Per diem meaning in shipping: two lines, one invoice

Two lines arrive on one invoice for one container. One says detention. One says per diem. A desk that has worked a single trade lane reads them as two charges and pays both.

Usually they are one charge under two names. Occasionally they are not, and the difference is written into American law rather than into the invoice.

The word

Per diem is Latin for by the day. In container shipping it is the daily rate a carrier charges for equipment you are still holding after free time runs out. In most of the world that invoice line reads detention. In United States trades it very often reads per diem, and the two point at the same thing: the carrier’s box, off the terminal, standing on your yard or your customer’s.

The regulator says so directly. Under 46 CFR 541.3, demurrage or detention mean “any charges, including ‘per diem’ charges, assessed by ocean common carriers, marine terminal operators, or non-vessel-operating common carriers related to the use of marine terminal space (e.g., land) or shipping containers, but not including freight charges”. The interpretive rule at 46 CFR 545.5(b) uses the same formula.

A line called per diem is therefore a detention line, for regulatory purposes, when it is charged for the container.

Where it stops being the same charge

That qualifier is doing a great deal of work.

When the Federal Maritime Commission finalised the rule it explained that the definition “encompasses all charges having the purpose or effect of demurrage or detention regardless of what those charges may be called by the billing party”, and then drew a boundary in the next sentence: “The definition excludes charges related to equipment other than containers, such as chassis, because depending on the context, ‘per diem’ can refer to containers, chassis, or both.”

A chassis is the road trailer the box rides on. On US intermodal moves it is commonly hired separately from the container, it bills by the day as well, and its line on the invoice also reads per diem.

So one American invoice can carry two lines wearing the same word:

The label does not tell you which one you are reading. What the charge is for does.

Why the distinction is worth ten minutes

Part 541 is not decoration. It attaches four things to a charge that falls inside it, and none of them to a charge that falls outside.

Thirty days to invoice. Section 541.7(a) requires the billing party to issue the invoice within thirty calendar days of the date the charge was last incurred, and says that if it does not, “the billed party is not required to pay the charge”.

A required content list. Section 541.6 sets out what the invoice must carry: the bill of lading and container numbers, the port of discharge on an import, the allowed free time in days, the start and end dates of that free time, the container availability date, the specific dates charged, the total, the rate, and the tariff rule or contract section the rate comes from.

A consequence for leaving any of it out. Section 541.5 says failure to include any of the required minimum information “eliminates any obligation of the billed party to pay the applicable charge”.

Thirty days to dispute. Section 541.8 gives the billed party at least thirty calendar days from issuance to request mitigation, refund or waiver, and requires the billing party to attempt to resolve the request within thirty days of receiving it.

The chassis line gets none of that. Same sheet of paper, same word, and one half carries a statutory content requirement while the other half is a pure contract question.

One further point dates the older explainers. Section 541.4, which limited who could be invoiced, was set aside by the D.C. Circuit in September 2025 and removed from the regulation on 29 December 2025; it now reads [Reserved] in the Code of Federal Regulations. What that took with it, and what it left standing, is on the demurrage invoice entry.

Outside the United States

The rights do not travel. Elsewhere a per-diem or detention charge is a term of the bill of lading and the carrier’s tariff, so the questions are contractual: what free time did we actually hold, when did the clock start, and what does the tariff say about the days being billed.

The arithmetic is the same everywhere. Detention runs from gate-out until the empty is returned and accepted, in bands that step up the longer the box is out. Free time is set per contract rather than per port, which is why two importers who came off one vessel can be charged differently for the same delay. And the port’s own terminal storage charge runs on a separate clock underneath, so a late clearance draws bills from two parties at once.

What a desk does with a per-diem line

Three things, in order.

Say which asset it is for. If the invoice does not make that plain, ask before paying, because the answer decides which rulebook applies to the rest of the argument.

Check the dates against your own record of the move. The fields section 541.6 requires are the ones a desk can actually verify: free time allowed, when it started, when it ended, the container availability date, the days charged. An invoice missing them is not merely unclear. In a US trade it is unenforceable on its face.

Then count backwards. Nearly every per-diem line exists because a document was late, not because a truck was. The demurrage and detention software worth paying for is the kind that knows the last free day and goes after the missing document ahead of it, not the kind that turns a container red once the meter has run. Disputing the invoice afterwards is the slower half of the same job.