Container loading guide
How to estimate how many cartons fit in a 20 ft, 40 ft or High Cube container using real geometry and payload constraints.
CBM is a useful check—not the loading plan
Cubic metres are useful for comparing total cargo volume with nominal container volume, but they do not answer the geometric question: do the cartons actually fit? Two cargo sets can have the same CBM and very different real capacities because carton side lengths interact differently with the container length, width and height.
A robust single-SKU container calculation therefore needs at least five things: internal dimensions, carton dimensions, allowed rotations, door feasibility and payload.
1. Use internal dimensions, not external container dimensions
A container is sold by an external class such as 20 ft or 40 ft, but loading happens inside it. PackMize presets therefore store representative internal length, width and height as well as door width, door height and maximum payload.
| PackMize preset | Internal dimensions used by PackMize | Calculated rectangular volume |
|---|---|---|
| 20 ft Standard | 5898 × 2352 × 2393 mm | 33.2 m³ |
| 40 ft Standard | 12032 × 2352 × 2393 mm | 67.7 m³ |
| 40 ft High Cube | 12032 × 2352 × 2698 mm | 76.4 m³ |
| 45 ft High Cube | 13556 × 2352 × 2698 mm | 86.0 m³ |
These are planning presets, not universal specifications. Real equipment varies, so the custom-container option is the correct choice when a carrier or container owner provides exact dimensions.
2. Test carton orientations geometrically
Consider a 40 ft High Cube and cartons measuring 400 × 300 × 250 mm. In one straightforward orientation, 400 mm runs along the container length, 300 mm across the width and 250 mm vertically.
An aligned layer can fit:
floor(12032 / 400) = 30 cartons along the length
floor(2352 / 300) = 7 cartons across the width
That gives 210 cartons per aligned layer. Ten 250 mm layers fit inside 2698 mm, so that simple orientation yields 2100 cartons before considering payload or more sophisticated layer patterns. Rotating the carton changes all three divisors, which is why the optimizer tests valid orientations rather than using one fixed grid.
3. Check the door separately
A box can fit inside the theoretical interior but still fail to pass through the door in the required orientation. PackMize performs a basic door-feasibility check against the carton orientations. It does not simulate the real loading motion, turning radius, forklift path or sequence needed to position every carton.
4. Apply payload after geometric capacity
A container can run out of allowable mass before it runs out of space. If the geometric layout can hold 2100 cartons at 8 kg each, cargo mass is 16,800 kg. That remains below the representative 26,580 kg payload in the PackMize 40 ft High Cube preset. If each carton instead weighs 15 kg, the same geometric count would weigh 31,500 kg and payload would become the limiting constraint.
PackMize therefore reports geometric capacity and then caps the final fitted quantity by payload and the quantity requested by the user.
5. Understand what utilization means
Volume utilization is the total carton volume loaded divided by the internal rectangular container volume. It is a useful efficiency indicator, but it should not be confused with load quality. Empty strips can remain even when utilization is high, and a dense load may still require specific weight distribution or blocking.
Standard vs High Cube
The extra height of a High Cube is only valuable when it creates another usable layer or enables a better carton orientation. If a carton is 600 mm high, the difference between 2393 mm and 2698 mm may or may not produce an extra complete layer depending on the orientation and how the layer stack is combined. This is why comparing nominal CBM alone can exaggerate or understate the practical benefit.
What PackMize does not model
The free Container Loading Calculator is designed for one rigid rectangular SKU. PackMize also provides a separate Multi-SKU Load Planner with Hybrid layer-safe/support-aware upright 3D planning, delivery-stop ordering and blocker auditing, geometric cargo center-of-gravity/balance diagnostics, validated manual editing including moves between loads and a palletized-container floor estimate. Vehicle Load Balance can additionally estimate static two-support reactions from user-entered geometry and optional limits. It still does not provide legal axle-load certification, physical stability or crush strength, friction, lashing, full forklift manoeuvring, dangerous-goods segregation, bracing, ventilation requirements or carrier rules. Those remain operational constraints outside the current geometric model.
Practical workflow
- Use actual external carton dimensions including packaging.
- Choose the exact container specification when available.
- Disable rotations prohibited by “this side up” or product handling rules.
- Enter real carton weight and requested quantity.
- Inspect fitted quantity, payload, utilization and the visual layer plan.
- Validate loading sequence, weight distribution and transport safety separately.
Example: prefabricated-building shipments
Prefabricated buildings can ship as folded units, flat-pack frames and panels, or separately packed components. The marketed or assembled room dimensions are not valid substitutes for the dimensions of the actual shipping packages. A room count is not necessarily a package count.
Before estimating capacity, request a configuration-linked packing list from the supplier. For each bundle, crate or accessory carton, confirm:
- Package identifier, contents and associated building or assembly reference.
- Actual external length, width and height in shipping condition, including wrapping, guards, pallets and protrusions.
- Gross package weight, quantity, permitted orientation and handling restrictions.
- Separately shipped stairs, fittings, hardware, utility equipment or other accessories.
- Document version or date, plus any provisional dimensions that must be rechecked after a design change.
Use the single-SKU calculator only for a set of comparable rigid rectangular packages. Multiple package sizes can be explored in the Multi-SKU Load Planner, but a geometric estimate does not validate irregular building modules, lifting points, load-bearing support, securing, transport stability or site unloading. Those require a configuration-specific plan from the responsible logistics and engineering professionals.
Optional supplier example: PQSE House's container-house loading and shipping guide illustrates how a manufacturer links a final room configuration to its packed components, protection, freight handover and receiving checklist. This is manufacturer-authored guidance, not an independent transport-safety standard or evidence that all prefabricated-building packages can be represented by PackMize's rigid-box model.
Assumptions and data sources
Published carrier dimensions are reference points, not universal constants. Compare PackMize presets with Hapag-Lloyd 20 ft Standard and 40 ft High Cube, then use custom dimensions for the actual equipment.
Frequently asked questions
Why is CBM not enough to calculate container capacity?
CBM compares total volumes but ignores whether carton side lengths tile the container dimensions. Real capacity also depends on orientation, door feasibility and payload.
What is the main difference between a 40 ft Standard and 40 ft High Cube?
A High Cube mainly provides more internal height. Whether that creates additional capacity depends on the carton dimensions and permitted orientations.
Do standard container dimensions vary?
Yes. Internal and door dimensions can vary by manufacturer, owner and equipment series. Use the actual equipment specification for operational planning.
Does PackMize calculate load safety?
No. The current engine estimates geometric and payload capacity for rigid rectangular cartons. It does not certify lashing, axle loads, center of gravity, dangerous-goods segregation or loading sequence.
Equipment reference: Hapag-Lloyd container specifications illustrate why internal dimensions, doors and payload must be checked for the actual equipment. PackMize presets are representative values and do not reproduce every carrier specification.