Container Loading Plans and CBM Optimisation: Why Your 68 CBM Container Holds 55

A 40HQ container has a nominal internal volume of about 76 cubic metres. Ship something that stacks perfectly and you will load 68. Ship cartons that do not stack, on pallets that do not fit, and you will load 52 and pay for 76. The gap between those two numbers is not a rounding error — on a China–Europe lane it is several thousand dollars of freight paid for air.

This article is about closing that gap. It is not about clever tricks; it is about three numbers, one rule, and doing the geometry before the cargo is packed rather than after.

The three numbers that decide what you pay

1. Volume, in cubic metres

Length × width × height of each carton, in metres, multiplied out. Measure the carton as packed, not as designed — a carton that bulges by 2 cm on each side loses 4% of its volume in a container.

2. Chargeable weight

For LCL sea freight the convention is weight or measure, whichever is greater, at a ratio of one cubic metre to one tonne. A cargo of 3 CBM weighing 400 kg is charged as 3 revenue tons. A cargo of 1.5 CBM weighing 2,200 kg is charged as 2.2 revenue tons. For air freight the ratio is different: one cubic metre equals 167 kg, so a light bulky cargo is charged on volume, not weight.

3. Payload

Every container has a maximum gross weight, and the payload is that minus the tare. A 20GP carries roughly 28 tonnes; a 40HQ roughly 26.5 tonnes, and less in countries with strict road weight limits. Heavy cargo runs out of payload before it runs out of space, and light cargo does the opposite. Knowing which constraint binds your cargo is the first decision in any loading plan.

Type	Internal dimensions (m)	Nominal CBM	Realistic usable CBM	Payload
20GP	5.90 × 2.35 × 2.39	33.2	27–29	~28,200 kg
40GP	12.03 × 2.35 × 2.39	67.7	56–60	~26,700 kg
40HQ	12.03 × 2.35 × 2.70	76.4	63–68	~26,500 kg
45HQ	13.56 × 2.35 × 2.70	86.0	71–76	~27,700 kg

The 'realistic usable' column is not conservative marketing. It reflects the fact that no cargo fills a rectangular box perfectly: there are door frames, corrugation ribs, floor rings and a ceiling that is lower at the edges.

The 85% rule

As a planning default, assume you will load 85% of nominal volume with cartons, and 75–80% if the cargo is palletised. This single rule prevents most of the unpleasant surprises in container shipping.

• Loose cartons, uniform size: 88–92% achievable with a good plan.
• Loose cartons, mixed sizes: 82–88%, because the small cartons have to fill the gaps around the big ones.
• Palletised cargo, standard 1.2 × 1.0 m pallets: 75–80%. A 20GP takes 10 pallets in a single tier, a 40GP takes 20–21. The pallet footprint wastes the space above it unless you double-stack.
• Drums and barrels: 70–78%, because cylinders do not tessellate.
• Machinery and irregular shapes: 55–70%, and often the binding constraint is lashing space rather than volume.

Loadability by cargo type

The same 40HQ holds wildly different quantities depending on how the cargo is presented. This table is worth keeping.

Cargo	Typical loading	What limits it
Standard export cartons, 60 × 40 × 40 cm	~1,150 cartons, 66 CBM	Ceiling height and stacking strength
Cartons on 1.2 × 1.0 m pallets, 1.6 m high	20–21 pallets, 40–45 CBM of cargo	Floor footprint and pallet height
200 L steel drums	80 drums in a 20GP	Payload and diameter
Bagged cargo, 50 kg bags	~26 t in a 20GP	Payload, not volume
Furniture, flat-packed	60–68 CBM	Carton length versus container width
Rolled goods	45–55 CBM	Cylindrical voids
Machinery on skids	30–50 CBM equivalent	Lashing points and centre of gravity

The six mistakes that cost the most

1. Designing the carton without designing the pallet

A carton of 60 × 40 cm fits a 1.2 × 1.0 m pallet exactly, five per layer with no overhang. A carton of 65 × 45 cm does not: you get four per layer with 15 cm of waste on one side. That single decision — made by a packaging designer who never sees a container — can cost 8% of the load.

