What is detention and demurrage, and why it eats your buffer
Every import planner has had the week where a shipment that was supposed to land clean instead shows up with a per diem invoice attached. Demurrage and detention are two different charges, and the difference matters because they're billed by two different parties for two different reasons. If you've ever tried to explain to finance why a $40,000 container suddenly carries a $1,800 line item nobody budgeted for, this is usually why.
Demurrage: the terminal's clock
Demurrage is what the terminal (or the carrier, depending on who owns the yard) charges you for leaving a container sitting on its property past the free time window. Free time is the number of days, often three to five at major gateways, that you get to pick the box up before the meter starts. Once the last free day (LFD) passes, you're billed per day, per container, until the box is gated out. It doesn't matter whose fault the delay is. A chassis shortage, a drayage carrier that's double-booked, a yard that's too congested to even schedule a pickup appointment: none of that stops the clock.
Detention: the carrier's clock
Detention is a separate charge, and it starts once the container leaves the terminal. The steamship line owns the box, and they give you a set number of free days to unload it and return the empty. Miss that window and you're billed again, this time by the carrier instead of the terminal. So a single container can rack up demurrage while it's stuck in the yard and then detention after it's finally out, if the empty return gets delayed on the back end too.
Both charges exist for the same reason: equipment and yard space are assets the terminal and the carrier want moving, not parked. The free time clock is designed to get you to act fast. The problem is that the clock doesn't adjust for conditions outside your control, and lately those conditions have been doing most of the damage.
Why this eats your buffer instead of just your invoice
Most import teams build a lead-time buffer to absorb normal variance: a few days for customs holds, a few for drayage scheduling, a few for the inevitable Monday-after-a-holiday backup. That buffer assumes the terminal is running at a normal pace. When a lane is backed up, with vessels queuing at anchor and the yard sitting at high fill, the free time clock keeps counting down on the terminal's posted schedule while your container waits its turn in a congested queue. You lose days to that congestion before your truck even has a pickup slot, and every one of those days comes straight out of whatever buffer you built.
This is the part that makes demurrage and detention feel unpredictable even to teams that have shipped the same lane for years. The charge itself is standard. What isn't standard is how much of your free time gets burned by yard conditions before you can act on it. A lane running two days behind on dwell turns a comfortable five-day free time window into a three-day one, and nobody tells you that until the invoice shows up weeks later.
Free time only helps if you know how much of it is already gone
The fix most planners reach for is padding every lead time with an extra few days as insurance. That works until it doesn't, because padding a number you can't tie to the lane's real conditions either wastes money on inventory sitting early or still comes up short when a specific lane is running worse than usual. What you need to know, per lane, per day, is how many days behind that lane is running right now, so you can move the pickup appointment before the LFD instead of after.
That's the gap Lead Time Risk is built to close: a daily slip-day number for the specific inbound lanes you register, based on vessel queues, terminal fill, and yard dwell, so you can see the free time squeeze coming instead of reading about it on an invoice.
If detention and demurrage have been showing up as surprises instead of line items you saw coming, it's worth putting your lanes on Lead Time Risk's waitlist.