Master carton dimensions are the numbers a buyer's freight forwarder asks for before anyone talks price per unit — and the numbers most supplier quotations still leave out. A master carton is the outer shipping box that holds multiple inner cartons or units; its external dimensions and gross weight drive the freight quote, the container loading plan, and whether the receiving warehouse accepts the delivery without a surcharge. Miss a field and the deal doesn't die — it stalls, while three people email you for the same missing number.
A master carton spec is the one set of numbers your buyer, their freight forwarder, and the receiving warehouse all read — get one digit wrong and all three call you.
This is a copy-paste template for the packing block of your quotation or spec sheet, a walkthrough of each field, and the container math to sanity-check your own numbers before a buyer does.
The Master Carton Dimensions Template (Copy-Paste)
Drop this table into your quotation, spec sheet, or catalog page and fill it per SKU:
| Field | Example value | Unit |
|---|---|---|
| Product model / SKU | LC-2041 lounge chair | — |
| Units per inner carton | 1 | pcs |
| Inner cartons per master | 2 | ctns |
| Units per master carton | 2 | pcs |
| Master carton dimensions (L × W × H, external) | 60 × 40 × 50 | cm |
| CBM per master carton | 0.120 | m³ |
| Gross weight (GW) | 14.5 | kg |
| Net weight (NW) | 12.8 | kg |
| Cartons per 20GP / 40GP / 40HC (practical) | 235 / 479 / 541 | ctns |
| Carton material & test strength | 5-ply corrugated, K=K, 32 ECT | — |
| Pallet / wood packing | ISPM-15 heat-treated, 1.2 × 1.0 m | — |
| Shipping marks | Main + side mark, see below | — |
Twelve fields. A buyer who receives all twelve can book freight the same day. A buyer who receives "carton size: about 60×40×50" starts a thread.
How to Fill Each Field
Dimensions: external, in centimeters, in L × W × H order
Freight is calculated on the outside of the box, so measure the assembled carton's external dimensions — bulge included if the pack genuinely bulges. State the unit every time; a spec that reads "60 × 40 × 50" with no unit gets re-confirmed by every careful forwarder, and a US buyer may quietly read inches. Keep the order L × W × H and keep it consistent across your whole catalog, because mixed conventions across SKUs are how loading plans end up 8% short.
Inner carton vs master carton: say both, even when they're the same
The inner carton is the sellable or handling unit inside; the master carton is what gets stacked in the container. If your product packs one unit per master with no inner, write that explicitly ("1 pc / master, no inner carton") — the absence of a line is ambiguity, not information. Buyers repacking for marketplace fulfillment care about inner dimensions too; those decide whether the unit clears a fulfillment center's parcel tiers.
CBM: three decimals, from external dimensions
CBM (cubic meters) = L × W × H in meters. The example carton: 0.60 × 0.40 × 0.50 = 0.120 CBM. Round to three decimals and multiply by carton count for the shipment total — that total is the number your buyer compares against container capacity and the number a consolidator bills against. Cross-check any hand math with a standard CBM calculator for sea freight once per catalog update; a transposed digit here multiplies across every line of the order.
Gross weight, net weight, and which limit binds
GW is carton plus contents; NW is contents only. Both matter because a container has two ceilings — volume and payload — and which one binds depends on your product. Furniture fills a 40HC's volume long before its ~26,000 kg payload; tiles and stone hit the payload wall with half the container still empty. Knowing which ceiling binds for your product is the difference between quoting "479 cartons per 40GP" and quoting a number the forwarder rejects on weight.
Loading quantity: quote practical, not theoretical
Divide container volume by carton CBM and you get the theoretical maximum; real loading loses 10–15% to carton geometry, pallet gaps, and door clearance. Industry practice is to plan at 85–90% utilization. Quote the practical number and buyers stop discovering the gap at booking time.
Container Quick-Reference Table
Internal capacity and typical maximum payload for the three standard dry boxes, per standard container capacity references:
| Container | Internal volume | Practical volume (85–90%) | Typical max payload | Example: 0.120 CBM carton |
|---|---|---|---|---|
| 20GP | ~33.2 CBM | 28–30 CBM | ~28,000 kg | ~235–249 ctns |
| 40GP | ~67.7 CBM | 57–61 CBM | ~26,500 kg | ~479–507 ctns |
| 40HC | ~76.4 CBM | 65–69 CBM | ~26,300 kg | ~541–572 ctns |
Always pair the volume answer with the weight check: 249 cartons × 14.5 kg GW = 3,611 kg — far under payload for the example chair, but run the same multiplication for a 28 kg carton of ceramic tile and the payload ceiling arrives first.
