Most hardware teams obsess over the unit cost of the PCB and ignore the box it ships in. That's backwards. On a sea-freight order, the carton and how it stacks into a container can swing your landed cost per unit more than a 10-cent BOM change ever will. Packaging is an engineering problem, and it has math.
The cartonization math
A 40-foot standard container holds about 67-68 CBM (cubic meters) of usable volume. Your job is to fill that volume — or hit the weight limit — with as many salable units as possible.
Work it from the product out:
- Inner box / retail unit — your product plus its retail packaging.
- Master carton — how many units per export carton (the count the factory ships in).
- Container — how many master cartons fit in 67 CBM (and under the ~26,000 kg payload).
A worked example. Say your master carton is 600 × 400 × 300 mm = 0.072 CBM, holding 20 units. A 40-ft container fits roughly 67 / 0.072 ≈ 930 cartons, so about 18,600 units per container. Now suppose you trim the retail box and fit 24 units in the same carton. Same 930 cartons, but now 22,320 units — a 20% jump in units per container with zero change to freight cost. If the ocean leg costs, say, $4,000, your freight-per-unit drops from about $0.22 to $0.18. Across multiple containers that's real money.
CBM vs weight: which one binds?
Containers cap out on whichever you hit first — volume or weight.
- Light, bulky goods (foam, plastics, soft goods) → you "cube out" before you hit the weight limit. Optimize for tighter packing.
- Dense goods (motors, batteries, metal enclosures) → you "weigh out" with the container half empty. There's no point cramming more volume; redesign for weight or accept the empty space.
Know which regime you're in before you redesign the carton. Squeezing volume on a product that weighs out is wasted engineering.
Protection: don't trade freight savings for breakage
Tighter packing is only a win if the product survives. Ocean containers see drops, edge crush, and stacking loads from cartons piled on top.
- Spec edge crush test (ECT) or burst strength on the corrugate, not just "double wall." A carton that fails ECT collapses mid-stack and crushes everything below it.
- Protect corners and edges — that's where drop energy concentrates.
- For a glass-front or screen product, run a drop test on the actual packed carton before the full run. One broken-sample test on a pre-production carton is cheaper than a container of cracked units.
Retail-ready vs bulk
Decide who opens the box. If units go straight to Amazon FBA or retail shelves, you need retail-ready packaging with barcodes, polybag/suffocation warnings, and FBA carton limits (typically ≤22.5 kg, specific dimension caps). If they go to your own warehouse for kitting, bulk-pack and skip the retail overhead — it's lighter and cheaper to ship.
The action: spec the carton before you tool the product
Put the carton config in your packaging spec at the same time as the product, not after. Specify units/carton, carton dimensions, ECT rating, and gross weight, then calculate units-per-container and divide your freight quote by it. That number — landed freight per unit — is what you actually optimize.
If you're shipping from China and don't want to manage cartonization, ECT specs, and the door-to-door customs leg yourself, an engineering-led agent like China Sourcing Agents handles logistics and packaging optimization as part of the run — including checking that the master carton fills the container instead of shipping you air.
Do the CBM math once, before tooling, and packaging stops being an afterthought and starts being a margin lever.
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