How to use the joist span calculator
Early in a project you need to know whether a room can be spanned with ordinary sawn joists, what depth to allow for in the section, and where a beam or bearing wall has to go. Code span tables answer that, but they run to dozens of pages. This tool gives you the same kind of answer for the common cases in a few clicks.
- Pick what the members carry. The presets match the load cases used in the International Residential Code (IRC) tables for floors, ceiling joists and rafters. Choose custom to enter your own live load, dead load and deflection limit.
- Choose the species and spacing. All values are for No. 2 grade, the most common framing grade in North America.
- Enter your clear span between supports to see the smallest size that works. For rafters, enter the horizontal projection.
Each result card shows the maximum span in feet and inches and in metres, and which check limits it: bending strength, deflection or shear.
How spans are worked out
A joist is a simple beam carrying a uniform load. The calculator finds the longest span that passes all three checks below, using the design values in the NDS Supplement for No. 2 lumber, with the repetitive member factor (1.15) and size factors applied, and a load duration factor of 1.15 for snow and 1.25 for roof live load.
Load per joist w = (Live + Dead) × Spacing
Bending: w L² ÷ 8 ≤ F′b × S
Deflection: 5 wlive L⁴ ÷ (384 E I) ≤ L ÷ limit
Shear: w L ÷ 2 ≤ F′v × b × d ÷ 1.5
Shallow, closely spaced joists are usually limited by deflection; deeper joists under heavier load are often limited by bending. Shear rarely governs uniformly loaded joists, but it matters near point loads and notches.
| Nominal size | Actual size | Typical floor span, DF-L No. 2 at 16" o.c. |
|---|---|---|
| 2x6 | 1½" × 5½" | about 9'-9" |
| 2x8 | 1½" × 7¼" | about 12'-9" |
| 2x10 | 1½" × 9¼" | about 15'-7" |
| 2x12 | 1½" × 11¼" | about 18'-1" |
Worked examples
A 14-foot living room
With No. 2 Douglas fir-larch at 16 inches on centre and the standard 40/10 psf floor load, 2x8s reach about 12'-9", which is short of 14 feet. 2x10s reach about 15'-7", so they work with some margin. Allow about 9¼ inches plus the subfloor and ceiling in the section.
Wider spacing to save material
The same floor at 24 inches on centre in hem-fir needs 2x12s, which reach about 14'-4". You use a third fewer joists but need deeper ones and a thicker subfloor rated for 24-inch spacing.
Rafters under snow
In a 30 psf snow area with a ceiling attached, No. 2 Douglas fir-larch 2x8 rafters at 16 inches reach about 15'-4" of horizontal projection. A 28-foot-wide gable, 14 feet each side, works with 2x8s; a 34-foot one would need 2x10s or a mid-span purlin.
Who this calculator helps
- Architects setting floor zones, ceiling depths and bearing wall positions at the concept stage.
- Architecture students learning how span, spacing and depth relate in timber framing.
- Builders and framers checking a proposed joist layout before ordering.
- Deck and renovation designers sanity-checking spans when removing walls or adding rooms.
- Homeowners who want to understand a framing plan before talking to an engineer.
Tips and limits
- Use these results for early design. Your adopted code tables or a structural engineer have the final word, especially for point loads, stair openings, tile or stone floors and heavy partitions.
- Choose the next size up where floors carry stone, large tiles or bathtubs. A stiffer floor (L/480 or better) also stops tile cracking.
- Wet service, notches, holes and cantilevers all reduce capacity and are not covered here.
- Turn the chosen size into an order with the board foot calculator, set roof slopes with the roof pitch calculator and plan stair openings with the stair calculator.
Turning a framing plan into something a client can picture? Convert the drawing into a finished interior with Sketch to Render.
