Zenmul
If you import into the US, you have probably seen both words on the same invoice. They sound alike and they are often billed together, but they measure different things. Knowing which is which is the first step to checking whether a bill is right.
Demurrage: the container is still at the terminal
Demurrage is a charge for a loaded container that stays inside the marine terminal after its free time ends. You are paying for the space the container takes up at the port. It can be billed by the ocean carrier or by the marine terminal operator.
Detention: the container is outside the terminal
Detention is a charge for keeping the carrier's container outside the terminal after free time ends. The clock usually runs from the time the container leaves the terminal until it is returned. Some carriers call this per diem.
The name on the invoice does not decide what the charge is
Carriers use different labels. The Federal Maritime Commission (FMC) looks at what a charge does, not what it is called. A charge for the use of terminal space or for the use of shipping containers is treated as demurrage or detention, whatever name appears on the bill.
Why the difference matters when you check a bill
Each charge runs on its own clock, and each clock is proven by different records. For demurrage, the key dates are when the container became available for pickup and when it left the terminal. For detention, the key dates are when the container left the terminal and when it came back empty. If an invoice charges both for the same days, or starts a clock on the wrong date, that is something to question.
Example only: if a container leaves the terminal on day 6, demurrage days and detention days should not overlap on day 7. The dates on your gate records are what settle it.
What the FMC billing rules require
Since May 28, 2024, demurrage and detention invoices on US trades must follow the FMC billing rules in 46 CFR Part 541. Two points stand out for importers:
- The invoice must include specific minimum information. If required information is missing, the FMC says the billed party does not have to pay that charge.
- The invoice must be issued within 30 calendar days of the last day the charge was incurred.
In September 2025 a federal court set aside one section of these rules, the section that limited who could be billed. The rest of the rules remain in effect.
Older charges may still be open
The FMC has confirmed that charge complaints about fees assessed on or after June 16, 2022 are not limited by a three year deadline. If you paid demurrage or detention since then, those bills may still be worth a look.
What to gather before you check an invoice
- The invoice itself
- Your contract or the carrier's tariff, which sets free time and rates
- Gate out and gate in records for each container
- Any notices about container availability
With those in hand, you can line up each charged day against what actually happened.
If you would like a second set of eyes, Zenmul reviews freight charges and gives you a written report. You decide what to submit to the carrier, and you submit it yourself.
This article is general information, not legal advice.