Floor Joist & Rafter Span Calculator
Pick floor joist, ceiling joist or rafter, your lumber and spacing. See how far a 2x6 to 2x12 can span, or enter your span and get the smallest size that works, with a 3D framing bay that turns green, amber or red.
- Allowed span—
- Your span—
- Joist—
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Joists turn green under the limit, amber within 15% of it and red over it · drag to turn · scroll or pinch to zoom
How this span calculator works
The method: the IRC span tables are built from the American Wood Council's Span Tables for Joists and Rafters. Each span is the smaller of two limits. Deflection: the member may sag no more than span divided by 360 for floors, 240 for ceiling joists and for rafters with a ceiling attached, and 180 for rafters with no ceiling attached, under the live load only. Bending: the live plus dead load must not stress the wood past its design value. This tool runs those same two checks with the AWC design values for No. 2 grade lumber (Table W-1) at each size and spacing, rounded to the nearest inch as the tables do.
Which IRC table: living-area floors use Table R502.3.1(2) (40 psf live, 10 psf dead). Sleeping rooms use Table R502.3.1(1) (30 psf live, 10 psf dead). Ceiling joists use Tables R802.4.1(1) and (2). Rafters use the R802.5.1 tables for 20 psf roof live load and for ground snow loads. The exact table numbers shift between code editions, so confirm against the edition your city enforces.
What is not covered: notches and holes, cantilevers, openings for stairs, point loads from posts or tubs, tile or concrete floors, spans over 26 ft, and any lumber below No. 2 grade. Snow loads vary by site, so ask your building department for the design snow load. Engineered lumber such as I-joists, LVL and open-web trusses is rated by its manufacturer and has its own span charts.
Tips before you build
- Stiffness: a floor that passes the table is code-legal, but near the limit it can feel bouncy. Dropping to 12 in spacing or going up one size makes a noticeably stiffer floor.
- Bearing: joists need at least 1.5 in on wood or metal and 3 in on masonry (IRC R502.6), plus hangers or ties where the joist ends at a beam.
- Blocking and bridging: joists deeper than 2x12 need blocking or bridging, and many builders add a row at mid-span of long floors to share the load.
- Permits: framing changes usually need a permit. Ask your building department before you cut anything.
This tool is a planning aid, not a design. The span tables are for new, dry-use, grade-stamped lumber in a typical house. Confirm sizes with your building department or a licensed engineer.
Sources
- 2021 International Residential Code, Chapter 5 (Tables R502.3.1(1) and (2) floor joist spans)
- 2021 International Residential Code, Chapter 8 (R802.4.1 ceiling joists, R802.5.1 rafters)
- American Wood Council, Span Tables for Joists and Rafters (AWC STJR 2021, the source of the IRC tables and the method used here)
- American Wood Council, Design Values for Joists and Rafters (Table W-1, No. 2 grade E and Fb values)
- Southern Forest Products Association, Maximum Spans: Southern Pine Joists and Rafters (cross-check for southern pine)
Estimates only — not engineering, legal or code advice. Check requirements with your local building department and a licensed professional before you build.
Common questions
How far can a 2x10 floor joist span?
With southern pine No. 2 at 16 in on center in a living area (40 psf live load, 10 psf dead load), a 2x10 can span 14 ft 0 in. At 12 in on center it reaches 16 ft 2 in and at 24 in on center 11 ft 5 in. In a sleeping room (30 psf live load) the same 2x10 at 16 in reaches 15 ft 8 in. These follow the method behind IRC Tables R502.3.1(1) and (2).
What size joist do I need for a 12 ft span?
For a living-area floor in southern pine No. 2 at 16 in on center, a 2x8 reaches 11 ft 10 in, which is just short of 12 ft, so a 2x10 (14 ft 0 in) is the smallest that works. At 12 in on center a 2x8 reaches 13 ft 6 in and is enough.
Does the spacing change the span a lot?
Yes. A southern pine No. 2 2x10 living-area joist spans 16 ft 2 in at 12 in, 14 ft 0 in at 16 in, 12 ft 10 in at 19.2 in and 11 ft 5 in at 24 in on center. Closer spacing also makes the floor feel stiffer.
Are floor joist spans measured from the center of the beam?
No. The tables use the clear span, the distance from the face of one support to the face of the next. Joists need at least 1.5 in of bearing on wood or metal and 3 in on masonry or concrete (IRC R502.6).
How far can a 2x6 ceiling joist or rafter span?
A southern pine No. 2 2x6 ceiling joist at 16 in on center, over an attic without storage (10 psf live, 5 psf dead), spans 16 ft 11 in. A 2x6 rafter at 16 in with a 20 psf roof live load and 10 psf dead load spans 12 ft 11 in measured on the horizontal projection, and 11 ft 2 in under a 30 psf snow load.
Do these tables work for I-joists or LVL?
No. I-joists, LVL, glulam and open-web floor trusses are engineered products rated by their manufacturer. Use the manufacturer's span chart or have the framing designed. Dimensional lumber tables also do not cover cantilevers, large openings, point loads or heavy tile and concrete floors.