Here is how to calculate demurrage charges: multiply the number of days a loaded import container stays inside the terminal after free time by the daily demurrage rate in the shipping line's tariff. The result is a per-container charge. Detention uses the same day-count idea after the full container leaves the terminal, and the two charges should stay separate on the customer invoice.
How to Calculate Demurrage Charges
A demurrage charge has three moving parts: free time, day count and daily rate. Start from the tariff, not from a previous invoice.
- Find the free time. The line or terminal tariff states how many calendar days the importer has after discharge to collect the loaded container. Free time varies by line, contract, terminal, equipment type and whether the unit is dry, reefer or special.
- Record two timestamps. Capture discharge and gate-out. A shipment tracking portal or the terminal's gate data gives both without relying on a driver's memory.
- Count the days beyond free time. If the container left within free time, demurrage is zero. If it overran, count the full days after the last free day. Check whether the tariff counts a part day as one day.
- Apply the tariff rate. Chargeable days multiplied by the daily rate for that container size and type equals demurrage for that unit. If the tariff steps, apply each band to its own days and add the totals.
- Repeat per container. Never average across a shipment. Two overrun containers can have different free time and rates.
A common error is to use the discharge date as day zero. The tariff may start free time from discharge or from vessel arrival, depending on the line. Another error is to include weekends or holidays as free days when the tariff says calendar days. The tariff wording decides both points.
Keep demurrage and detention in separate calculations
| Point | Demurrage | Detention |
|---|---|---|
| Where the container is | Inside the port terminal | Outside the terminal, with the consignee |
| Clock starts | After free time expires, measured from discharge | After gate-out of the loaded container |
| Clock stops | Gate-out | Empty return to the depot |
| Who charges | Shipping line or terminal | Shipping line or container owner |
The table matters when a customer says one charge is wrong. A single bundled amount hides which clock ran.
Read the tariff table before you quote or invoice
A line tariff usually gives free days and then a daily rate, sometimes in steps. Record these fields for every import shipment that is likely to overrun:
| Tariff element | What it tells you |
|---|---|
| Free time | The last day the container can leave without demurrage |
| First band daily rate | The charge per day for the first overrun days |
| Later band daily rate | The rate after the first band, if the tariff steps |
| Equipment type | Whether a reefer or open top has less free time or a higher rate |
Keep the tariff pages in a shared place. When the line updates a rate in high season, the desk must use the version that applied on the discharge date, not the current one.
Evidence to attach when you recharge demurrage
The customer did not see the terminal clock. Prove the days.
- Attach the carrier's or terminal's demurrage invoice or debit note.
- Attach the tariff page or contract quote that shows free time and daily rates.
- Attach the terminal or line event report with discharge, last free day and gate-out timestamps.
- If the container overran because of a hold, attach the hold code or release delay record.
- State the per-container calculation on the invoice or debit note: a day count times the tariff rate, one line per container.
If the shipment moved through a port community system, export the event log. That record is also what you show when the customer asks why the clock ran.
Invoice demurrage and detention as separate lines
Do not write one 'local charges' line. Demurrage and detention have different start events, different clocks and different evidence. Separate lines let the customer match each charge to its proof, and they let your finance team keep the VAT category and exemption code correct per line. A freight invoicing system can carry each line's VAT category, rate and exemption code from rules the finance team edits, instead of the operator choosing them by hand.
If your quotation says demurrage is recharged at cost, attach the carrier invoice. If the quotation allows a markup or handling fee, the quotation must say so before the charge lands. Recharging without a quotation term is how a valid cost becomes a dispute.
VAT treatment differs by country in the Gulf. Kuwait has no VAT in force. In Saudi Arabia, the UAE, Bahrain and Oman, the charge follows the supply rules for the underlying freight or service. Do not copy a VAT code from another charge. The finance team's category rules decide each line.
What a desk should do this week
- Pull the current demurrage and detention tariffs for the lines you move most.
- For every import job, record discharge, free time expiry, gate-out and empty return as milestones with actual times.
- Build a small table of the jobs that overran last month, one row per container.
- Check that the invoice line matches the tariff band and the number of days you recorded.
- Stop keeping tariff versions in scattered spreadsheets. A single reference table per line with an effective date prevents a desk quoting last month's rate.
Detention is often recharged after the empty container is returned, when the line issues its final charge. Issue the customer debit note at the same time, not before, because the line may correct the empty return date.
Demurrage is a calculation, not a guess. The days must come from terminal events, the rate must come from the tariff in force on discharge date, and the customer invoice must show the arithmetic. That is how the charge survives a question.
