Cargo does not simply get loaded into an aircraft's hold piece by piece. On most wide-body aircraft, and increasingly on narrow-body freighters, cargo is first built up into unit load devices, standardized containers or pallets sized to fit the specific aircraft's hold contours. Understanding the basics of how ULDs work explains a surprising amount of what shapes an air freight quote and schedule.
What a ULD actually is
A unit load device is a container or pallet, built to standardized dimensions set by IATA, that consolidates individual pieces of cargo into a single unit for loading. This matters for two reasons: it lets ground handlers load and unload an aircraft far faster than piece-by-piece handling would allow, and it means cargo dimensions have to be planned against the ULD's actual internal shape, not just the aircraft's overall hold dimensions, since a ULD's contoured shape (matched to the curve of the aircraft fuselage) reduces usable space compared to its outer footprint.
Containers versus pallets
ULDs broadly split into two categories:
- Containers (sometimes called cans, with codes like AKE or LD3) are enclosed units, typically used for general cargo, that protect contents from the elements during ground handling and offer a defined maximum weight and volume.
- Pallets (flat aluminum platforms, often coded PMC or PAG) carry cargo built up and secured under a net or tarpaulin, and are generally used for larger, heavier, or irregularly shaped cargo that would not fit efficiently inside an enclosed container.
The specific type used for a given aircraft depends on that aircraft's hold configuration; a widebody passenger aircraft's lower deck and a dedicated freighter's main deck are built for different ULD sizes, which is one reason the same cargo can have different volumetric efficiency depending on which aircraft type actually operates the flight.
Why tare weight matters
The ULD itself has weight, and that tare weight is added to the cargo's actual weight when calculating total chargeable weight for the shipment. A larger container-type ULD can have a tare weight running into several hundred kilograms. For a heavy shipment that fills a ULD close to its maximum payload, tare weight is a small percentage of the total and rarely changes the economics. For a lighter shipment, particularly one being quoted close to a minimum chargeable weight threshold, tare weight is worth asking about directly rather than assuming only the cargo's declared weight is what gets billed.
Why cargo sometimes cannot use the ULD it seems to fit
Volumetric fit is not the only constraint. Weight limits per ULD position are set by the aircraft's structural loading limits and the specific position within the hold, not just the ULD's own rated maximum payload. Dangerous goods, cargo requiring temperature control, and oversized cargo that cannot be contoured to fit a standard ULD shape all have their own handling rules that can rule out certain ULD types or aircraft entirely, regardless of whether the cargo would technically fit the dimensions on paper.
What shippers should actually check before booking
| Question to ask | Why it matters |
|---|---|
| Is my cargo going on a passenger belly hold or a dedicated freighter? | Available ULD types and volumetric efficiency differ significantly between the two |
| What is the tare weight being added to my chargeable weight? | Matters most for lighter shipments quoted close to a rate break |
| Does my cargo's shape actually use the ULD's contoured space efficiently? | Irregular shapes can waste billable volume even within stated dimensional limits |
| Are there handling restrictions (dangerous goods, temperature, orientation) that limit which ULD or aircraft type is usable? | Can restrict routing options or require specific booking well in advance |
ULD mechanics are one of those details air freight quotes rarely explain but that quietly shape both cost and schedule. For the surrounding rate mechanics, see our guide to how freight rates are calculated, or post a freight request to compare air freight quotes for your next shipment.
