Pitch & Span

Joist Span Calculator

This tool looks up the maximum allowable floor joist span from published American Wood Council and Southern Forest Products Association span tables, by species, grade, size, and spacing — for planning reference, not as a substitute for an engineered design.

This is reference information from published span tables, not an engineered design. Every span shown must be verified against your local building code and, where required, a licensed engineer or building official before any joist is installed.

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      Joist Span Calculator

      This tool looks up the maximum allowable floor joist span from published American Wood Council and Southern Forest Products Association span tables, by species, grade, size, and spacing — for planning reference, not as a substitute for an engineered design.

      Scenario presets

      Douglas Fir-Larch and Southern Pine No.2 are the most common framing lumber grades in the U.S.

      Nominal dimension lumber size.

      16" on center - Standard residential · 24" on center - Lighter loads or engineered spacing.

      The distance this joist needs to clear, wall to wall or support to support.

      Roof rafter profile diagram Live-updating roof profile showing run, rise, rafter length, and cut angles.
      Allowable span 15'-5"
      Your span
      14 ft
      Status
      Within published span

      40 psf live load, 10 psf dead load, L/360 deflection — the standard residential floor-joist ("living areas") load case. Sleeping rooms, decks, and other load cases use different published spans not covered here.

      How it works

      Unlike the geometry calculators on this site, joist span is not computed from a formula — it is looked up directly from published span tables, which come from engineering analysis of each species/grade combination's bending strength and stiffness.

      • Species and grade — determines the lumber's design values (bending strength, stiffness).
      • Size — deeper joists (2x12 vs. 2x8) allow longer spans at the same species/grade.
      • Spacing — wider spacing (24" vs. 16" on center) reduces the allowable span for the same size.
      • Load case — every span shown assumes 40 psf live load, 10 psf dead load, L/360 deflection — the standard residential floor "living areas" case.

      Full span reference table

      40 psf live load, 10 psf dead load, L/360 deflection.

      Species/Grade Size 16" o.c. 24" o.c.
      Douglas Fir-Larch No.2 2x8 12'-7" 10'-3"
      Douglas Fir-Larch No.2 2x10 15'-5" 12'-7"
      Douglas Fir-Larch No.2 2x12 17'-10" 14'-7"
      Southern Pine No.2 2x8 11'-10" 9'-8"
      Southern Pine No.2 2x10 14'-0" 11'-5"
      Southern Pine No.2 2x12 16'-6" 13'-6"

      Sources: Southern Forest Products Association, Maximum Spans: Southern Pine Joists & Rafters, 2013 Edition (New Design Values, effective June 1, 2013), Table 2 · American Wood Council, Span Tables for Joists and Rafters, 2021 Edition, Table F-2, cross-referenced with Design Values for Joists and Rafters, 2021 Edition, Table W-1

      Worked example

      A floor needs to span 13 ft between supports. The framer has Southern Pine No.2 2x10 in stock and plans 16" on-center spacing.

      Look up the allowable span. Southern Pine No.2, 2x10, 16" o.c. = 14'-0" per the reference table above.

      Compare. 13 ft desired ≤ 14'-0" allowable — within the published span.

      If the same floor needed to span 15 ft instead, it would exceed the 14'-0" allowable for this size — the calculator would flag it, and the fix would be a deeper joist (2x12, allowing 16'-6" at 16" o.c.), tighter spacing, a mid-span beam, or an engineer's alternative design.

      Tips from the field

      • Confirm the actual species and grade stamped on your lumber — yards sometimes substitute species based on regional availability.
      • Measure span as the clear distance between supports, not including bearing length on each end.
      • When a span is close to the published limit, err toward the next size up rather than cutting it exactly to the allowable number.
      • Always check with your local building department — some jurisdictions adopt amended span tables or additional load requirements.

      What this tool does not do

      • Does not replace an engineer's stamped design for anything outside a standard residential floor load case.
      • Does not cover rafters, decks, sleeping-room-specific loads, or engineered lumber (I-joists, LVL) — each uses different tables.
      • Does not cover species/grades beyond Douglas Fir-Larch No.2 and Southern Pine No.2, or sizes beyond 2x8-2x12.

      For rafter geometry (a different structural element with its own span considerations), see the Rafter Length Calculator.

