A Docket shipment flagged critical: demurrage in 7 days, EPR registration missing and a fumigation stamp query, above the invoice, route, free-time and CHA details. — A container against its clock — free time ends 14 Jul, two documents unresolved.

How to calculate demurrage charges, on one container

By Balkrishna Agarwal, Founder, Docket ·2026-09-28 · updated 2026-10-08

Balkrishna Agarwal has worked on and managed an export–import trade desk for more than 20 years — the supplier chasing, the document packs, the demurrage invoices — which is why Docket is built around the desk work rather than the record-keeping.

Demurrage on one container is day zero, plus the free days, then a rate per day that rises in bands by container size. The arithmetic is trivial; the inputs are where it goes wrong. Jebel Ali allows 10 free days then charges AED 164 a day on a 40ft, rising to AED 306 from day 16; Mundra allows 2 and charges US$16, then US$30, then US$42. In US trade the invoice must show the dates it charged.

The demurrage sum, in the order the tariff applies it Four inputs and one lookup. Every error on an invoice is in the first two.

Every explainer on this subject stops before the sum. It tells you demurrage starts when free time ends, which you knew, and then offers a formula with letters in it. So here is the sum, done twice, on one 40ft dry import container, against two tariffs that are published documents anybody can read.

What the sum is

Four inputs and one lookup:

1/ Day zero. The event the tariff counts from. 2/ The free days. How many, and whether non-working days are among them. 3/ The chargeable days. From the day after the last free day, up to and including the day the box leaves. 4/ The band and the column. The rate rises in periods, and there is a separate column per container size.

Nothing in that list is arithmetic. All four are readings of a document, which is why two people with the same container and the same calculator reach different answers. Settle the four readings and the rest is mechanical: a demurrage calculating system adds the bands once you hand it your own free days and rates. What no system does is make the four readings for you.

Worked once: Jebel Ali, 18 days

One 40ft dry import container, not hazardous, discharged and then collected on day 18.

DP World’s UAE tariff allows 10 free days from the date of discharge. Storage then runs in two bands: AED 164 a day for anything over 20ft in days 11 to 15, and AED 306 a day from day 16 onward.

Days 1–10: free.
Days 11–15: 5 days × AED 164 = AED 820.
Days 16–18: 3 days × AED 306 = AED 918.
Total: AED 1,738.

Two things in that sum are easy to get wrong. The bands are periods, not a rate that reprices the whole stay: the first five charged days stay at AED 164 after the clock crosses into the higher band. And the column matters more than the band: the same 18 days on a 20ft is AED 869, exactly half, because the tariff runs two size columns and a third, higher one for out-of-gauge.

The same 18-day stay, under two published tariffs One 40ft dry import container, discharged day zero, collected on day 18. Both tariffs are current published documents.

Worked again: Mundra, the same 18 days

The same box at Mundra, evacuated by road, is a different shape of bill. Adani’s tariff for the three container terminals allows 2 free days from the day of landing, counted “including Sundays and holidays”, and then charges in four bands.

Days 1–2: free.
Days 3–5: 3 days × US$16 = US$48.
Days 6–15: 10 days × US$30 = US$300.
Days 16–18: 3 days × US$42 = US$126.
Total: US$474.

Sixteen charged days against Jebel Ali’s eight, from the same 18-day stay, because the free allowance differs by eight days. If you are modelling a lane you have not shipped before, the free-day count moves the answer far more than the rate does.

The trap that is not in either sum

Both of those are terminal storage, a charge from the terminal operator under a tariff that is published and identical for everyone. Your carrier’s demurrage is a separate charge on a separate clock, set in your service contract, and two importers on the same vessel can hold different free time for the same box. An importer who clears late pays both, and the two allowances rarely line up.

Singapore is the clean illustration, because there the carrier’s tariff is published too. Ocean Network Express’s Singapore agency tariff allows 4 working days of inbound demurrage, excluding Saturday, Sunday and the public holidays the Ministry of Manpower declares. Four working days is anywhere from four to ten calendar days depending on the sailing, which is a different question from the one the rate table answers. It is also the reason demurrage and detention software that stores free time as a single number per shipment is guessing. Which day is day zero, whether weekends count, and whose holiday calendar applies are three separate settings per lane.