ISPM-15: the wood-packing line item
If anything wooden ships with the goods — pallets, crates, dunnage — it falls under ISPM-15, the international standard for treating wood packaging. Heat treatment to a core temperature of 56°C for at least 30 minutes is the common route, and the IPPC mark must be stamped, legible, on at least two opposite sides. Manufactured boards (plywood, OSB, particleboard) and wood under 6mm thick are exempt. Non-compliant wood at the destination port means treatment, destruction, or re-export of the packaging at the shipper's expense — one line in your packing spec ("ISPM-15 HT pallets") tells the buyer you already know this. Full standard at the IPPC's ISPM 15 publication page; the ISPM 15 overview on Wikipedia is a readable summary of country adoption.
Where the Numbers Go Wrong
| Mistake | Consequence | Fix |
|---|---|---|
| Internal carton dims quoted as external | Loading plan overbooks; forwarder re-measures and re-quotes | Measure outside faces, state "external" |
| No units on dimensions | cm read as inches (or vice versa) | Write the unit in the header and the cell |
| Theoretical container count quoted | Buyer plans revenue on cartons that never fit | Quote at 85–90% utilization |
| Assembled product dims used in the packing block | Freight quoted on the wrong volume entirely | Packed dims in the packing block, assembled dims in the product spec — labeled |
| GW/NW swapped or estimated | Customs declarations mismatch weighbridge | Weigh a real production carton, not the sample |
| Wood packing unmentioned | ISPM-15 question at booking, or worse, at the port | One line: pallet type + treatment |
The assembled-vs-packed confusion deserves the extra flag because it burns suppliers twice: in freight, when the forwarder discovers the "0.35 CBM" chair is actually a 0.12 CBM flat-pack (annoying but survivable), and in reverse when a marketplace listing shows packed dimensions as product size — the buyer receives a chair smaller than the number on the listing, and that's a return. If you sell direct on marketplaces as well as B2B, the return cost calculator puts a number on what that mix-up costs per hundred orders.
Put the Packing Spec on the Image Itself
The template above answers the forwarder. But the earliest place a buyer looks for packing information isn't your quotation — it's photo 6 of your listing, and for most suppliers that slot is either empty or a bare photo of a brown box.
The fix costs one image: a photo of the actual master carton with its external dimensions, GW, and units-per-carton marked directly on it. Tools like a dimension and spec annotation tool draw the measurement lines and labels on the photo in minutes, so the numbers a buyer needs sit on the image they're already looking at instead of three emails away. Suppliers who structure their whole quotation this way close faster — the mechanics are covered in how spec sheets win B2B orders, and the field-by-field logic in product spec sheet buyers actually read.
Shipping marks belong on that image too. Standard practice: a main mark (buyer name or logo, PO number, destination) on one face, a side mark (carton number "12/235", quantity, GW/NW, external dimensions, country of origin) on an adjacent face, and handling marks where the product needs them. Photograph the marked carton once per order template and the "please confirm shipping mark" email disappears from your pre-shipment checklist.
FAQ
What is the difference between an inner carton and a master carton?
The inner carton is the smaller box holding one sellable unit or set; the master carton is the outer shipping box holding one or more inner cartons. Freight and container planning run on the master carton's external dimensions and gross weight; marketplace and fulfillment decisions often run on the inner carton's. A complete packing spec states both.
How do I calculate CBM for a master carton?
Multiply external length × width × height in meters. A 60 × 40 × 50 cm carton is 0.60 × 0.40 × 0.50 = 0.120 CBM. Multiply by carton count for shipment volume, then check the total against the container table above at 85–90% utilization — never at 100%.
How many cartons fit in a 20ft container?
Divide ~33.2 CBM by your carton's CBM, then take 85–90% of the result. For a 0.120 CBM carton that's roughly 235–249 cartons. Confirm the weight side too: carton count × gross weight must stay under the ~28,000 kg payload, and for dense goods the weight ceiling arrives before the volume one.
Do all export pallets need ISPM-15 treatment?
Solid-wood pallets, crates, and dunnage shipped internationally need ISPM-15 treatment and the IPPC mark in most trading countries. Plywood, OSB, and other manufactured boards are exempt, as is wood under 6mm thick. If the wood carries no legible mark, the destination port can order treatment, destruction, or re-export at the shipper's cost.