Packaging guide
What is a master carton?
The outer box nobody sees — what it does, how to size one around your inner packs, and what has to be printed on it.
Published · Updated
In short
- A master carton is the outer box that holds a number of inner boxes or retail units for shipping. Also called an outer, a shipper or a master box.
- It is sized around the arrangement of the inner units, not around a single one.
- It is normally a heavier board than the inners it carries — usually 5-ply, and 7-ply for export.
- Weight is the practical limit on how many units go in. Most stay under about 20 kg so they can be lifted safely.
- It must carry contents, quantity and carton number at minimum, plus shipping marks and weights for export.
The box nobody is meant to see
A master carton is the outer corrugated box that a quantity of smaller boxes or retail units travel in. You will also hear it called an outer, a shipper, or simply a master box — they all mean the same thing.
It is the one part of a packaging set that is not trying to sell anything. Its job is practical: protect what is inside, turn many items into one handleable unit, and carry the information a warehouse, a customs officer and a courier each need.
Where it sits in the packaging set
Most shipments have two or three layers of packaging, and the vocabulary is worth knowing:
| Layer | Also called | What it does |
|---|---|---|
| Primary | Retail pack | Touches the product. The pack a customer buys and takes home |
| Inner carton | Inner, intermediate | Groups a small number of retail packs. Often printed |
| Master carton | Outer, shipper | Groups the inners for transport, storage and stacking. Plain or lightly printed |
A shipment of 240 units might be packed as 24 inner cartons of 10, and those 24 inners packed into 4 master cartons of 6 inners each. The master is the only one the freight company ever handles.
Glass tableware makers print this on their price lists as a formula. A line reading (1 × 6) × 12 = 72 means one inner box holds 6 glasses, 12 inner boxes go into one master carton, and the master carton holds 72 glasses. Match factories count the same way in packets and cartons — see glassware and crockery boxes and matchbox master cartons.
Sizing one
A master carton is sized around the arrangement of the inner units, never around a single unit. The method is straightforward:
- Measure one inner carton, the right way — external dimensions this time, because the inners are what has to fit inside.
- Decide the arrangement — how many along the length, how many across the width, how many layers high. Pick an arrangement that tiles cleanly with no wasted gap.
- Multiply out to get the internal size of the master.
- Add a small clearance so the inners load and unload without forcing.
Two constraints usually decide the arrangement for you. The first is weight — see below. The second is whether the master fits your pallet: a carton that leaves a gap on every pallet layer wastes freight on every shipment you ever make.
Weight is the real limit
The temptation is to pack as many units as will fit. The limit in practice is what a person can lift repeatedly and safely, which is why most master cartons are kept under about 20 kg.
A master that is too heavy gets dropped, dragged and split, and the damage lands on the goods inside. If your unit count pushes the weight past comfortable lifting, the answer is usually more cartons of fewer units, not a stronger box.
Choosing that count — and working out how many fit a carton you already have — is covered in how many pieces fit in a carton.
Which board to use
A master carton carries the weight of everything inside it, gets stacked in a warehouse and takes all the handling the inner boxes are being protected from. It is normally a heavier board than the inners it contains.
| Shipment | Typical master board | Why |
|---|---|---|
| Local delivery, light contents | 3-ply | Short journey, little stacking |
| Domestic freight, warehouse stacking | 5-ply | Stacked storage and repeated handling |
| Export, heavy or high-value contents | 7-ply | Container stacking, long journey, humidity |
Board grade matters as much as ply here — GSM and BF decide whether a 5-ply master actually performs like one.
What goes on the outside
A master carton is read, not admired. At minimum it should say what is inside, how many, and which carton of how many it is in the shipment.
Export shipments usually add:
- Gross and net weight
- External dimensions, and the CBM your forwarder will charge on — the carton CBM calculator works it out from inches
- Country of origin
- The buyer's shipping marks, exactly as they specify them
- Handling symbols — fragile, this way up, keep dry
- A barcode, where the buyer's system requires one
Ask your customer for their marking requirement before anything is printed.Buyers, and particularly export buyers, are often specific about placement and wording, and a rejected consignment over a mark is an expensive way to learn that.
Printing: keep it cheap
One or two colours is almost always right. Text, a logo and handling marks do everything a master carton needs, and the box will be opened and discarded in a warehouse.
Full-colour print belongs on the inner or the retail pack, where a customer actually sees it. Spending it on the outer is spending it on the bin.
Where to go next
- How to measure a carton — get the inner dimensions right before you size the outer.
- 3-ply, 5-ply or 7-ply — choosing the board for the weight it carries.
- Corrugated cartons — masters and inners, made to your sizes.
Common questions
What is a master carton?
A master carton is the outer corrugated box that holds a number of smaller inner boxes or retail units for shipping and storage. The customer never sees it — its job is to protect the units inside, to make them handleable as one item, and to carry the information a warehouse needs. It is also called an outer, a shipper or a master box.
What is the difference between a master carton and an inner carton?
The inner carton is the box around a single unit or a small group of units, and it is often printed for retail. The master carton is the plain outer box that a quantity of inner cartons travel in. A shipment of 240 units might be 24 inner cartons of 10, packed into 4 master cartons of 6 inners each.
How many units go in a master carton?
Whatever divides cleanly and gives a carton that a person can lift. Common counts are 6, 10, 12, 20, 24 and 48, chosen so the inner units tile the base without gaps. Weight is the practical limit — a master that a warehouse worker cannot lift safely gets damaged, so most stay under about 20 kg.
What ply should a master carton be?
Usually 5-ply, and 7-ply for export or heavy contents. A master carton carries the weight of everything inside it and takes the handling that the inner boxes are protected from, so it is normally a heavier board than the inners it contains.
What should be printed on a master carton?
At minimum: what is inside, how many, and the carton number in the shipment. Export shipments usually add gross and net weight, dimensions, country of origin, the buyer’s shipping marks, and handling symbols such as fragile or this way up. Many buyers also require a barcode. Ask your customer for their marking requirement before printing.
Can master cartons be printed?
Yes. Most are printed in one or two colours — text, a logo and handling marks are enough for a box nobody is meant to display. Full-colour printing on a master carton is usually money spent on something that will be opened and discarded in a warehouse.
