Demurrage and per diem invoices arrive weeks after the fact, which is what makes them feel arbitrary. They are not arbitrary — they are arithmetic, and the arithmetic can be run in advance. Knowing the number before the container is late changes decisions that otherwise get made on instinct.

First, the two meters

These get conflated constantly. They are separate charges from separate parties measuring separate things.

Demurrage is charged for the container sitting inside the terminal past its free time. It is rent on terminal space, and it stops the moment the box goes out the gate.

Per diem (also called detention, depending on the carrier’s vocabulary) is charged for the container being outside the terminal past its allowed free days. It is rent on the steamship line’s equipment, and it stops when the empty is returned and accepted.

Our full breakdown of demurrage vs. detention vs. per diem covers the terminology differences between carriers. For this article, what matters is that they are sequential meters with a handoff at the gate.

The five numbers you need

To estimate exposure on any container, find these before it discharges:

1. Last free day at the terminal. Usually three to five calendar days after discharge or availability, depending on the carrier and the terminal. This is on your arrival notice. See how last free day is set.
2. Demurrage tier schedule. Per container, per day, escalating. Published in the carrier’s tariff or negotiated into your service contract.
3. Free days on the equipment. Separate allowance, often three to five days, running from when the container leaves the terminal.
4. Per diem tier schedule. Same escalating structure, different rates.
5. Container size and type. Reefers, flat racks and specialized equipment carry higher rates than standard dry boxes, sometimes several times higher.

All five live in documents you already receive. The work is collecting them once per carrier rather than hunting for them during a crisis.

Running the estimate

The calculation is a two-stage sum:

Demurrage = for each chargeable day past last free day, the rate at that day’s tier, added together, multiplied by the number of containers.

Per diem = the same structure, counted from the day free time on the equipment runs out to the day the empty is accepted back.

Two practical notes. First, the tiers escalate, so linear estimates understate badly — a container seven days late does not cost seven times a one-day charge, it costs considerably more. Second, multi-container shipments multiply. A single delayed customs release on a five-container booking is five parallel meters.

What this number is actually for

The estimate is a decision input, and it is usually compared against one of three alternatives.

Pre-pull vs. let it sit

If demurrage is accruing and your warehouse cannot take delivery for four days, pulling the container to a yard stops the terminal meter. You pay a pre-pull charge plus yard storage, and you start the per diem meter — but per diem is usually cheaper than demurrage, and yard storage is usually cheaper than both. Run the numbers rather than assuming.

Expedited unload vs. normal schedule

If per diem is running and the container is sitting at your dock waiting for labor, the cost of bringing in extra hands for one shift is often less than two more days of per diem on a reefer.

Transload vs. hold the box

Moving cargo out of the ocean container into a domestic trailer returns the equipment immediately and stops per diem cold. Our transloading guide covers when that math works.

Reducing the exposure structurally

• File your ISF and clear customs early. Most demurrage traces to a container that was available and not released. ISF timing is the cheapest lever you have.
• Book the appointment the day availability posts, not the day before last free day. Appointment scarcity in South Florida is real.
• Negotiate free time in your service contract. Two extra days across a year of volume is worth more than most shippers realize — and it is negotiable.
• Return empties promptly and confirm acceptance. Per diem does not stop when the truck leaves your yard; it stops when the terminal takes the box. See stopping per diem charges.
• Audit every invoice. Billing errors are common. Our invoice audit checklist and guide to disputing demurrage cover the grounds that actually work.

Build it once

A simple spreadsheet with your carriers’ tier schedules, updated annually, turns a guess into a number in thirty seconds. That is usually enough to make the right call on the day it matters. For help stopping the terminal clock on containers you cannot take yet, talk to us about pre-pulls and yard storage in South Florida.

Frequently asked questions

How are demurrage tiers usually structured?

Most ocean carriers and terminals use escalating daily tiers – a lower rate for the first few chargeable days, then a higher rate, then higher again. The tier structure and dollar amounts are published in the carrier’s tariff or your service contract and vary by carrier and terminal.

Do demurrage and per diem ever run at the same time?

Yes. If a container is still in the terminal past last free day you accrue demurrage. Once it leaves, per diem starts on the container itself. There are situations, particularly around disputed holds, where a shipper ends up exposed to both within the same week on the same box.

Are weekends and holidays counted as chargeable days?

It depends on the carrier’s tariff and the terminal’s schedule. Some exclude days the terminal is closed, some do not. Because gate closures are exactly when you cannot act, this is one of the more commonly disputed line items.