One centimeter on a carton decides your freight bill
Container loading optimization and CBM design - from interior-width math to the loading plan and the stuffing floor

Hello, this is GreenFrog Seoul.

"You said it would all fit in one 40HQ. Why are 300 cartons sitting on the dock?"
"My cargo is 58 CBM and the container holds 68. Where did the other 10 go?"
"The factory made the carton 2cm bigger and my freight bill jumped by three thousand dollars. How?"
"Should I go LCL, or just book a 20FT?"

Ocean freight is usually the second biggest line in your landed cost after the goods themselves, and the number is set by volume, not by what the cargo is worth. That volume does not come from your product. It comes from the box your product sits in. Two importers ship the same 5,000 units; one closes out a single 40HQ and the other has to add a 20FT. The gap usually starts with a few centimeters the factory picked without thinking about it.

Today we are on container loading optimization. How CBM is actually calculated, why the capacity on the container spec sheet and the capacity you get are two different numbers, how to size the master carton backwards from the container's interior width, what you gain and give up with pallets versus floor loading, where LCL stops being cheaper than FCL, and how to hand the factory a loading plan that actually controls the floor.

Most people go after freight savings by squeezing the forwarder's rate. The bigger money moves somewhere else entirely: on the carton drawing. Shaving 5% off a rate is hard work; redesigning a box so one container covers the whole order is easier and pays more.

1. Freight is priced on volume, not weight

For ocean LCL and air freight, the billing unit is not actual weight but the revenue ton (R/T): the carrier takes whichever is larger, your actual weight in tons or your volume in CBM, and charges on that. For almost every consumer good coming out of China, volume wins. Apparel, plastic housewares, lighting and toys run about 150-250kg per CBM, which means in practice you are paying purely for air and cardboard.

The CBM math itself is simple. Convert one carton's length, width and height to meters, multiply them together, then multiply by your carton count.

CBM = (L m × W m × H m) × number of cartons
Example: a carton measuring 60 × 40 × 35cm, 400 cartons total
0.6 × 0.4 × 0.35 = 0.084 CBM → 0.084 × 400 = 33.6 CBM

One thing here matters more than it looks. The dimensions that go into that formula are not your product size and not the carton's inside dimensions. They are the measured outside dimensions of the master carton, corrugate thickness and post-taping bulge included. A carton the factory drew at 60cm routinely comes out at 61.5cm, and that 1.5cm can cost you an entire row in the container.

Air works a little differently. Volumetric weight is L cm × W cm × H cm ÷ 6000, converted to kilos and compared against actual weight. Couriers such as DHL, FedEx and UPS often divide by 5000 instead, so the same carton bills about 20% heavier. That is why the quote for a sample shipment or a first small run lands higher than you expected.

2. The spec sheet capacity is not what you get

Plenty of importers plan against the spec sheet and then get a surprise at the port. A 40HQ having 76 CBM of interior volume does not mean you can load 76 CBM. Unless your cartons are perfect cubes there will be gaps, the door end needs working space, and the last row needs clearance so it does not topple.

The working numbers are 25-28 CBM for a 20FT, 54-58 for a 40FT, 60-68 for a 40HQ. When your carton dimensions line up with the interior width you land at the top of the range; when they do not, you will not even reach the bottom. Closing that 8 CBM gap is what this article is about.

Loading efficiency is usually read as actual loaded CBM ÷ theoretical volume. A well-planned floor load gets 85-90%, pallets get 70-80%, and stuffing with no plan at all drops below 65%. On a 40HQ, the difference between 65% and 88% is 17 CBM — roughly a quarter of the box.

3. Size the carton backwards from the interior width

If you take one thing from this article, take this. Most factories build a carton around whatever size the product needs and never check how many of those cartons fit a container. Reverse the order. Start with a dimension that divides cleanly into the 235cm interior width, then fit your pack quantity to it.