      Did this calculator help you?

      Frequently asked questions

      These are the questions that come up when a published span table needs to become an actual framing decision — and where this tool's reference numbers stop and an engineer's judgment starts.

      Where do these span numbers come from?

      Southern Pine No.2 spans come from the Southern Forest Products Association's "Maximum Spans: Southern Pine Joists & Rafters" (2013 Edition, current design values). Douglas Fir-Larch No.2 spans come from the American Wood Council's "Span Tables for Joists and Rafters" (2021 Edition), cross-checked against AWC's own design values.

      What load case do these spans assume?

      40 psf live load and 10 psf dead load, with an L/360 deflection limit — the standard assumption for residential floor joists in "living areas" (as opposed to sleeping rooms, which sometimes use a different live load in some code editions).

      Is this calculator a substitute for an engineer or my local building department?

      No. This is reference information from published span tables, not an engineered design. Your local building code, any site-specific conditions, and load cases beyond the standard 40/10 psf assumption can all change what is actually allowed — always confirm with your local building official or a licensed engineer before framing.

      Why only two species/grades and three sizes?

      Douglas Fir-Larch No.2 and Southern Pine No.2 are the two most common structural framing grades used in U.S. residential construction, and 2x8/2x10/2x12 cover the sizes used for the great majority of floor joist spans. Other species, grades, and sizes exist in the full AWC tables but are not included here yet.

      What happens if my span exceeds the allowable number?

      You have a few standard options: step up to a deeper joist, tighten the spacing (e.g., from 24" to 16" on center), add a mid-span beam or wall to break up the span, or consult an engineer for an alternative (engineered lumber, steel, or a custom design).

      Does 24" spacing always mean a shorter allowable span than 16"?

      Yes, for the same size, species, and grade — wider spacing puts more load-sharing responsibility on each joist, which reduces the allowable span.

      Why does Douglas Fir-Larch allow a longer span than Southern Pine at the same size?

      The two species have different design values (bending strength and stiffness) at the same nominal grade — Douglas Fir-Larch No.2 has a higher modulus of elasticity in the published AWC design-value tables, which increases its allowable span.

      Does this apply to roof rafters as well as floor joists?

      No — this table is specifically for floor joists under the 40/10 psf living-area load case. Rafters use different load assumptions (snow load, roof dead load) and different deflection limits (L/240 or L/180), and are not covered by this calculator.

      Can I use these spans for a deck?

      No — decks typically use a different live load and are governed by separate deck-specific span tables (and often different lumber, like pressure-treated Southern Pine). Do not apply these interior floor-joist numbers to an exterior deck.

      Why does the calculator round the allowable span down to a whole inch?

      Because that is how the source tables themselves publish the figures — to the nearest inch, not a fraction. There is no additional rounding happening on top of the published table value.

      What if I'm using an engineered joist (I-joist, LVL) instead of dimension lumber?

      Engineered lumber spans come from the manufacturer's own published span tables, not the AWC/SFPA sawn-lumber tables used here — check your specific product's literature.

      Is "No.2" grade the only option?

      No.2 is the most commonly available structural grade at lumber yards and the one most published tables default to. Higher grades (Select Structural, No.1) allow longer spans; this calculator currently covers No.2 only.

      How we checked this page

      Written by
      Pitch & Span Editorial
      Reviewed by
      Span values checked directly against the Southern Forest Products Association's 2013 Southern Pine span table and the American Wood Council's 2021 Span Tables for Joists and Rafters and Design Values for Joists and Rafters.
      Last reviewed
      2026-07-28
      Publisher
      Pitch & Span
      Scope
      Residential floor joist allowable span reference (not an engineered design)
      What we checked
      • Southern Pine No.2 spans verified against SFPA's primary-source 2013 table.
      • Douglas Fir-Larch No.2 spans cross-checked against AWC's 2021 design values for consistency.
      • Deflection limit (L/360) confirmed against AWC's own published Table F-2 header.
      • Worked example (13 ft span, Southern Pine No.2 2x10, 16" o.c.) verified against the live calculator.

      Methodology

      Cite this calculator

      Pitch & Span. "Joist Span Calculator." Pitch & Span, https://pitchandspan.com/joist-span-calculator/. Accessed .

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