Joist Span Calculator
This free joist span calculator answers both framing questions: "how far can this joist span?" and "what size joist do I need?" Floor spans are computed with the exact American Wood Council method behind the IRC tables — the L/360 live-load deflection limit and the NDS bending check with the repetitive-member factor — using NDS 2018 design values for Douglas fir-larch, Hem-fir, Southern pine (post-2013 SPIB values), and spruce-pine-fir, so the outputs reproduce IRC Tables R502.3.1(1) and R502.3.1(2) value for value.
Pick sleeping-area (30 psf) or living-area (40 psf) loading, standard or heavy dead load, then your species, grade, size, and spacing. Enter the clear span your floor must cross and the calculator checks pass/fail, shows what percentage of the allowable span you are using, and renders the full size-by-spacing table with every combination that carries your span highlighted. A deck mode looks up pressure-treated joist spans from IRC Table R507.6 with its wet-service reductions built in.
Free and no signup, with the governing limit named (deflection vs. strength) so you know whether a deeper joist or a better grade fixes a short span. These are the IRC prescriptive values for uniformly loaded single spans — point loads, cantilevers, and spans past the table belong to a design professional. When your joists work, size the rest of the frame with the Framing, Deck, or Subfloor calculator.
Joist Span Calculator
Look up the maximum allowable clear span for floor and deck joists — by species, grade, size, and spacing — straight from the IRC R502.3.1 floor tables and the R507.6 deck table. Enter your span and it checks pass/fail, shows what percentage of the limit you are using, and finds the smallest joist that works. Free, no signup.
What are you framing?
Floor use & loads
Lumber
Joist size & spacing
Your span (optional)
What the span tables are really saying
The IRC tables this calculator reproduces are built from two engineering checks, and misreading them is how joists end up undersized. These engineering-style diagrams show what clear span actually measures (and the bearing the code adds on top), which of the two limits — stiffness or strength — sets your span, and how little the species stamp moves a #2 joist.
The clear-span diagram is the first thing to get right: the tables answer the open air between supports, not the length of the stick. Each end also needs bearing per IRC R502.6, so a 14′-0″ clear span means 16-ft lumber.
The two-checks diagram is the engine of the whole calculator. Every tabulated span is the smaller of a deflection limit (L/360 — the bounce test) and a bending-strength limit — and because stiffness grows with depth³, going one joist size deeper buys far more span than upgrading to the best grade.
The species chart answers “does it matter what the lumberyard stocks?” Less than most people think — the full spread across the four species is about a foot and a half — and Southern pine’s post-2013 design values mean older intuition about it leading the table is now backwards.
Calculation Formulas
A uniformly loaded simple-span joist deflects Δ = 5wL⁴/384EI. Setting Δ = L/360 under live load only and solving for L gives the stiffness-limited span — the limit that controls floor bounce.
Example:
SPF #2 2×10 at 16″ o.c., 40 psf: w = 40 × 16 ÷ 144 = 4.44 lb/in; L = ∛(384 × 1,400,000 × 98.93 ÷ (1800 × 4.44)) ≈ 188″ — but strength governs first (see below).
Maximum moment in a uniform simple span is M = wL²/8. Setting M equal to the adjusted bending capacity Fb′·S and solving for L gives the strength-limited span, computed on total (live + dead) load.
Example:
SPF #2 2×10 at 16″ o.c., 50 psf total: Fb′ = 875 × 1.1 × 1.15 = 1107 psi; L = √(8 × 1107 × 21.39 ÷ 5.56) ≈ 185″ = 15′-5″ — the governing (and published) value.
Whichever limit is smaller governs, matching the IRC table construction. High grades at close spacing are usually deflection-limited; low grades and wide spacing go strength-limited.
Example:
DFL SS 2×10 at 12″ o.c.: deflection gives 19′-1″, strength gives more — the table prints 19′-1″.
NDS adjustment factors as the IRC tables apply them: the size factor CF by depth, the 1.15 repetitive-member factor for three-plus joists sheathed together, and a 1.0 load-duration factor for floor live load.
Example:
DFL #2 2×8: Fb′ = 900 × 1.2 × 1.15 = 1242 psi. Southern pine skips CF — its published values are already size-specific.
Each joist carries a strip of floor as wide as its spacing. Converting psf and inch spacing to pounds per lineal inch feeds the two span formulas.
Example:
40 psf at 16″ o.c.: w = 40 × 16 ÷ 144 = 4.44 lb per inch of joist length (53.3 lb per foot).
Moment of inertia (stiffness) and section modulus (strength) from the actual dressed size — a 2×10 is really 1½″ × 9¼″. Depth enters cubed for stiffness, squared for strength: deeper beats wider.
Example:
2×10: I = 1.5 × 9.25³ ÷ 12 = 98.9 in⁴; S = 1.5 × 9.25² ÷ 6 = 21.4 in³.
