Structural Calculators
Sizing a structural member is almost never a calculation — it is a table lookup, and the whole job is knowing which table applies.
The International Residential Code carries several span tables that look interchangeable and are not. A deck joist falls under Table R507.6, at 40 psf live load and wet service. A floor joist inside the building envelope falls under R502.3.1, at 30 or 40 psf depending on whether the room is a sleeping area, and dry service — which yields longer spans for the identical piece of lumber. Rafters and ceiling joists are R802.4 and R802.5, and they bring snow load into it. Pick the wrong table and you get an answer that is confidently wrong by feet.
That is the thing most calculators will not tell you. A tool that reports a span without naming its table has hidden the only decision that mattered. Every page here names the table, prints it in full, and states the load assumptions it was built on, so you can see whether your situation is the one the table describes.
Where the tables stop
Prescriptive tables cover ordinary conditions and nothing else. A hot tub, a planter wall, a roof over the deck, a species not in the list, a load above 40 psf, or a second storey all put you outside the table and into engineered design. Each calculator says where its edge is rather than extrapolating past it — a span table that silently interpolates is more dangerous than one that refuses.
Local amendments are the other limit. Jurisdictions modify these sections routinely; Rapid City amends the stair section outright. Adoption of a given IRC edition is state by state and county by county, so treat every number here as a planning figure to confirm with your building department.