Keep beam deflection under span/250 generally, span/360 where people will notice a bouncy floor.
Serviceability limits from building codes. A 4 m joist may sag 16 mm at span/250, or no more than ~11 mm at span/360 where people will feel it — stiffness governs long before strength does.
A beam under a brittle plaster ceiling can crack at span/400, while a garden pergola at span/150 is perfectly serviceable. The limit is set by what the movement damages or who notices it, not by the beam — span/250 is a default for when nobody has told you which case you are in.
Estimate with the rule, then check it against the calculator that models it properly.
Open Beam Deflection & Stress Calculator →