Demurrage and detention are the two charges importers confuse most — and the confusion is expensive, because both accrue per container, per day, with escalating tiers. The distinction is actually simple: it is the same container measured on two different clocks, one running while the box is inside the terminal, the other while it is outside. This guide pins down each charge, walks one container through both clocks, summarizes what the US FMC billing rule changed, and lists the moves that keep both charges at zero.
The one-line difference
| Charge | The clock runs while… | Starts | Stops | Typical free time |
|---|---|---|---|---|
| Demurrage | the loaded container sits inside the terminal | at container discharge from the vessel | at gate-out (pickup) | ≈3–7 days |
| Detention (per diem) | the carrier's container is outside the terminal | at gate-out | when the empty is returned to the depot | ≈2–5 days |
| Storage | the container sits at an off-dock depot or CFS | per the facility's tariff | at release from that facility | facility-specific |
Two things follow directly from the table. First, the charges are not alternatives: one container can incur demurrage and then detention on the same import if it is picked up late and returned late. Second, free times are separate allowances on separate clocks — using none of your demurrage days does not add days to your detention allowance unless your contract explicitly merges them into a combined free-time pool.
On the export side the same logic applies in mirror image: a container gated in before the earliest receiving date or left standing after cut-off can draw terminal charges, and the empty picked up at the depot for stuffing is on a detention clock until it is gated in full.
One container, two clocks: a worked example
Say a container discharges on Monday, the carrier's tariff gives 5 demurrage free days and 4 detention free days (calendar days, for simplicity), and the tariff escalates in tiers.
Demurrage clock: free time covers Monday–Friday. The box is picked up the following Wednesday — 5 chargeable days. At a tariff of $150/day for days 1–3 and $300/day from day 4, that is 3 × $150 + 2 × $300 = $1,050.
Detention clock: starts at Wednesday's gate-out with 4 free days, so free time runs through Sunday. The empty is returned the next Thursday — 4 chargeable days. At $100/day for days 1–5, that is another $400, for $1,450 total on one box.
This tiered per-day structure is exactly what the demurrage & detention calculator models: enter your free days and your contract's tier rates, and it returns the exposure for any pickup and return date — useful both for checking an invoice after the fact and for deciding whether paying a trucker's weekend rate beats a tier-two demurrage day.
Free time: what is typical, what is negotiable
Free time is a tariff term, not a law of nature. Standard (non-contract) allowances at most major ports cluster around 3–7 days for demurrage and 2–5 days for detention, but the details differ by carrier, port, direction, and equipment type — reefers and specials usually get less. Three details are worth checking in your own tariff rather than assuming:
Calendar vs working days. Some tariffs count weekends and holidays toward free time, some pause for them, and some pause only for the terminal's non-working days. A "5-day" allowance can differ by half a week between two carriers on the same port pair.
When the clock starts. Most import demurrage clocks start at container discharge — not at vessel arrival, and not when your customs broker gets the delivery order. If the vessel works cargo over several days, boxes discharged first start burning free time first.
What is negotiable. Shippers with volume negotiate extended free time in service contracts — 10, 14, sometimes 21 combined days — and this is routinely worth more than an equivalent rate reduction, because free time is insurance against the delays you do not control. How these charges sit alongside the rest of the invoice is covered in the freight surcharges guide.
The FMC billing rule (US trades)
After pandemic-era congestion turned demurrage and detention into a dispute machine, the US Federal Maritime Commission issued a final rule on D&D billing that took effect in May 2024. For shipments in US trades it changed three practical things:
Who can be billed. The invoice may go only to the party that contracted with the carrier for the transport (or the consignee), not to whichever intermediary is easiest to find — and never to two parties at once. Truckers who never contracted for the ocean move cannot be billed.
Deadlines. The carrier must issue the invoice within 30 calendar days after charges stop accruing, and must give the billed party at least 30 calendar days to request mitigation, a refund, or a waiver.
Invoice contents. The rule prescribes minimum information — container number, billing dates, free time start and end, the applicable rate, and more. An invoice missing required contents is not a properly issued invoice under the rule, which is a concrete, citable basis for a dispute. Keep your own gate-out and empty-return records (EIRs); disputes are won with timestamps.
Keeping both charges at zero
Every effective tactic amounts to the same thing: compress the time between discharge and empty return, and start compressing before the vessel arrives.
Before arrival: pre-file customs entries so the box is released before discharge, and book the trucker and chassis against the expected discharge date, not the vessel ETA. Chassis shortage is a classic way to burn free days while the container is technically available.
During free time: pull the container early in the window rather than on the last free day — one gate congestion event or missed appointment on the last day converts directly into tier-one demurrage. If your warehouse cannot unload immediately, a drop-and-hook (leaving the container on a chassis at your yard) trades trucker chassis rent against carrier detention; run the numbers in the calculator rather than guessing.
After unloading: return the empty promptly, and confirm the return location first — depots restrict which box types they accept on which days, and a rejected empty return costs both the wasted trip and more detention days. For high-volume lanes, shipper-owned containers (SOCs) remove the detention clock entirely, which is why they appear on chronically congested trades. Whether your shipment should be in a container at all is a different question — see FCL vs LCL.
What is the difference between demurrage and detention?
Demurrage is charged while a loaded container sits inside the port terminal beyond its free time. Detention (also called per diem) is charged after the container leaves the terminal, for keeping the carrier's equipment out — until the empty box is returned to the depot. Same container, two separate clocks.
How many free days do you usually get?
Typical standard tariffs allow about 3–7 days of demurrage free time at the terminal and about 2–5 days of detention free time for the equipment, but the exact numbers vary by carrier, port, and contract. Shippers with volume routinely negotiate 14–21 combined days in their service contracts.
Who charges demurrage — the port or the shipping line?
Usually the ocean carrier bills demurrage under its tariff, even though the container is sitting on terminal ground; the terminal's own storage tariff may apply in parallel or instead, depending on the port. Detention is always a carrier charge, because it is rent on the carrier's container.
What does per diem mean in container shipping?
Per diem is the North American trucking term for detention: a per-day charge for keeping the carrier's container (and sometimes chassis) outside the terminal beyond free time. On an invoice, per diem and detention refer to the same charge.
Can demurrage and detention charges be disputed?
Yes. In US trades, the FMC's 2024 billing rule requires the invoice to go only to the party that contracted for the transport, to be issued within 30 days of when charges stopped accruing, and to contain specific minimum information; the billed party then has at least 30 days to dispute. An invoice that does not meet the rule's requirements is not properly billed.
How do you avoid demurrage and detention?
Start the clock conversation before the vessel arrives: pre-clear customs, book trucking and a chassis against the discharge date rather than the ETA, pull the container early in the free-time window, unload promptly, and confirm the empty-return depot accepts your box type before the driver leaves.