Checking the invoice instead of believing it

If your trade touches the United States, you do not have to reconstruct any of this from first principles, because the carrier is required to show you its working. 46 CFR § 541.6 sets out what a demurrage or detention invoice must contain, and the timing fields are the ones that let you redo the sum:

the allowed free time in days; the start date of free time; the end date of free time; for imports, the container availability date; … and the specific date(s) for which demurrage and/or detention were charged.

The rate paragraph goes further. The invoice must let the billed party “readily ascertain how that amount was calculated”, and must name the applicable tariff rule and the specific rate or rates it comes from. That is the regulator saying, in plain terms, that the arithmetic is yours to check.

Outside the US nothing compels any of it — but the five fields are still the right five things to ask for, and a carrier that will not state its own day zero in writing is a carrier whose invoice you cannot audit. Where you do get them, recompute in this order, because that is the order the errors occur in:

Check	What it catches
Free-time start against your discharge date	Day zero a day early, which shifts every later date
Allowed free time in days against the tariff or contract	The wrong lane’s allowance, or a volume relief not applied
Free-time end against the two above	An arithmetic slip inside the carrier’s own three fields
The specific dates charged	Days billed twice, or billed after the box had gone
The named tariff rule and rate	A superseded edition, or the wrong size column

Four of those five are date comparisons, and every one of them needs a document you already hold: the arrival notice, the tariff, the delivery order. That is the whole audit.

Why the sum is the least useful part

A demurrage invoice arrives at the end of a story that was decided weeks earlier. Packs routinely run about eight days late against free time, and a week of demurrage on a single container has cost US$12,000. By the time there is something to calculate, the only remaining question is whether the amount is correct.

So calculate it — and then look at which of the eight days was yours. Our demurrage calculator takes your free days and your own rate bands, because nobody can publish those for your lane. What it cannot do is send the fourth email to the supplier who has not attached the certificate of origin, on the Tuesday, while the last free day is still four days out. That is the part that changes the number.

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Questions this raises

How do you calculate demurrage charges?

Take day zero from the tariff, add the free days, and count every day from the day after the last free day up to and including the day the container leaves. Then split those days across the tariff's rate bands and read the column for your container size. On a 40ft at Jebel Ali collected on day 18: 10 free days, days 11 to 15 at AED 164 and days 16 to 18 at AED 306, which is AED 820 plus AED 918, AED 1,738 in total.

How to calculate demurrage charges when the container sits past several bands?

Band by band, not at the highest rate. The bands are cumulative periods, not a single rate that changes retroactively, so a container collected on day 18 at Mundra is billed 3 days at US$16, 10 days at US$30 and 3 days at US$42 — US$474 on a 40ft — and not 16 days at the top rate. Reading a banded tariff as a flat rate is the most common overstatement on a self-calculated estimate.

Is demurrage charged per day or per container?

Per container per day, and the rate depends on the container's size and often its cargo class. Both tariffs here charge roughly double for anything over 20ft, Mundra charges hazardous containers at twice the normal rate and over-dimensional containers at four times, and Jebel Ali gives IMDG class 5 no free time at all. Two boxes on the same bill of lading can carry different daily rates.

What is the difference between demurrage and terminal storage on my invoice?

They are two charges from two parties on two clocks. Terminal storage is billed by the terminal operator under a published tariff that is the same for everyone. Demurrage in the carrier's sense is billed by the shipping line under your service contract, and two importers on the same vessel can have different free time. An importer who clears late pays both, and the free-day counts rarely match.

How do I check a demurrage invoice is right?

Recompute it from the invoice's own fields. A US demurrage or detention invoice must carry the allowed free time in days, the start date of free time, the end date of free time, the container availability date and the specific dates charged, and must let the billed party "readily ascertain how that amount was calculated" by naming the tariff rule and the rate. Where the rule does not apply, ask for the same five fields anyway: a carrier that will not state its own day zero is a carrier you cannot audit.

Why is my calculated demurrage lower than the invoice?

Usually one of four things: day zero is a day earlier than you assumed, weekends counted when you thought they did not, the stay crossed into a higher band, or a second charge is on the same invoice — a terminal storage line beside the carrier's per diem, or a reefer monitoring fee. Work the four in that order and the gap almost always closes on the first two.

Does a demurrage calculator give me the right answer?

Only with your numbers in it. Free time and per-diem rates vary by carrier, terminal, direction, equipment and cargo class, so no calculator can supply them and be right for your lane. Our demurrage calculator asks for your free days and your rate bands for exactly that reason. It also cannot fix the cause: the sum is only ever the price of a document that arrived late.