Packaging Fundamentals · Quick guide
Internal vs External Box Dimensions
Published Reviewed
Internal dimensions are the space inside a box; external dimensions are its overall size, including the board. Custom packaging is specified by internal dimensions, because that is what has to fit the product, while external dimensions decide postage, shelf fit and how many boxes go in an outer carton. The gap between the two is roughly twice the board thickness on each dimension, and more for some styles.
Internal and external dimensions defined
Both are written as length × width × depth in millimetres, with length and width describing the opening. The difference is only where you measure from.
- Internal dimensions run from the inside face of one wall to the inside face of the opposite wall. They are the space available for the product, its wrapping and any insert.
- External dimensions run from outside face to outside face. They are what a courier measures, what sits on a shelf and what you stack in a carton.
A third figure sometimes appears on quotations and dielines: the blank size, the flat sheet the box is cut from before it is folded. It is far larger than either and is not a box dimension at all. Our guide on how to measure a box covers measuring the product and choosing clearance.
When to use internal and when to use external
Start from internal dimensions when you are designing a box around a product. Switch to external dimensions when you are fitting the finished box into something else.
| The question you are answering | Use | Why |
|---|---|---|
| Will the product fit? | Internal | The product sits inside the walls |
| Will the insert fit? | Internal | The insert fills the internal space |
| Which postal or courier size band will it fall into? | External | Carriers measure the outside of the parcel |
| Will it fit the shelf or a counter display unit? | External | The shelf sees the outside of the box |
| How many boxes fit in an outer carton or on a pallet? | External | Boxes stack by their outside size |
| Will the lid go over the base? | Lid internal against base external | The lid slides over the outside of the base |
| Will the box slide into a sleeve? | Sleeve internal against box external | The sleeve wraps the outside of the box |
How board thickness converts one to the other
For a simple box, each external dimension is the internal dimension plus two board thicknesses, one for each wall. Depth works the same way, with the base and the lid or flaps adding a layer each.
The table shows the effect on a box with an internal size of 200 × 150 × 80 mm. Thicknesses are approximate and vary by mill and grade, which is why the exact figures come from the dieline.
| Board | Approximate thickness | Approximate external size |
|---|---|---|
| Folding boxboard carton | About 0.5 mm | About 201 × 151 × 81 mm |
| E flute mailer | About 1.5 mm | About 203 × 153 × 83 mm, more across folded side walls |
| B flute box | About 3 mm | About 206 × 156 × 86 mm |
| BC double wall box | About 6.5 mm | About 213 × 163 × 93 mm |
| Rigid base in 2 mm greyboard with wrap | About 2.3 mm | About 205 × 155 × 85 mm for the base alone |
The difference looks small on a carton and large on double wall. On a postal parcel, those extra millimetres can be the difference between two size bands. Our comparison of E, B and C flute explains how to choose the thinnest board that still protects the product.
Styles where the difference is larger
The two-walls rule assumes each side is a single layer of board. Several common styles double the board somewhere, and the external size grows accordingly.
Roll-end tuck front mailers
The side walls of a roll-end tuck front mailer are folded over on themselves, so each side wall is two layers thick. Across the width of the box that adds four board thicknesses, not two. In B flute that is about 12 mm instead of 6 mm.
Lid and base boxes
On a lid and base box the lid is a second shell that sits over the base. Its internal size is the base's external size plus a small clearance, so the closed box's external footprint is the lid's, which is larger again. Depth depends on whether the lid covers the base fully or partly.
Magnetic closure and book-style boxes
A magnetic closure box has a wrap-around cover that adds a layer on the back, top and front, plus the thickness of the magnets and the flap. The closed box is noticeably bigger than the inner tray.
Drawer boxes and sleeves
A drawer box puts a tray inside a sleeve. The sleeve's internal size must clear the tray's external size, and the whole pack's external size is the sleeve's. A printed sleeve over a mailer or carton works the same way.
Tuck flaps and inserts
Tuck flaps and dust flaps add a little to the depth of a folding carton. Inserts work the other way: they reduce the usable internal space, so the box is sized around the insert rather than the bare product.
Stock boxes and supplier listings
When you compare an existing stock box with a custom quotation, check which dimensions each quotes. Many stock box listings give internal sizes, some give external, and a few do not say. A box that looks a few millimetres larger on paper may simply be quoted the other way.
If you are replacing a stock box that works, measure its inside. If you are replacing one that does not, measure the product. Either way, label the figures as internal or external when you send them. Our printed corrugated boxes and mailer boxes are both specified and quoted by internal dimensions.
Common mistakes
- Sending external dimensions without saying so. The supplier treats them as internal and the finished box is larger than intended on every side.
- Designing to a postal band using internal dimensions. The box fits inside the band on paper and falls outside it once the board is added.
- Forgetting the double-thickness walls of a mailer. The width grows by four board thicknesses, not two.
- Measuring a lid and assuming it is the base. On two-piece boxes, the lid and the base have different sizes on purpose.
- Changing the board late. Moving from E flute to B flute after the size is fixed adds about 3 mm to each dimension and can push a parcel into a higher band.
What to tell us
Send your product's size in millimetres and its weight, or the internal size you want if you already know it. If the finished box has to fit inside something, such as a size band, a shelf, an outer carton or a sleeve, send that limit as an external maximum. We show both internal and external sizes on the dieline so you can check them before approving the proof, and every size can be discussed through the quote form.