Inputs
Tip: Change any value and press Calculate. Results include pass or fail checks for bending and deflection, plus allowable span by joist size.
Results
Pending
- Allowable span for selected size: ft
- Bending check: PASSFAIL
- Deflection check: PASSFAIL
- Recommended action:
| Joists required | — |
|---|---|
| Total linear feet | — |
| Total board feet | — |
| Estimated weight | — |
| Estimated cost | — |
Allowable span by joist size (current loads & spacing)
Detailed values
| Span input (ft) | Spacing (in) | Size | Species | Allowable span (ft) | Bending OK | Deflection OK | Joists | LF total | BF total |
|---|
Example scenarios
These examples use typical loads of 10 psf dead and 40 psf live with 16 in spacing. Always verify local requirements.
| Room width (ft) | Span (ft) | Spacing (in) | Size | Allowable span (ft) | Pass? | Joists |
|---|
How to use this calculator
- Enter the clear span along the joists and the room width across the joists.
- Choose joist spacing and size. If needed, pick a different species or enter custom E and Fb in Advanced.
- Adjust dead and live loads as required by your use and local code.
- Click Calculate. Review PASS/FAIL for bending and deflection, allowable span, quantity, board feet, and weight.
- Use the chart to compare sizes, and export your data to CSV or PDF.
Disclaimer: Results are approximate and for planning only. Consult local building codes and a qualified professional before construction.
Formulas used
For a simply supported joist with uniform load:
- Uniform line load per joist:
w = (DL + LL) × s / 12(lb/ft), whereDLandLLare psf andsis spacing (in). - Convert to lb/in:
win = w / 12. - Maximum moment:
M = win L2 / 8(lb‑in). - Bending stress check:
M / S ≤ Fb, whereS = b h2 / 6. - Deflection:
Δ = 5 win L4 / (384 E I), whereI = b h3 / 12. - Allowable deflection limit:
Δ ≤ L / R(e.g.,R = 360for floors). - Allowable span by bending:
L = √(8 Fb S / win). - Allowable span by deflection:
L = ³√((384 E I)/(5 win R)).
Section properties use dressed sizes (in): b = 1.5; h = 5.5, 7.25, 9.25, 11.25 for 2×6, 2×8, 2×10, 2×12.
FAQs
Commonly 10 psf dead and 40 psf live, but verify with your local building code and use.
It means mid‑span deflection should not exceed span divided by 360. Stricter limits like L/480 reduce flex and floor bounce.
Yes. Select 19.2 in spacing to align with sheathing panels laid out for five spaces over eight feet. Check allowable span results.
We use typical values for E and Fb for No.2 grade. Lumber varies by source and grade. Enter custom values if you have certified data.
No. It is a planning tool. Always confirm with code‑approved span tables or a licensed professional.
We divide room width by spacing in inches and add one for the far edge. This approximates field practice; adjust for framing details.