QUICK ANSWER Container per-diem days are counted from two timestamps: the Equipment Interchange Receipt (EIR) created when the container gates out of the terminal, and the one created when the empty gates back in. Between them, most carrier tariffs count calendar days against 3 to 5 days of free time, then bill $100 to $200 per day. The clock is stopped by an empty return, which makes the return leg, appointments, gate hours, and acceptance rules the whole game.
What container detention is, how it differs from demurrage, and what the FMC billing rule requires are covered in our demurrage and detention guide; this page is about the machinery. Where the days come from, why empties are hard to give back, and how to audit the invoice when it lands, because a meaningful share of per-diem bills do not survive one.
Where the days come from
Every container interchange produces an EIR, the record of which piece of equipment crossed which gate, when, in what condition. The out-gate EIR starts the equipment clock described in our drayage explainer; the in-gate EIR for the empty stops it. Between those records, the carrier's tariff does the counting: most count calendar days, so a Friday pickup burns free time across the weekend before Monday's delivery, and free time itself commonly runs 3 to 5 days. Nobody phones you as the days accrue. The first notice is often the invoice, weeks later, built from two timestamps you never saw.
The empty return: the move that stops the clock
Deliveries get planned; returns get assumed, and that asymmetry generates most per-diem in the trade. Returning an empty is its own move with its own frictions: many terminals require an empty-return appointment just like a pickup, the appointment systems run out of slots at peak, and carriers restrict which locations accept which boxes, so "take it back where it came from" is frequently wrong. A trucker holding your empty with nowhere to put it is not an anomaly; it is Tuesday at a busy port.
TAKEAWAY Treat the empty as a shipment of its own. The return appointment should be booked when the delivery is booked, not after the unload, because the days between those two decisions are exactly the days the carrier bills.
When the terminal will not take your empty
The clock's ugliest failure mode: the equipment is ready to go back and the system will not take it. Return gates close, terminals declare no-empty days when yards run full, and appointment portals show nothing available. Document those days as they happen, screenshots of the portal, the refused appointments, the carrier notice, because they are the core of a dispute. California made the principle law for its ports: carrier tariffs now state that detention there is assessed only on days the return gate is actually open for empties. Outside California that standard is not automatic, but a documented no-appointment day is still the strongest line in any dispute letter, and carriers know it.
Street-turns and dual transactions
Two dispatch moves shorten the clock from the trucking side. A street-turn hands your emptied container directly to another shipper who needs one, an exporter loading nearby, with the carrier's approval, so the box never returns to the terminal at all: fewer miles, no return appointment, clock stopped at interchange. A dual transaction pairs the return with the next pickup in one terminal visit, one queue instead of two. Both depend on the carrier, the port, and the day, which is why they are a dispatcher's craft rather than a checkbox, and why carriers who run dense port operations pull them off more often.
Who bills whom on the way to you
The ocean carrier does not bill you directly at first. Under the UIIA interchange agreement, the equipment moves on the motor carrier's account, and the UIIA is itself a billing contract for the equipment, which is why per-diem can land on the trucker even though the FMC rule bars carriers from billing truckers under the ocean tariff. The trucker then passes the charge through to the importer under the terms of your drayage agreement. That chain matters when something goes wrong: the trucker disputes with the carrier on facts it holds (EIRs, appointment logs), while you dispute the rebill on terms you hold. The FMC's billing requirements, who may be billed and what the invoice must show, are covered in the demurrage guide's FMC section.
Auditing a per-diem invoice
The five checks before paying a per-diem bill
| Check | What to compare | What kills the charge |
|---|---|---|
| Dates | EIR out-gate and in-gate vs billed days | Days billed outside the actual interchange window |
| Free time | Days allowed in the governing tariff vs days granted | Free time shorted on the invoice |
| Closure days | Billed days vs documented no-appointment or closed-gate days | Days the empty could not physically return |
| Billed party | Who the invoice names vs who holds the contract | Bills sent to a party with no billing relationship |
| Arithmetic and rate | Daily rate and tier vs the tariff | Wrong tier, wrong rate, doubled days |
Run the five checks in order and a surprising share of invoices lose weight before anyone argues about fairness. The chassis line deserves the same reading on the same bill, since the frame's days usually mirror the box's.
How Platton keeps the equipment clock short
Per-diem prevention is dispatch discipline plus paperwork discipline, and both happen before the invoice exists.
The empty leg dispatched, not assumed
In our drayage dispatch the empty holds a confirmed return slot before the container is even unloaded, and long unload dwell gets flagged the same day it happens, while rebooking still costs nothing but a phone call.
Closure days on file for the dispute
No-appointment days and gate closures are logged per container as they occur, alongside the shipment tracking record. When a per-diem invoice arrives, the dispute references dates and evidence, not recollections, which is the difference between a waived charge and a shrug.
Street-turns where the lane allows
Where export demand and the carrier's rules line up, empties go to the next load instead of the terminal queue. On lanes with steady matchback the equipment clock barely gets started.
Have your last per-diem invoice audited with your next quote
Common Questions About Container Per-Diem
How are container detention days calculated?
From the interchange records: the out-gate EIR date to the in-gate EIR date for the empty, minus the free time in the carrier's tariff, times the daily rate for each tier. Every element of that formula is checkable, so an audit against the EIRs is the first response to any bill.
Do weekends count toward per-diem?
Under most carrier tariffs, yes: counting is calendar-day, so a Friday gate-out spends free time on Saturday and Sunday. A few tariffs and some state rules carve out days the return gates are closed. Read the governing tariff's counting clause; it decides more than the daily rate does.
What if the terminal will not accept my empty container?
Document it the same day: portal screenshots, refused appointment records, any carrier notice. Days the empty physically could not return are the strongest dispute grounds available, and in California carriers may not assess detention for days the return gate was closed at all. Then keep trying dates, because the clock argument works best alongside an actual return.
Can a per-diem invoice be disputed?
Routinely, and successfully when the facts are held. Disputes on dates, shorted free time, documented closure days, and wrong billed parties succeed often enough that auditing every invoice pays for the habit. The chain matters: your trucker disputes with the carrier on the interchange facts while you dispute the rebill.
What is a street-turn?
A carrier-approved move where your emptied container goes directly to another shipper's next load instead of back to the terminal. The interchange closes at handoff, so per-diem stops early, the trucker saves a terminal round trip, and the exporter gets equipment without queueing for it. Everyone wins, so dispatchers hunt for them.
How long do I have to return an empty container?
Read the governing tariff's counting clause, because that is what decides your real deadline: calendar or working days, and what stops the count. Free time commonly runs 3 to 5 days from gate-out. The practical answer is shorter than either: return it as soon as the unload allows, because free time exists to absorb friction, not to be scheduled against.
Related Trucking Guides
Demurrage vs detention vs per-diem: the definitions, rates, and the FMC billing rule.
Chassis fees explained: the frame whose days mirror your container's.
What is drayage: the move that starts the equipment clock.
Transloading explained: the routing that sends equipment home in days.
