Cardboard boxes are almost always measured internally, in the order length x width x depth, and that internal figure is the space your product actually gets. The external dimensions are slightly larger because they include the thickness of the board itself, and it is the external figure that your courier measures when working out which postage band you fall into. Get these two numbers mixed up and you either order a box your product will not fit inside, or you tip a cheap parcel into an expensive size band.
This guide explains how box dimensions are quoted in the UK, how to measure a box properly, and how to choose a stock size that protects your product without inflating your shipping bill. If you are buying custom printed boxes for the first time, this is the single most useful thing to get right before you order.
Internal vs external dimensions: what is the difference?
Internal dimensions describe the usable cavity inside the box once it is assembled. External dimensions describe the box from the outside, including the board on both sides.
The gap between the two depends entirely on the thickness of the material. A single wall corrugated box built from B flute adds roughly 3mm of board per wall, so a box with 200mm of internal length measures around 206mm externally. Double wall board is thicker again, typically adding around 7mm per wall. Our guide to corrugated flute types explains how much each grade adds and how much protection it buys you.
As a rule of thumb:
- Internal dimensions tell you whether your product will fit.
- External dimensions tell you what the courier will charge you.
Reputable UK packaging suppliers quote internal dimensions by default, but not universally, so check the product page before you commit to a run. If a listing does not say, assume nothing and ask.
The correct order: length, width, depth
UK packaging is quoted as length x width x depth (sometimes written L x W x D or L x W x H). The convention is straightforward once you know it:
- Length is the longer side of the box opening.
- Width is the shorter side of the box opening.
- Depth is the measurement from the opening down to the base.
So a 200 x 150 x 100mm box has an opening of 200mm by 150mm and is 100mm deep. Quoting those three numbers in a different order is one of the most common ordering mistakes we see, because a 100 x 150 x 200mm box is a completely different shape.
How to measure a box you already have
If you are trying to reorder something you already use, do it this way:
- Assemble the box fully, with the base taped as you would ship it.
- Measure the internal opening across the longest side, from the inside face of one wall to the inside face of the opposite wall. That is your length.
- Repeat across the shorter side for your width.
- Measure from the top edge down to the inside of the base for your depth.
- Measure the same three axes on the outside of the box, and note those separately as your external dimensions.
Work in millimetres rather than centimetres. Packaging tolerances are usually plus or minus 2 to 3mm, and rounding to the nearest centimetre hides exactly the kind of error that stops a product fitting.
Why external dimensions decide your postage
Carriers measure the outside of your parcel, including tape, labels and any bulge in the sides. Royal Mail's published bands are unforgiving: a Large Letter must be within 353 x 250 x 25mm and 750g, a Small Parcel within 45 x 35 x 16cm, and a Medium Parcel within 61 x 46 x 46cm and 20kg. You can check the current thresholds on the Royal Mail size and weight guide, and we break the bands down further in our article on Royal Mail parcel sizes.
The 25mm thickness limit on a Large Letter is where most sellers get caught. A product that is 22mm thick sits comfortably inside the band until you add 3mm of board on each face, at which point the parcel is 28mm externally and has jumped a price tier. Choosing a purpose-made large letter box that is engineered to stay under the threshold once assembled is far cheaper than discovering the problem at the counter.
On larger, lighter consignments the reverse problem appears: the box is well within the weight allowance but is charged on volume instead. Our guide to volumetric weight explains how that calculation works and why oversized boxes quietly erode margin on every order.
How much clearance should you leave?
Aim for roughly 5 to 10mm of clearance around a rigid product on each axis. That is enough room to insert the item without forcing it, and enough space for a thin layer of protection, while keeping the parcel snug enough that the contents do not slide.
For anything breakable you will want more. Fragile goods generally need 30 to 50mm of cushioning between the product and every wall of the box, which means adding 60 to 100mm to each internal dimension. Our guide on packaging fragile items for posting covers how to build that protection without overshooting a size band.
Soft goods such as clothing are more forgiving, so measure the product folded as you would actually pack it, not laid flat.
Stock sizes and why they matter when ordering printed boxes
At Fast Printed Boxes we print onto pre-made stock boxes rather than manufacturing bespoke sizes, so measuring accurately is about choosing the closest stock size rather than specifying your own. In practice this is a good thing: stock sizes are far cheaper per unit, carry no tooling cost, and ship quickly, because the box already exists and only the print is made to order.
It does mean the sizing decision sits with you. Our small parcel box range and wider e-commerce box range list internal dimensions for every option, so compare your measured product against those figures rather than against an existing supplier's external quote.
If you are between two sizes, order a sample and pack a real product into it before you commit to a full run. Ten minutes with a tape measure and a sample box is cheaper than a pallet of boxes that are 5mm too small.
Frequently asked questions
Are cardboard boxes measured internally or externally?
Cardboard boxes in the UK are normally quoted using internal dimensions, because that is the space available for your product. External dimensions are larger by the thickness of the board on each side, typically around 3mm per wall for single wall and around 7mm per wall for double wall. Always confirm which figure a supplier is quoting before ordering.
What order do box dimensions go in?
Box dimensions are written as length x width x depth. Length is the longer side of the opening, width is the shorter side of the opening, and depth is the distance from the opening down to the base. A 300 x 200 x 100mm box therefore has a 300mm by 200mm opening and is 100mm deep.
How much bigger should a box be than the item inside it?
For a rigid, non-fragile product, allow around 5 to 10mm of clearance on each axis. For fragile items, allow 30 to 50mm of cushioning on every face, which adds 60 to 100mm to each internal dimension. Too much space is as risky as too little, because unsupported contents move in transit.
Which dimensions does Royal Mail use to price a parcel?
Royal Mail measures the outside of the finished parcel, including tape, labels and any bulging. A Large Letter must be within 353 x 250 x 25mm and 750g, a Small Parcel within 45 x 35 x 16cm, and a Medium Parcel within 61 x 46 x 46cm and 20kg. Always check your packed parcel externally rather than relying on the internal size quoted by your box supplier.
Does the printing add to the size of the box?
No. We print water-based CMYK inkjet directly onto the surface of the box, which adds no measurable thickness and does not change the dimensions or the fold lines. The printed design sits within a defined print area on the top face, with an unprinted margin to the edges, so the box performs exactly as an unprinted one would.
What if my product falls between two stock box sizes?
Size up rather than down, then use void fill to take up the slack. Going up one size costs a little more in materials and possibly postage, whereas a box that is even 3mm too small is unusable. Ordering a sample first is the safest way to settle it.
Measure once, order with confidence
Accurate measuring is the least glamorous part of packaging and the part that saves the most money. Take internal and external figures, work in millimetres, allow sensible clearance for your product type, and check the external size against your carrier's bands before you place a bulk order.
When you are ready, browse the full range at Fast Printed Boxes, where every product lists its internal dimensions clearly, or order a printed sample so you can measure the real thing before you commit.