Establish structural spacing first
Joist spacing is not chosen from material quantity alone; species, grade, size, span, loads, decking, and local requirements matter.
Use the applicable design table or engineered design before counting members.
Count joist spaces
Divide the framed width perpendicular to the joists by the maximum on-center spacing and round spaces up.
Add one joist to convert spaces into boundary-to-boundary joist lines for a simple rectangular frame.
Add special framing
Stairs, guard posts, beam details, openings, and picture-frame decking can require extra or doubled joists.
Count these explicitly instead of increasing the base count by an unexplained percentage.
Convert to stock lengths
Joist count and joist length are separate purchasing decisions.
Account for bearing, cantilevers, splices, and available lumber lengths when building the stock list.
Verify before ordering
A takeoff does not establish structural capacity.
Confirm the framing plan, connectors, treatment, fasteners, and local code requirements before construction.