How to split 235cm

Look at 60cm against 58cm. Two centimeters off the width takes you from 3 cartons per row to 4. That is a third more cargo in the same container. Asking the factory to hold the carton to 58cm costs essentially nothing, and it uses less corrugate besides.

Run the length the same way: take the 12.03m interior and divide by your carton's other dimension to see what is left over. On the floor, crews often turn some rows 90 degrees and mix orientations, so the ideal is a carton whose two dimensions both divide reasonably well into 235cm and 1203cm.

Height counts too

Interior height is 239cm on a DC and 269cm on an HQ. Subtract pallet height (usually 14-15cm) if you are palletizing, or about 5cm of top clearance if you are floor loading.

The 45cm row stands out. DC and HQ both take 5 tiers, so there is no reason to book the HQ. Either trim the carton to 42cm and get 6 HQ tiers (252cm), or drop to a DC and keep the surcharge. You pick the container type after you fix the carton height, not the other way round.

4. Pallets or floor loading

Pallets speed up handling and protect the goods, but the pallet itself eats volume. Sometimes a domestic distribution center or an Amazon FBA receiving requirement takes the choice away from you — so at least know what each option gains and gives up before you decide.

Here are the common pallet standards and how many fit a container. Korea's T11 (1100×1100), Europe's EUR (1200×800) and the US GMA (1219×1016) come up most often.

If you are palletizing, deal with overhang — cartons sticking out past the pallet edge — first. A single centimeter of overhang touches the neighboring pallet and costs you an entire row, and in the warehouse it fouls the racking. Underfilling the pallet footprint wastes the same space in the other direction. When you set carton dimensions, check that they divide into the pallet top surface as well as the container width.

5. Where LCL flips to FCL

This is the most common question when the volume sits in between. LCL bills by the CBM; FCL bills for the whole box regardless. Past a certain CBM, taking a 20FT outright comes out cheaper.

LCL looks cheap on the per-CBM rate, but the invoice tells another story. CFS charges, THC, destination D/O fees and bonded storage pile on, and those extras sometimes rival the freight itself. Consolidated cargo also gets handled alongside other shippers' goods, so damage and delay risk run higher. Once you are over 15 CBM, put a 20FT quote side by side with the LCL number.

There is one more test worth using: divide the all-in 20FT price by your expected CBM and compare it against the LCL all-in rate. A 900-dollar 20FT carrying 20 CBM works out to 45 dollars per CBM; if LCL all-in is 60, take the container. And if you can put next season's order in the leftover space, the gap widens further.

6. Weight limits and uneven loading

Focus only on volume and weight will catch you out. Dense cargo — tile, hand tools, metal parts, liquids — hits the weight ceiling before filling half the container.

Two separate limits apply. The first is the container's own max payload, marked on the door: roughly 28 tons for a 20FT, 26 for a 40FT. The second is road transport regulation. In Korea, gross combination weight limits covering tractor, chassis and container mean the practical cutoff is 20-22 tons on a 20FT and 24-26 on a 40FT. You can end up with cargo that is legally inside the container but cannot legally move on the road.

Before loading you also have to declare the verified gross mass under SOLAS VGM. If the declared figure and the measured one diverge badly, the shipment gets refused or fined. Check the gross weights on the factory's packing list against reality.

7. Handing the factory a loading plan

You can get all the math right and still lose it on the floor, for a simple reason: the workers stuffing the container have never seen your calculation and will load it the way they always do. Hence the loading plan. Nothing elaborate — one A4 page does the job.

Put these on it.

• Container type and count — spell it out, "one 40HQ."
• Cartons and outer dimensions per item — use a table if you have multiple SKUs.
• Stacking pattern — give numbers: 4 across × 2 deep × 7 tiers = 56 cartons per block.
• Loading sequence — whatever gets unloaded first at destination goes at the door end.
• Weight distribution — instruct that heavy SKUs go on the bottom and toward the center.
• Securing — state whether airbags, straps or dunnage are used when voids appear.
• Photos — require four: first tier complete, half loaded, just before the doors close, and the seal number.

