The short answer: a shipping container holds whichever of two limits you hit first — volume (CBM) or payload (kg) — and for most custom packaging the binding constraint is set before freight is ever booked, in the master-carton dimensions. Carton volume is length × width × height in metres, multiplied by carton count. A 20ft general-purpose (GP) container gives you roughly 33 m³ of internal volume and about 28 tonnes of payload; a 40ft GP gives roughly 67 m³; a 40ft high-cube gives roughly 76 m³. A realistic plan is 85–90% of that nominal volume.
The two limits that decide your container
Volume: the CBM math
CBM (cubic metre) is the measure ocean freight is planned in:
CBM = length (m) × width (m) × height (m) × number of cartons
A 60 × 40 × 40 cm master carton is 0.6 × 0.4 × 0.4 = 0.096 m³. One hundred of them is 9.6 m³ — about a quarter of a 40ft container.
Payload: when weight fills before space
Flat, light packaging such as courier bags and folding cartons shipped flat is almost always volume-bound: you run out of space before you run out of weight. Dense rigid boxes or heavy molded pulp trays can be weight-bound. Always check both.
| Container type | Nominal internal volume | Typical payload | Best for |
|---|---|---|---|
| 20ft GP | ≈ 33 m³ | ≈ 28,000 kg | Small lots, first full-container order |
| 40ft GP | ≈ 67 m³ | ≈ 26,500 kg | Most full-load packaging orders |
| 40ft HC | ≈ 76 m³ | ≈ 26,500 kg | Bulky or tall-carton formats |
These are reference values from carrier container specifications and vary slightly by manufacturer and operator (Hapag-Lloyd container specification).
How to estimate cartons per container in five steps
- Write down the finished master-carton dimensions in centimetres.
- Convert each to metres (divide by 100) and multiply length × width × height to get CBM per carton.
- Multiply by the number of cartons in the order to get total CBM.
- Compare total CBM against usable container volume — plan on 85–90% of the nominal figure.
- Multiply gross weight per carton by carton count and confirm it stays under payload.
A flat-pack example: courier bags
A brand orders 100,000 custom courier bags, packed 500 per master carton:
- Master carton: 40 × 30 × 25 cm = 0.03 m³
- 200 cartons × 0.03 m³ = 6.0 m³
- 6.0 m³ against a 20ft's ~30 m³ usable → about 20% fill
Six cubic metres does not justify a dedicated 20ft; this is a less-than-container (LCL) shipment or a consolidation. The same math in reverse is why a full container only makes sense once your flat-pack volume approaches the container's usable volume.
A bulky example: rigid boxes
The same unit count in a rigid format behaves differently. 20,000 rigid boxes at 10 per carton, in a 50 × 40 × 40 cm carton:
- 50 × 40 × 40 cm = 0.08 m³
- 2,000 cartons × 0.08 m³ = 160 m³
- 160 ÷ 76 (40ft HC) ≈ 2.1 high-cubes by volume
Identical unit count, very different freight: this is where folding cartons vs rigid boxes becomes a freight decision, not just an unboxing decision.
Why your packaging format changes the container count
Two formats can hold the same product at very different freight economics:
- Flat or nested formats — folding cartons, mailers, pouches and bags shipped flat — compress into cartons with almost no void and fill containers densely.
- Rigid or pre-formed formats — rigid boxes, molded pulp trays and any pre-erected structure — keep their shape in the carton, so air ships for free and volume fills far faster.
This is the "size = freight" lever we care about most: choosing a format that ships flat, or right-sizing the master carton, can remove whole containers from an annual volume. For the outbound side of the same problem, see our guide on right-sizing e-commerce packaging and dimensional weight.
The 10–15% you always leave on the floor
Nominal container volume is not loadable volume. Cartons rarely pack perfectly because of:
- Carton dimensions that don't divide evenly into the container's internal length, width or height
- Palletisation, which consumes height and floor area
- Headspace and load-locking gaps that keep cargo stable in transit
A realistic plan is 85–90% utilisation. If your estimate says "it just fits at 100%", it doesn't fit — drop a layer or resize the carton.
What we calculate at quoting — and what we won't guess
Because our packaging is made to order, the numbers that feed this math are confirmed during quoting, not published as fixed figures:
- We confirm master-carton dimensions and gross weight from your width × height + quantity.
- We provide a container-loading estimate so you can compare formats before committing.
- We don't publish fixed prices, MOQs, lead times or freight rates — these are confirmed in the formal quotation, since they depend on the structure, factory and route.
If you already have a size and quantity in mind, send them through our RFQ form and we'll run the cartonization and container math with you — before you commit to a format.