet service adjustment applied, not the minimum modulus that belongs to stability checks.
The dressed thickness of the joist, across the grain.
The narrow dimension of the section, which for nominal two-inch framing is one and a half dressed. Doubling the breadth doubles the strength; going one size deeper does far more, which is why depth rather than breadth is the variable that gets adjusted.
The deflection limit applied to the joist under load.
L/360 is the usual floor criterion and is what most people expect underfoot. A long-span deck that satisfies L/360 can still feel lively, because comfort is governed by frequency and damping rather than by static deflection, and tightening the limit is the practical cure.
The dressed depth of the section you are considering.
The utilisation figures below are reported against this depth, so it is the field to move when you want to test a size rather than be told one. Dressed depths are appreciably less than nominal ones, and using the nominal figure here overstates capacity by a wide margin.
How far the joist runs past its outer support, if it overhangs.
A cantilever is limited to a quarter of the adjacent span, checked against the span the joist actually has rather than the span it could have had — and, under IRC R507.6, to the table's cantilever for the joist where that is shorter, which this page does not hold. Enter zero for a deck whose joists stop on the beam.
Required joist depth
7.64 in
The proposed section satisfies both checks, with deflection governing the required depth. Design values must be the adjusted ones — a reference value used without its wet service, load duration and size factors overstates a deck joist substantially.
- Depth required for bending
- 7.6 in
- Depth required for deflection
- 7.64 in
- Design bending moment
- 1,203 lbf·ft
- Line load carried by one joist
- 66.83 lbf/ft
- Bending capacity used by the proposed section
- 67.44 %
- Deflection allowance used by the proposed section
- 56.3 %
- Deflection of the proposed section
- 0.23 in
- Largest permitted cantilever
- 36 in
They open the calculator with your figures already in it
Deck Joist Span and Size Calculator: 7.64 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- ANSI/AWC NDS, National Design Specification for Wood Construction: allowable stress design for sawn lumber bending members, with reference design values and adjustment factors published in the NDS Supplement per species, grade and size
- Simple-span mechanics: bending moment M = wL²/8, required section modulus S = M/Fb′, and deflection Δ = 5wL⁴/(384EI) checked against the span divided by the deflection ratio
- IRC R507.6 (Deck joists) limits a deck joist's cantilever to one quarter of the adjacent joist span, or the shorter cantilever its Table R507.6 gives for the species, size and spacing, whichever is less; the deck live load is set by the adopted residential code and is entered here rather than assumed
Inputs used
- Clear Span Between Supports
- 12 ft
- Joist Spacing
- 16 in
- Combined Live and Dead Load
- 50.13 psf
- Adjusted Bending Design Value Fb′
- 1000.76 psi
- Modulus of Elasticity E
- 1399614.17 psi
- Joist Breadth
- 1.5 in
- Deflection Limit
- L/360
- Proposed Joist Depth
- 9.25 in
- Cantilever Beyond the Support
- 2 ft
Intermediate steps
- Depth required for bending
- 7.6 in
- Depth required for deflection
- 7.64 in
- Design bending moment
- 1,203 lbf·ft
- Line load carried by one joist
- 66.83 lbf/ft
- Bending capacity used by the proposed section
- 67.44 %
- Deflection allowance used by the proposed section
- 56.3 %
- Deflection of the proposed section
- 0.23 in
- Largest permitted cantilever
- 36 in
Confidence note: The proposed section satisfies both checks, with deflection governing the required depth. Design values must be the adjusted ones — a reference value used without its wet service, load duration and size factors overstates a deck joist substantially.
What this calculation does not cover
- Uniform load only. A hot tub, a large planter, a masonry feature or drifted snow is a concentrated case that governs on its own and is not covered here.
- The joist alone. Ledger attachment, beam and post sizing, footings, lateral load connections and guard posts are separate checks, and the ledger is where deck failures actually begin.
- Does not check bearing length at the supports, nor lateral stability of the compression edge, which relies on the decking and on blocking at the ends.
- The cantilever is checked against the quarter-span rule only. IRC R507.6 also caps it at the cantilever its Table R507.6 lists for the species, size and spacing, which can be shorter; read that table before building an overhang.
Add the equipment this sizes
This result is a specification — 7.64 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-25 · v1.0.0
Regulatory standards & verification citations3
- ANSI/AWC NDS, National Design Specification for Wood Construction: allowable stress design for sawn lumber bending members, with reference design values and adjustment factors published in the NDS Supplement per species, grade and size
- Simple-span mechanics: bending moment M = wL²/8, required section modulus S = M/Fb′, and deflection Δ = 5wL⁴/(384EI) checked against the span divided by the deflection ratio
- IRC R507.6 (Deck joists) limits a deck joist's cantilever to one quarter of the adjacent joist span, or the shorter cantilever its Table R507.6 gives for the species, size and spacing, whichever is less; the deck live load is set by the adopted residential code and is entered here rather than assumed
Which documents these citations point at
- International Residential Code — R507.6 (United States)One- and two-family dwellings and townhouses up to three storeys — structure, fire safety, energy, plumbing, mechanical and electrical in a single document.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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