2. Ignoring the door opening

The door is narrower and lower than the interior. A 40HQ door is about 2.34 m wide and 2.58 m high, against an interior 2.35 m wide and 2.70 m high. Machinery that fits inside will not go through the door if it is loaded on a skid that adds 12 cm of height.

3. Stacking beyond the carton's compression strength

A carton rated for 300 kg stacked eight high with 40 kg inside is carrying 280 kg at the bottom. It will crush, and the crush propagates through the whole stack. Always calculate the bottom-layer load, and specify the stacking strength in the packaging specification rather than assuming it.

4. Forgetting the weight distribution

A container loaded heavily at one end and lightly at the other creates a handling problem and, on some routes, a refusal. Keep the centre of gravity near the middle and avoid concentrating more than about 60% of the payload in half the container length.

5. Planning volume but not lashing

Lashing points, dunnage and airbags consume space that was counted as cargo. For a machinery shipment, budget 3–5% of the volume for securing material, and remember that the surveyor will want to see the lashing plan before the doors close.

6. Not allowing for humidity

On a 30-day sea voyage a container can accumulate significant condensation. For moisture-sensitive cargo, add desiccant and a container liner, and leave the top layer slightly lower than the ceiling to allow air movement. This costs a little volume and prevents a total loss claim.

How to build a loading plan

1. List every SKU with carton dimensions, gross weight and quantity.
2. Calculate total CBM and total weight. Compare against the container payload and usable volume — one will bind first.
3. Choose the container type based on which constraint binds. Heavy cargo: 20GP. Light and bulky: 40HQ or 45HQ.
4. Test the carton-to-pallet fit. If the carton does not divide evenly into the pallet footprint, redesign the carton or go loose.
5. Test the pallet-to-container fit. A 40GP holds 20 pallets of 1.2 × 1.0 m in a single tier, or 40 if double-stacked and the cargo allows.
6. Draw the load: a simple elevation and plan view, showing which SKU goes where and at what height.
7. Check the door opening against the largest unit, including any skid or crate.
8. Calculate the bottom-layer compression load and confirm the carton specification.
9. Add lashing, dunnage and desiccant to the plan, and reduce the cargo volume accordingly.
10. Send the plan to the warehouse with photographs of the reference load, not just a text description.

A worked example

A shipment of 900 cartons, each 60 × 40 × 35 cm, 22 kg gross. Total volume is 900 × 0.084 = 75.6 CBM. Total weight is 19,800 kg.

Volume says 40HQ (76.4 nominal, 63–68 usable) is too small at 75.6 CBM, so a naive calculation would book a 45HQ. But the cartons are 60 × 40 cm, which fits a 1.2 × 1.0 m pallet exactly at five per layer, and the 35 cm height means four layers per pallet at 1.4 m. That is 20 cartons per pallet, 45 pallets for the shipment.

A 40HQ holds 20 pallets in a single tier — so 45 pallets needs either three containers single-tier, or two containers with double-stacking. Double-stacking to 2.8 m is not possible in a 2.70 m container, so the pallets must be single-tier at 1.4 m, which wastes the top 1.3 m.

The correct answer is to load loose, not palletised. 900 cartons at 0.084 CBM is 75.6 CBM of cargo, which fits a 45HQ at 88% utilisation, or a 40HQ at 99.5% — too tight for a real load. So: one 45HQ, loaded loose, 900 cartons, 19.8 tonnes. The naive answer (45HQ) and the correct answer coincide, but only after the pallet arithmetic showed that palletising would have required two 40HQs and cost roughly 60% more.

That is the whole discipline: the answer is rarely the first number you calculate.

Checklist before you book

• Every SKU listed with measured carton dimensions and gross weight
• Total CBM and total weight calculated, and the binding constraint identified
• Container type chosen against the binding constraint, not against habit
• Carton-to-pallet fit tested, and palletising costed against loose loading
• Pallet-to-container fit tested, including double-stack height
• Door opening checked against the largest unit including skid or crate
• Bottom-layer compression calculated against the carton specification
• Lashing, dunnage and desiccant volume deducted from usable CBM
• Loading plan drawn and sent to the warehouse with reference photographs
• Container weighed at origin and the ticket retained

CBM optimisation is not a trick and it is not a negotiation with the carrier. It is arithmetic done at the right moment. Done before the cartons are made, it is free. Done after the container is at the port, it is a rework invoice.