Demurrage and detention charges escalate fast and carrier invoices often contain errors. This calculator counts only the chargeable days beyond your granted free time, applies tiered per-diem rates, and totals both charges so you can check an invoice line by line before paying it.
Demurrage and detention are among the most disputed line items in international freight. Studies by shipping industry groups have found that a significant proportion of container invoices contain billing errors — either wrong free-time applied, wrong start date used, or wrong tier rate applied. Running the calculation yourself before paying is the single most reliable way to catch overcharges.
How it works
Held days are measured from the dates, free time is removed, and the remainder is split across two rate tiers:
held days = return/pickup date − available date
chargeable days = max(0, held days − free days)
tier-1 days = min(chargeable days, tier-1 window)
tier-2 days = chargeable days − tier-1 days
charge = tier-1 days × rate-1 + tier-2 days × rate-2Demurrage and detention are computed independently with their own free time and rates, then summed for the total exposure.
The distinction in plain terms
Demurrage is a port charge. It accrues when a full, loaded container sits inside the terminal or port yard longer than the carrier’s free time allows. The port needs that slot for the next vessel; demurrage is the cost of occupying it beyond the agreed window. You cannot pick up a container until you have paid (or arranged to pay) the outstanding demurrage.
Detention is an inland charge. It accrues when you keep the carrier’s empty or laden box at your premises — warehouse, factory, or farm — beyond the free time allowed for unpacking and returning the equipment. The carrier needs the container back in its rotation; detention compensates for the delay.
The two charges can run concurrently if you are slow to collect from the port and slow to return the empty. They can also arise independently — a fast customs clearance but a slow unloading generates detention without much demurrage.
Why tiered rates matter
Carriers set escalating per-diem rates deliberately. The first few chargeable days attract a lower rate to give importers a grace period for genuine operational delays. Once a container has been overdue for a week, the rate typically jumps to a higher tier — sometimes three to five times the tier-one rate — to create strong financial pressure for return. A container held two weeks beyond free time can cost far more than the goods inside it.
Enter your carrier’s specific tier break-point and rates accurately. Using the wrong tier-one window is a common error that makes a self-calculated bill diverge from the carrier’s invoice.
Calendar days versus working days
Whether free time and chargeable days count calendar days or working days is the single most common source of demurrage and detention disputes. Many carrier tariffs count every calendar day including weekends and public holidays. Some, especially in markets with shorter port hours, exclude weekends from the count. This calculator treats all days as calendar days; if your tariff says “working days,” subtract weekends and any applicable national holidays from your date range manually.
Worked example
For example: a container becomes available on a Monday. Free time is 5 days. You pick it up Friday (day 4 — no demurrage) but return the empty the following Thursday (day 11 — 6 days after free time expired). Tier 1 detention rate is $75 per day for the first 3 days; tier 2 is $110 per day after that.
- Chargeable days: 6
- Tier-1 days: 3 × $75 = $225
- Tier-2 days: 3 × $110 = $330
- Total detention: $555
If the carrier’s invoice says $660, you have grounds to dispute it — likely a day-count discrepancy or wrong tier-break applied.
Tips for reducing exposure
- Request an arrival notice as soon as the vessel berths and act immediately — free time starts regardless of whether you noticed.
- Pre-clear customs while the vessel is still at sea wherever possible so customs release does not eat into free time.
- Negotiate extended free time on large shipments before the cargo ships, not after it arrives.
- Keep the gate-in and gate-out timestamps from the terminal — they are your primary evidence in a dispute.