"载货容积" can be translated into English as "cargo volume" or "load-carrying capacity (in terms of volume)".
"Cargo volume" is a direct and commonly used term to refer to the space available for carrying goods in a vehicle, container, or other cargo-carrying units.
"Load-carrying capacity (in terms of volume)" provides a more detailed description, emphasizing that it refers to the volume-based capacity for carrying loads.
In most contexts, especially in transportation and logistics, "cargo volume" is sufficient and widely understood.