What a 40ft Container is used for
A 40ft container suits general merchandise of medium density: furniture, household goods, textiles or mixed consumer products. It doubles the volume of a 20ft, but not the payload, so it is the better choice when cargo fills space before weight.
Cartons rarely fill the full 67.7 m³. Fixed carton sizes leave gaps against the walls and ceiling, and stacking limits, fragile goods and the payload reduce what fits. That is why a carton-by-carton load plan is more reliable than dividing the container's CBM by the carton CBM.