Same days, different party billed
Per diem and detention often measure the same thing: days a container spends outside the terminal past free time. The main difference is who receives the bill. Lines typically send detention to the cargo side, while per diem usually goes to the motor carrier that signed for the equipment. Some lines use the words interchangeably, and the FMC's demurrage and detention rules expressly include per diem charges. Read the basis printed on the invoice rather than relying on its label.
The interchange agreement behind most U.S. bills
Many U.S. motor carriers interchange equipment under the Uniform Intermodal Interchange and Facilities Access Agreement, known as the UIIA, plus each line's own addendum. The addendum sets free time, per diem rates and the records that prove pickup and return, usually the terminal's equipment interchange receipts. Truckers then decide whether to pass the charge to their customer, which is why drayage quotes often list per diem as a possible extra. Disputes tend to turn on those interchange timestamps.
Dates a dispatcher should log for every box
Per diem exposure opens at the gate-out milestone and closes when the empty is returned, so log both for every container on the board. Also note when the consignee finished unloading, because that is when the empty became ready to go back. Chassis charges usually run on their own clock, billed by the pool or lessor rather than the line, so track the chassis return separately.
Example
A drayage company keeps TCNU8173559 in its yard over a long weekend until the consignee's dock reopens, then receives a per diem bill from the line for the days past its interchange free time.
Track a container
Save the reference now and create your account; tracking starts once your 30-day trial is active.
Official sources
Questions
What does Per Diem mean?
Per diem is a daily charge for keeping a carrier's container, and sometimes a chassis, past the allowed free time. In U.S. drayage it usually means the equipment charge billed to the trucking company that holds the box.
Why is my trucker billing me per diem?
Usually because the line billed the trucker for keeping its container past free time, and the trucker's rate agreement lets it pass that cost on. Ask for the line's invoice and the interchange receipts to confirm the dates.
Does per diem stop on weekends?
Terms differ: some lines count every calendar day, and others count only working days, so a long weekend can change the bill. The addendum or tariff states the counting rule.