IRC Table R301.5 sets 30 psf live load for sleeping rooms and 40 psf for other living areas. Dead load is the floor itself — 10 psf standard, 20 psf for heavy finishes like mortar-bed tile.
Example:
A bedroom floor designs at 30 + 10 = 40 psf total; a living-room floor with a mortar-bed tile finish at 40 + 20 = 60 psf.
Deck spans come straight from the code table rather than the dry-service engine, because exterior lumber takes wet-service reductions (CM) and incising reductions on treated Hem-fir/SPF that the table already builds in.
Example:
Southern pine #2 2×8 at 16″ o.c.: 11′-10″ per R507.6 — about 10% under the equivalent dry interior span.
How much of the allowable span your design uses. Passing at 98% is code-legal but leaves no margin; floors near the deflection limit feel springy even though they are safe.
Example:
A 14′-0″ span on a 15′-5″ allowable is 91% — legal, but one size deeper drops it to about 75% and noticeably stiffens the floor.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Deflection limit, floors | L/360 (live load) | IRC Table R301.7 limit for floor members — the basis of every span in R502.3.1. |
| Repetitive-member factor Cr | 1.15 | NDS 4.3.9 — applies to three or more members at ≤ 24″ o.c. joined by sheathing, as in every joisted floor. |
| Size factor CF (Fb) | 2×6: 1.3 · 2×8: 1.2 · 2×10: 1.1 · 2×12: 1.0 | NDS Supplement Table 4A adjustment for visually graded dimension lumber (not applied to Southern pine, whose values are size-specific). |
| Live loads | 30 psf sleeping / 40 psf living | IRC Table R301.5 minimum uniformly distributed live loads for residential floors. |
| Dead loads | 10 psf standard / 20 psf heavy finish | The two dead-load columns the IRC prints; 20 psf covers mortar-bed tile, stone, and doubled subfloor. |
| DFL #2 design values | Fb 900 psi · E 1.6×10⁶ psi | NDS 2018 Supplement Table 4A, Douglas fir-larch No. 2, 2″–4″ thick, 2″ & wider. |
| SPF #1/#2 design values | Fb 875 psi · E 1.4×10⁶ psi | Spruce-pine-fir No. 1 and No. 2 share identical design values in NDS Table 4A. |
| Southern pine #2 2×10 | Fb 800 psi · E 1.4×10⁶ psi | SPIB values effective June 1, 2013 — the post-2013 reduction that shortened many published SP span tables. |
| Minimum bearing | 1½″ wood/steel · 3″ masonry | IRC R502.6 — the ends of each joist need this much bearing surface. |
| Table span cap | 26′-0″ | IRC floor span tables stop at 26 feet; anything longer is engineered territory. |
| Dressed sizes | 2×6 = 1½×5½ · 2×8 = 1½×7¼ · 2×10 = 1½×9¼ · 2×12 = 1½×11¼ | PS 20 American Softwood Lumber Standard dressed dimensions used for all section properties. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
International Residential Code — Allowable Joist Spans(IRC R502.3.1, Tables R502.3.1(1) & R502.3.1(2))
View StandardThe prescriptive floor-joist span tables: Table (1) for sleeping areas and attics (30 psf live load), Table (2) for other living areas (40 psf), each with 10 and 20 psf dead-load columns, L/360 deflection, spans as clear span in feet-inches.
Key Requirements:
- •Sleeping areas: 30 psf live load; living areas: 40 psf (Table R301.5)
- •Deflection limited to L/360 under live load (Table R301.7)
- •Spans are clear spans between faces of supports
- •Tables assume repetitive members and uniform loads only
International Residential Code — Deck Joists(IRC R507.6 / Table R507.6)
View StandardSpan table for pressure-treated deck joists at 40 psf live / 10 psf dead load in wet service, including allowance for a cantilever up to one-fourth of the joist back-span.
Key Requirements:
- •40 psf live, 10 psf dead, wet-service design values
- •#2-grade lumber; incised treated Hem-fir/SPF grouped with DFL
- •Cantilever ≤ ¼ of the actual joist back-span
AWC Span Tables for Joists and Rafters(AWC STJR 2021)
View StandardThe American Wood Council document that defines the calculation procedure behind the IRC span tables — the exact deflection and bending formulas this calculator implements, with design values from the NDS Supplement.
Key Requirements:
- •Deflection span from Δ = 5wL⁴/384EI at the code deflection limit
- •Strength span from M = wL²/8 against Fb′ with NDS adjustments
- •Smaller of the two governs
National Design Specification for Wood Construction(ANSI/AWC NDS-2018 + Supplement)
View StandardReference design values (Fb, E) for visually graded dimension lumber by species and grade, and the adjustment factors (size CF, repetitive member Cr, load duration CD, wet service CM) the span calculations apply.
