A charge based on a fixed rate per day, which a carrier makes against another carrier or customer for use of its containers or trailers. What are per diem charges? Per diem charges, also known as ‘detention’, is a fee that an ocean carrier or shipping line charges for each day exceeding the number of free days a container spends away from the port. This acts as a penalty for holding onto a container for too long. Now, you might be wondering who would put this charge in place and why? When it comes to per diem charges, the main players calling the shots are shipping lines and container leasing companies. They provide the containers for shipment and dictate the terms of container usage. It is to discourage importers from storing. See also ‘Detention’.