Read this before you use the number below: These are rounded, commonly-published reference figures for a standard residential deck load (40 psf live + 10 psf dead, L/360 deflection) — not a calculation of your specific joists, grade, or local code. Actual allowable spans depend on exact grade, moisture condition, and amendments your jurisdiction may have made. For anything you're actually going to build, pull the current span table from the
American Wood Council's DCA6 guide (free PDF) or your local building department, or have a permit-holding contractor or engineer confirm it.
Why this isn't just a bigger table
Real span tables (like AWC's DCA6, which most US building departments reference for deck construction) break spans down by exact grade, wet-service vs. dry-service lumber, cantilever length, and multiple-span conditions — dozens of combinations per species. Compressing that into a quick lookup means picking one representative grade and load case and rounding generously, which is exactly why this tool says "estimator," not "calculator." It's meant to answer "is a 2×10 in the right ballpark for this span, or do I need a 2×12" before you go pull the real table — not to be the last word.
What actually changes your real answer
- Grade: No.1 and Select Structural grades span further than No.2; a lower grade (No.3, stud grade) spans less.
- Wet vs. dry service: Decks are exterior, wood-to-weather structures — tables for wet service conditions are typically more conservative than interior floor tables, which is part of why deck joist spans look shorter than floor joist spans of the same size.
- Cantilevers: Overhanging the beam past the last support changes the math and typically shortens the allowable back-span.
- Local amendments: Snow load, seismic considerations, and local code amendments can all tighten these numbers further depending on where you live.