Do not skip that last one. When quantities come up short or damage turns up at destination, stuffing-day photos sort out responsibility fast. The seal number gets checked against the number on the B/L, which is how you tell whether the box was opened en route.

With large volumes or many SKUs, container load simulation software is worth a look. Commercial tools draw a stacking diagram from your carton dimensions and quantities, and there are free web calculators too. Just remember that whatever the program spits out is a theoretical figure. Plan against 90-95% of it to stay safe.

8. What the improvement looks like in numbers

Here is a worked example: 5,000 units of a housewares item moving from Ningbo to Busan. The figures illustrate the structure rather than quote any real shipment.

Before

After

Carton count went up from 417 to 500, yet total volume dropped from 47.3 to 37.3 CBM. The box shrank to fit the product and the dead air inside it disappeared. The order now closes out in a single container, so the extra LCL shipment and everything attached to it vanish from the bill. Total cost of the change: one email asking the factory to redraw the carton.

Of course, moving from 12 per carton to 10 moves your selling unit and your retail price with it. This is not a decision logistics makes alone. What you should know before you place the order is that carton dimensions are a variable you can negotiate with the factory. For where this sits inside the wider cost structure, read it alongside the cost breakdown in Episode 97.

9. Common mistakes

These come up again and again in loading consultations.

• Calculating CBM from the dimensions on the factory's drawing without measuring an actual carton
• Planning against theoretical spec-sheet volume (40HQ = 76 CBM) as if it were usable capacity
• Paying the HQ surcharge when the carton height yields the same tier count on a DC
• Not checking pallet overhang and losing a full row because of it
• Skipping the IPPC fumigation mark on wood pallets and paying for it at destination
• Watching volume only and running into the payload limit or road weight regulations
• Stuffing with no loading plan and leaving the pattern to whoever is on the floor
• Not collecting stuffing photos and the seal number, then having no evidence in a damage or shortage dispute
• Leaving space empty in an FCL and sending the rest by separate LCL next month

10. Container loading checklist

Carton design

• Checked that the carton width divides cleanly into the 235cm interior width
• Checked whether the carton height gains a tier at DC 234cm or HQ 264cm
• If palletizing, checked that it also divides into the pallet top surface
• Asked the factory for board grade and compression strength given the planned stack height
• Approved the master carton drawing at the sample approval stage

Shipment planning

• Collected measured carton dimensions, pack quantity, carton count, gross weight and pallet plan from the factory
• Calculated total CBM and total weight and chose container type and count
• Over 15 CBM, compared the LCL all-in quote against the total 20FT price
• For dense cargo, confirmed road transport weight limits with the forwarder
• Reviewed what else could ride along if space is left over

Stuffing day

• Sent the loading plan to the factory and got written confirmation back
• Instructed weight distribution and securing method (airbags, straps)
• Requested all four photos: first tier, half loaded, before closing, seal
• Checked the seal number against the number on the B/L
• Confirmed the packing list gross weight matches the VGM declaration

Closing: freight savings come from design, not negotiation

Everyone compares three forwarders and works a few percent off the rate. Put that same effort into the carton drawing and far more money moves. Take 2cm off a carton width so four fit across instead of three, and that one decision keeps paying on every shipment you make afterwards.

Two things to carry away. First, carton dimensions get set backwards from the container's interior width, not forwards from the product. Second, no amount of good math survives a factory floor that never saw it, so finish the job with a loading plan and stuffing photos. Those two habits alone lift loading efficiency by 10-20 points on most cargo.

GreenFrog Seoul helps importers buying from China design freight out at the carton stage, control the stuffing floor, and save a container. For dangerous goods or cargo needing special equipment, bring in a specialist and your forwarder alongside this.