Key Requirements:
- •Table 4A design values for DFL, Hem-fir, SPF
- •Cr = 1.15 for three or more sheathed members at ≤ 24″ o.c.
- •CF by member depth for non-Southern-pine species
SPIB Southern Pine Design Values(SPIB / NDS Supplement Table 4B (effective June 1, 2013))
View StandardSouthern pine design values are published size-specific by the Southern Pine Inspection Bureau. The June 2013 revision lowered Fb and E for visually graded SP — spans from older tables can overstate current lumber by a full size.
Key Requirements:
- •Size-specific Fb (e.g. No. 2: 1000 psi at 2×6 down to 750 psi at 2×12)
- •E = 1.4×10⁶ psi for No. 2, all widths
- •No additional size factor — CF is built into the published values
IRC — Bearing(IRC R502.6)
View StandardJoist ends must bear at least 1½ inches on wood or metal and 3 inches on masonry or concrete, or hang in approved joist hangers.
Key Requirements:
- •1½″ minimum bearing on wood or metal
- •3″ minimum bearing on masonry or concrete
American Softwood Lumber Standard(PS 20-20)
View StandardDefines the dressed (actual) sizes of nominal dimension lumber — the 1½″ × 9¼″ reality of a 2×10 — used for every section property in the span math.
Key Requirements:
- •Nominal 2× stock dresses to 1½″ thick
- •Widths dress to 5½″, 7¼″, 9¼″, 11¼″
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Which species is on the shelf
Framing lumber is regional
The species you can actually buy varies by region: Southern pine dominates the Southeast, Douglas fir-larch the West, and SPF the North and big-box stores everywhere. The same 2×10 #2 spans 15′-7″ in DFL but 15′-5″ in SPF and 14′-0″ in post-2013 Southern pine at 16″ o.c. living-area loads.
Regional Examples:
Pre-2013 Southern pine tables still circulate
Old printouts overstate SP spans
SPIB lowered visually graded Southern pine design values effective June 2013, and the IRC tables from 2015 on reflect it. Older span cards, 2009/2012 IRC printouts, and many websites still show the old, longer spans — a #2 SP 2×10 at 16″ dropped from about 16′-1″ to 14′-0″.
Regional Examples:
Local live-load amendments
Some jurisdictions design all floors at 40 psf
The 30 psf sleeping-area allowance is an IRC baseline. Some states and cities amend residential floors to a flat 40 psf, and specific occupancies (home offices with heavy files, waterbeds historically) may warrant more.
Regional Examples:
Decks in snow country
Ground snow load can exceed the 40 psf deck table
IRC 2021 provides separate deck-joist span tables for 40, 50, 60, and 70 psf ground snow loads. Where ground snow exceeds 40 psf, the standard table this calculator uses is not conservative — spans shorten one to two sizes.
Regional Examples:
Old-house joists don't match modern tables
Full-dimension and ungraded lumber
Pre-1960s houses often have true 2″ × 10″ rough-sawn joists of old-growth timber — stronger and stiffer than modern dressed stock, but ungraded, so the tables technically don't apply. Attic conversions hit this constantly: the existing 2×6 or 2×8 "ceiling joists" were designed for 10–20 psf, not the 30 psf a bedroom floor needs.
Regional Examples:
Engineered lumber follows manufacturer tables
I-joists and LVL are outside the IRC tables
Once spans pass the solid-sawn tables (about 18–20 ft for deep joists), floors move to wood I-joists or LVL. Those span tables come from each manufacturer's ICC-ES evaluation report, not the IRC — spacing, bearing, web holes, and rim details all follow the manufacturer's literature.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
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How to Use This Calculator
- Pick the joist type: floor joists (interior, IRC R502.3.1) or deck joists (pressure-treated, IRC R507.6).
- Floor joists: choose the floor use — sleeping areas design at 30 psf live load, all other living areas at 40 psf — and the dead load (10 psf standard, 20 psf for heavy finishes like mortar-bed tile).
- Select the lumber species and grade from the grade stamp: SPF #2 is typical big-box stock in the North, Southern pine #2 in the South, Douglas fir-larch in the West.
- Pick the joist size (2×6 through 2×12) and on-center spacing (12", 16", 19.2", or 24").
- Enter the clear span your joists must cross — the open distance between the faces of the supports, not the joist length.
- Click Calculate to see the maximum allowable span, whether your span passes, the percentage of the limit you are using, and the full size-by-spacing table with passing combinations in green.
- If your span fails, read off the smallest size that works at your spacing — or a tighter spacing that saves the size you have.
