The Fold · From the factory floor
What size should your packaging be?
Packaging is measured from the inside. Almost every expensive sizing mistake comes from forgetting that one sentence.
· Nick Wahl, founder
The short answer: size the cavity, not the box. Measure your product at its widest real point on each axis, add the clearance you need to load it, and give those numbers as internal dimensions in the order length × width × depth. Board thickness makes the outside bigger and is decided by the material you pick — it is an output, not something you specify.
“12 × 8 × 4” describes two different boxes. On one reading it is the space your product sits in; on the other it is the footprint the box takes up on a pallet. The gap between them is two wall thicknesses on every axis, and on corrugated that is enough that the product does not go in.
This is the most common way a packaging order goes wrong, and it is the most expensive, because nothing catches it. The artwork is fine. The print is fine. The boxes arrive beautifully made and the product will not fit inside them.
The five ways a size goes wrong
| The mistake | What it looks like | When you find out |
|---|---|---|
| Measured the old box, not the product | A tidy set of numbers copied off packaging that already exists | On delivery — the new box inherits every compromise of the old one, including the empty space you were paying to ship |
| Gave outside dimensions | Correct to a millimetre, taken with calipers across the finished box | When the product will not go in. On board, outside minus inside is two wall thicknesses |
| No clearance at all | A cavity that matches the product exactly | On the packing line, where a glove-tight box takes three times as long to fill and scuffs the product going in |
| Too much clearance | A comfortable, easy-to-pack box | On the freight invoice, and again in reviews, when the product arrives having rattled across the country |
| Rounded the conversion | “8 inches, call it 200mm” | At 8 inches the drift is 3.2mm — more than a whole wall of B-flute. Compounded across three axes, the product no longer fits |
Measure the product, not the box it came in
The box you already have is a record of decisions someone else made — often to fit a size that was already on a shelf. Copying it copies the empty space you have been paying to ship, and any compromise made for a product that has since changed.
Measure the item itself, at its widest real point on each axis. That means over the cap, not the neck. Over the label seam, not the bare bottle. Over the bulge of a filled pouch, not the flat one. Fill a pouch and measure it full — an empty one measures like a sheet of film and lies to you about every dimension.
Add clearance for how it is actually packed
A cavity that matches the product exactly is a box nobody can fill quickly. On a pack line, a glove-tight fit means a person wrestling each unit in, scuffing the product on the way, and taking several times as long per box. Too much room is the opposite failure: the product travels, arrives rattled, and you pay freight on air.
Retail cartons that sit on a shelf can be snug — a couple of millimetres per axis is often enough. Anything shipped on its own needs room for whatever protects it, and that protection is part of the measurement, not an afterthought.
Inside is the number. Outside is a consequence.
Every dieline is built from the internal cavity. That is the number your product has to live in, and it is the one you should be quoting to anyone who asks. What the box measures on the outside follows from the material: corrugated B-flute runs about 3mm of caliper, so a 100mm internal wall lands near 106mm across the outside once both walls are counted.
This is also why swapping material after the size is locked is not a free change. Move from a thin folding carton board to corrugated and the cavity stays exactly where you put it — but the finished box grows, and it may no longer fit the shelf, the shipper or the pallet pattern you planned around.
Convert exactly. Round once, at the end.
One inch is exactly 25.4 millimetres. Not 25. Not “about 25 and a half.” The rounding looks harmless on a single number and stops being harmless the moment it is applied three times.
| Inches | Exact | What rounding costs you |
|---|---|---|
| 2″ | 50.8 mm | Rounding to 50 loses 0.8mm |
| 3″ | 76.2 mm | Rounding to 75 loses 1.2mm |
| 4″ | 101.6 mm | Rounding to 100 loses 1.6mm |
| 6″ | 152.4 mm | Rounding to 150 loses 2.4mm |
| 8″ | 203.2 mm | Rounding to 200 loses 3.2mm |
| 12″ | 304.8 mm | Rounding to 300 loses 4.8mm |
At 8 inches the drift is 3.2mm — more than an entire wall of B-flute. Do it on all three axes and a product that fits on paper does not fit in the box.
Artwork has its own edge, and it is not the cut line
Sizing the cavity is only half of it. Once the die exists, the artwork has a smaller boundary to respect: a safe area held in from every cut and crease, because both cutting and folding have tolerance. Anything that must be read — a barcode, an ingredient panel, legal copy — belongs inside that margin, and none of it should ever cross a fold. On a narrow panel that margin tightens toward a 4mm floor rather than vanishing, so a slim panel still keeps a usable live area instead of silently losing one.
For the one element that has to satisfy a machine rather than a person, see how to put a barcode on your packaging so it actually scans.
Check it folded, not flat
A flat sheet is a poor judge of size. Panels that look generous laid out can close into a box that is taller than the shelf, or a depth that swallows the product. Fold it before it is printed — the same numbers, standing up, answer a different question than the same numbers lying down.
On TWOFORM you set the internal dimensions and watch the die rebuild around them, then fold it in 3D at that exact size before anything is committed. There are over 500,000 dielines behind that, so the size you need is a number you type rather than a template you settle for.
Common questions
Are box dimensions inside or outside measurements?
Inside. Packaging dimensions are the internal cavity — the space your product occupies. The outside is larger by roughly two board thicknesses on each axis, and it is an output of the material you choose, not something you specify.
What order do box dimensions go in?
Length × Width × Depth, where length and width are the two dimensions of the opening and depth is measured from the opening down. Writing them in a different order is one of the most common ways a box is ordered wrong.
How much clearance should I leave around my product?
Enough to load it without forcing, and not so much that it moves in transit. A snug retail carton is typically a couple of millimetres per axis; a shipper that needs void fill is sized around the product plus the protection. Measure the product at its widest real point, including any cap, label or seam.
Does board thickness change the size I order?
It changes the outside, never the cavity. Corrugated B-flute is around 3mm of caliper, so a box with a 100mm internal wall measures roughly 106mm across the outside. Specify the inside; let the material decide the outside.
Why does converting inches to millimetres matter so much?
One inch is exactly 25.4mm. Rounding 8 inches to 200mm rather than 203.2mm loses 3.2mm — more than an entire wall of B-flute corrugated. Do the conversion once, exactly, and round only at the very end.
How close to the edge can artwork go?
Not to the edge. Every panel carries a safe area inset from the cut and crease lines, because cutting and folding both have tolerance. Anything that must be read — text, a barcode, legal copy — belongs inside it, and it never crosses a fold.
Keep reading
- What is a dieline?
- How to put a barcode on your packaging so it actually scans
- How to get custom packaging made
Try the size you actually need. Type your internal dimensions, watch the dieline rebuild around them, and fold it in 3D before anything is printed.
Start designing →