What did the overstay actually cost?

Four steps, and the whole calculation of demurrage is arithmetic once you have them. One: find the last free day — discharge date plus the free days your contract allows, counted on your tariff's basis, calendar or working. Two: count the chargeable days, from the day after the last free day up to and including the day the box gated out. Three: price those days against your tariff's tiers, because the rate almost always steps up — days 1 to 5 at one rate, 6 to 10 at a higher one, and so on, so you price each band separately rather than multiplying by the headline rate. Four: multiply by the number of containers. That is what the fields above do; the only figures we cannot supply are the free days and the rates, which come off your own contract.

Yes, and that is deliberate. Free time and per-diem rates vary by carrier, port, terminal, direction and equipment, and by what you negotiated. We do not publish a tariff table because we do not hold sourced rows for your lane, and a confident wrong free-time figure is worse than a blank one — you would plan against it. Take the free days and the rates off your service contract or the invoice in front of you.

It calculates one clock at a time, and most people arriving at a demurrage and detention calculator need both. This is demurrage: the charge for the box sitting inside the terminal past free time. Detention is the carrier's charge for its own equipment after the box leaves the gate, on a separate clock, with its own free days and its own rate, and one late document pack commonly triggers both. Run it twice — once with your demurrage allowance and rate, once with your detention allowance and rate — and add the two. The arithmetic is identical; only the numbers off your contract change.

Because it changes the last free day, and every figure below is derived from it. Most container tariffs count calendar days, straight through weekends and public holidays. Some service contracts count working days. Public holidays are not modelled here at all, because the calculator cannot know your port's calendar — check the allowance against your own tariff.

No. Charter-party demurrage, what a voyage charterer pays a shipowner for exceeding laytime, is a different charge with different parties and a different calculation, done from the statement of facts. This calculator is container demurrage on an import. If you are working a charter, this is not your tool.

Two things. Check it against the invoice you were sent, field by field, because the start date is the one most often wrong. And look at the per-day figure: that is what one day earlier would have saved, and it is the number that makes the case for working the document pack backwards from the last free day instead of forwards from whenever the supplier replies.