Clear Span, Bearing, and What the Tables Assume
Every span in this calculator is a CLEAR span — the open distance a joist crosses between the faces of its supports. The joist itself is longer: IRC R502.6 requires at least 1½ inches of bearing on wood or steel (3 inches on masonry), so a rough length is clear span plus 3 inches, and lumber is bought in 2-foot increments. The tables also assume repetitive members — three or more joists at 24 inches or closer, tied together by subfloor — which is where the 1.15 bending bonus comes from, and uniformly distributed loads. A joist carrying a point load (a kitchen island, a tub, a post from above), a doubled joist under a partition, a cantilevered end, or a notched and drilled member is outside the tables and needs its own check. Deflection matters as much as strength: most passing floors that still feel bouncy are riding near the L/360 limit, and the cheap fix is one size deeper, not a higher grade.
Frequently Asked Questions
How far can a 2x10 floor joist span?
At 16 inches on center under living-area loads (40 psf live / 10 psf dead), a #2-grade 2×10 spans 15'-7" in Douglas fir-larch, 15'-2" in Hem-fir, 15'-5" in spruce-pine-fir, and 14'-0" in Southern pine, per IRC Table R502.3.1(2). Bedrooms (30 psf) allow roughly a foot more; 24-inch spacing takes two to three feet away. That spread is exactly why the calculator asks for species and grade before answering — there is no single "2×10 span."
What size floor joist do I need for a 14-foot span?
At 16 inches on center under 40 psf living-area loads, a 14'-0" clear span needs a 2×10 in Douglas fir-larch #2 (15'-7" allowable), Hem-fir #2 (15'-2"), or SPF #2 (15'-5") — but Southern pine #2 tops out at 14'-0" exactly, with zero margin. At 12-inch spacing a 2×10 works in every species; a 2×12 works everywhere with room to spare. Enter your span in the calculator and the size-by-spacing table shows every passing combination in green.
Are these the actual IRC span table values?
Yes. Floor spans are computed with the same procedure the American Wood Council uses to build the IRC tables — the L/360 live-load deflection limit and the NDS bending check with the 1.15 repetitive-member factor and size factors, on NDS 2018 design values — and the output was verified value-for-value against published IRC Table R502.3.1(2) rows. Deck spans come straight from IRC Table R507.6. Computing rather than transcribing also lets the calculator show WHY a span is limited: deflection (bounce) or bending strength.
What's the difference between the 30 psf and 40 psf tables?
IRC Table R301.5 assigns sleeping rooms a 30 psf design live load and all other living areas 40 psf, and the code prints a span table for each — R502.3.1(1) and R502.3.1(2). The lighter bedroom load buys roughly 6–12 inches of extra span for the same joist. Some jurisdictions amend residential floors to a flat 40 psf, so check locally before using the sleeping-area numbers. Attics matter here too: an attic with a fixed stair counts as habitable and needs at least the 30 psf table, which is the core structural question in any attic conversion.
Why does Southern pine span less than Douglas fir now?
Because the Southern Pine Inspection Bureau lowered visually graded Southern pine design values effective June 1, 2013, after in-grade testing — a #2 2×10's bending value dropped to 800 psi with E of 1.4 million psi, and the values became size-specific. The IRC tables from 2015 onward reflect it, but older charts still circulate showing SP out-spanning everything. This calculator uses the current SPIB values; if a table you find disagrees by more than an inch or two on Southern pine, it predates 2013.
Does span mean the length of the joist?
No — span is the CLEAR span: the open distance between the faces of the supports, measured face-to-face, not center-to-center and not the lumber length. The joist itself must be longer, because IRC R502.6 requires a minimum of 1½ inches of bearing on wood or steel and 3 inches on masonry or concrete at each end. As a rule of thumb, joist length ≈ clear span + 3 inches minimum, bought in the next even 2-foot increment.
My floor passes the table but feels bouncy — is it safe?
Almost certainly safe, but working near its deflection limit. The tables cap live-load deflection at L/360 — about half an inch over a 15-foot span — which is a code minimum, not a comfort guarantee; floors near 100% utilization can feel springy, and tile and stone want stiffer (L/480 to L/720 is a common tile-industry recommendation). The fix is stiffness, not strength: one size deeper cuts deflection roughly 40–50% (depth enters the math cubed), while a higher grade barely moves it. The calculator names the governing limit so you can see when you're deflection-bound.
Can I use this for deck joists, ceiling joists, or rafters?
Deck joists, yes — the deck mode uses IRC Table R507.6, which builds in the wet-service and incising reductions pressure-treated lumber needs outdoors (an exterior span runs about 10% shorter than the same joist indoors). Ceiling joists and rafters, no — those use their own tables (R802.5.1 and R802.4.1) with different loads and deflection limits, and roof spans depend on snow load. For a full deck frame — beams, posts, and hardware, not just joists — use the Deck Calculator, which applies the same R507.6 data plus the beam and footing tables.