Identify what the charge covers
In a typical import movement, demurrage concerns keeping the container at the terminal beyond the agreed free time. Detention concerns using it outside the terminal, for example between collecting the full container and returning it empty. The contract and applicable trade tariff determine the exact rules.
Storage is a charge for occupying storage space. Do not assume it is always included in demurrage. Check who charges it and avoid counting the same item twice.
Separate periods or combined?
Combined demurrage and detention uses a common period for container use inside and outside the terminal. Do not automatically add two separate calculations. Maersk distinguishes demurrage, detention, combined charges and storage in its terms.
Record the tariff name, container type, trade and validity period before calculating. A rate for a different country or equipment type may produce an irrelevant result.
Prepare the calculator inputs
You need the events and dates that start and end the period, the free days and the rates for each chargeable band. Check whether the tariff uses calendar or working days and how it treats collection and return days.
Keep collection and return receipts. The calculation follows the event defined in the tariff, not necessarily the day the forwarder receives a document.
Check a simple example
Illustration only: a period contains 8 counted days, of which 5 are free. That leaves 3 chargeable days. At a flat EUR 40 per day for one container, the total is EUR 120.
If rates change over time, split the days into bands. Show the assumptions alongside the amount so the customer can reproduce the calculation. Example rates are not a carrier quotation.