Two different clocks share this arithmetic. Demurrage runs while the container sits inside the terminal past its free time; detention runs while the container is out with you — at the warehouse, unpacked or not — past its free time. Same tariff shape, same tiers, separately billed, and a box can incur both on one trip. This tool computes either; run it twice for a shipment caught on both clocks.
The tariffs are tiered on purpose: a few days at a tolerable rate, then a jump designed to hurt. The commercial meaning of the jump is "the line wants its box back", and the operational meaning is that the marginal cost of delay is not constant — the day that crosses a tier boundary can cost double the day before it. Averages hide this; the per-day delta is the honest number.
For a forwarder the money is usually not even the worst part — the worst part is whose money it is. Charges accrue against the party on the line's records, which is often you, while the delay belongs to a consignee who has not cleared or a warehouse that has no slot. The defence is speed of information: know the number daily, tell the customer with tomorrow's figure attached, and get the protest on record before the invoice exists.
Where the container is. Inside the terminal past free time is demurrage; outside with the merchant past free time is detention. A box discharged Monday, picked up the following Friday and returned empty two weeks later can owe demurrage for the terminal days and detention for the street days, each on its own tariff. Some lines merge them into one "combined" free time — the tariff wording decides, so read it before assuming.
Before the fact, yes — extended free time is a standard ask in any volume negotiation and costs the line little to grant. After the fact, sometimes: lines do reduce charges where the delay was demonstrably theirs (late availability, terminal congestion they controlled) or where a good customer asks once, politely, with the timeline documented. What never works is silence followed by refusing the invoice — by then the leverage has left the building.
Tier boundaries. The tariff might run 60 a day for three chargeable days and 120 thereafter; the fourth chargeable day costs 120 while the third cost 60. The calculator computes the next-day figure as a true delta across the boundary rather than quoting yesterday's rate, because the day it matters most is exactly the day the rate jumps.
Usually yes — most lines count calendar days, so a box discharged on Thursday can burn most of its free time over a weekend nobody works. Some tariffs and some charters count working days instead. It is a definition, not a custom: the tariff's own wording governs, and the difference routinely decides whether a shipment pays nothing or pays the top tier.
Cargo-Stack reads the enquiry, prices it against your own rates, and drafts the quote — with this arithmetic underneath it. Same maths, no retyping.
Try it on a real enquiry