Per Diem vs. Detention: The Container Fees Nobody Explains Until You Owe Them

You pull a container, run it to your warehouse, unload it, and send it back to the port a few days later. Then two invoices show up that you weren't expecting: one for per diem and one for detention. You already paid demurrage. Weren't those the same charge?

They're not, and mixing them up is one of the fastest ways for a shipper or forwarder to eat fees that were avoidable. Each one is billed by a different party, starts its clock at a different moment, and is triggered by a different piece of equipment sitting somewhere it shouldn't. Once you can tell them apart, you can actually plan around them instead of getting surprised.

The confusion mostly comes from the fact that all three fees punish the same thing — equipment not moving fast enough — but each one belongs to a different party in the supply chain, with its own free-time allowance and its own invoice.

• Demurrage — charged by the marine terminal. The container is still sitting inside the port, past its free time, because it hasn't been picked up yet.
• Per diem — charged by the ocean carrier (the steamship line). You picked the container up, but you're holding the carrier's box (and often its chassis) past the number of free days your contract or bill of lading allows.
• Detention — the broader industry term for holding any piece of equipment — container, chassis, or trailer — beyond agreed free time, wherever it's sitting. In everyday use it usually refers to time the box sits at your dock or yard being loaded or unloaded, rather than moving.

Ocean carriers own or lease the containers and, in many cases, the chassis that carry them inland. When you take possession, the carrier gives you a set number of free days to return the empty equipment. Per diem is what accrues once you blow past that number — literally a per-day rental charge for keeping their asset out in the world.

The clock usually starts the day the container leaves the terminal, not the day you actually unload it. That's the detail that trips people up. A container can clear the gate on Monday, sit in transit or on your lot through the weekend, and already be a day or two into per diem before anyone at your warehouse has touched it. Chassis splits — where the chassis provider is separate from the steamship line — add another layer, since chassis per diem and container per diem can run on different clocks entirely. Our container & chassis 101 breakdown covers how those pieces of equipment are tracked separately.

Detention is a wider term than per diem, and that's exactly why it gets confused with everything else. Strictly speaking, detention is any charge for holding equipment past its agreed free time, outside the terminal. Depending on who you ask, that includes per diem itself, chassis detention from a chassis pool provider, and street/dock detention that a trucking company or intermodal marketing company might bill you directly if their equipment sits at your facility too long.

In practice, most shippers use "detention" specifically for the days a container sits at their own dock or yard being loaded or unloaded, as opposed to demurrage (sitting at the port) or per diem (a carrier-billed rental charge). The line between per diem and detention can genuinely blur — some carrier contracts use the words interchangeably — so the only reliable fix is reading what your specific bill of lading or service contract calls it and how many free days it grants.

Demurrage is the one most shippers learn about first, because it's the fee that stops a container from ever leaving the terminal. It's charged by the marine terminal operator, not the carrier, and it starts counting from the moment the container is discharged from the vessel — regardless of whether you've even arranged trucking yet. We go deeper on how that clock works, and how to fight an unfair demurrage bill, in demurrage explained. The short version for this article: demurrage ends the moment the container leaves the terminal gate. Per diem and detention pick up from there.

A single import move can rack up all three fees without anything going dramatically wrong — just a chain of small delays. Say a container sits at the Port of Oakland for six days before a chassis is available, past the terminal's free time: that's demurrage. It finally moves to your warehouse, but a produce backlog means it isn't unloaded for four more days: that's detention. Meanwhile the empty isn't returned to the carrier's yard until two days after that: per diem. Three separate invoices, three separate clocks, one container.

1. 01Know your free days for each stage — terminal, carrier, and chassis pool — before the vessel even arrives.
2. 02Have your trucking arranged and dispatched ahead of the discharge date, not after.
3. 03Unload and return equipment on a fixed schedule rather than whenever the dock has a gap.
4. 04Track the container's real-time status so you know exactly which clock is running. Our tracking page shows live status once a load is dispatched.
5. 05When delays are unavoidable, use a pre-pull to get the container off the terminal clock and onto a yard clock you control — see pre-pull and yard storage for how that trade-off works.

We run container drayage in and out of the Port of Oakland and the Northern California rail ramps every day, and the fee that catches shippers off guard almost never comes from the leg we control — it comes from equipment sitting still before or after our part of the move. Our dispatch desk is staffed 24/7 specifically so a truck can be moving the moment a container is available, and our fleet is GPS-tracked and ELD-equipped so you can see exactly where a box is against its free-time clock. Every driver carries a TWIC card, so there's no credentialing delay at the terminal gate either.

If you're planning a move with a tight free-time window, or you're not sure which clock a fee is coming from, get a quote and we'll walk through the timeline before the container even lands.