The maximum Span Calculator for Wood Joists and Rafters can be found on the RoofCalc.org website.
The allowable lengths of joists and rafters can be determined using the span tables below.The American Wood Council website has a Wood Beam Calculator that can be used to determine maximum rafter and joist lengths.
There are different lengths and sizes of joists depending on the lumber used.You can span a #1 grade no knots southern yellow pine a greater distance than a #2 grade with knots.
If you can use fewer or smaller #1 grade joists or rafters in your design, you may be able to save money.
Rafter spans can be extended slightly beyond what the rafter tables suggest.
This table can be used to determine the maximum lengths of the ceiling.
The maximum lengths of the floor can be determined using this table.