Detention calculation
Detention calculation: how to bill it
A detention calculation is free time subtracted from total time, multiplied by the rate in the contract: chargeable hours = hours on site minus the free period, times the hourly or daily rate. If a truck arrives at 08:00 for an 08:00 appointment and leaves at 13:30, with two hours free and a $75 per hour example rate, the bill is (5.5 - 2) x $75 = $262.50. Demurrage and per diem use the same arithmetic against a terminal clock instead of a dock.
By Richa Myria, Microsive
Three charges, one arithmetic
| Charge | Measures | Clock starts |
|---|---|---|
| Detention | A truck and driver held at a shipper or receiver beyond the free time | Arrival or appointment time, as the contract names |
| Demurrage | A container left inside the port or terminal beyond its free days | When the container is available for pickup |
| Per diem | A container, and sometimes its chassis, kept outside the terminal beyond its free days | Gate-out from the terminal |
All three are the same shape: a free window, then a rate per hour or day, applied to time beyond the window. The differences are the start event and who owes whom, which is why the contract wording matters more than the math.
The calculation, step by step
- Write down the start event and the end event, with times and time zone.
- Subtract the free period the contract grants (hours or days).
- Round the way the contract says: by hour, by fraction of a day, with a stated minimum.
- Multiply by the contracted rate, per hour or per day.
- Attach the evidence: gate times, appointment record, signed pickup and drop paperwork.
Worked example
| Step | Math | Result |
|---|---|---|
| Total time on site | 08:00 to 13:30 | 5.5 hours |
| Less free time | 5.5 - 2 | 3.5 chargeable hours |
| Charge | 3.5 x $75 | $262.50 |
Where the disputes come from
- Clock disagreement: appointment time versus gate time versus ready time.
- Rounding: half a day billed as a full day with no clause that says so.
- Missing free time: the contract grants two hours, the invoice assumes none.
- Evidence: a charge with no times attached is a charge that gets written off.
Billing it cleanly
- Capture gate-in, ready and gate-out times where the event happens, not from a driver's message later.
- Put free time, rate, rounding and minimums in the contract before the first shipment.
- Show the arithmetic on the invoice: times, free window, chargeable hours, rate, total.
- Report unrecovered detention weekly, while the evidence is still fresh.
Port and terminal rules change and are published by the terminal itself: cite the terminal's own page and its date wherever a port clock is involved.
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