How to Measure a Box for Shipping: A UK Guide to Internal vs External Box Size

Every box has two sizes. The internal size decides whether your product fits – measure it empty, wall to wall. The external size decides what the courier charges – measure it packed, sealed, and taped. Board thickness adds roughly 6 to 14mm to every outer dimension depending on flute type, and even a few millimetres over a courier’s limit can tip a parcel into a more expensive band.

A candle maker in Leeds is ordering 1,000 boxes. Her jars are 11cm tall, so she’s choosing boxes with a 15cm internal height, which leaves a bit of room for padding. The jar fits, with room for a wax melt and some tissue. Then the first parcel gets flagged at the courier depot. Packed and taped, the box measures 16.4cm deep. That’s 4mm over the Small Parcel limit, and every order now ships at a higher price band.

She had measured properly. She had only measured the inside. Every box has two sizes. The inside size decides whether your product fits. The outside size decides what the courier charges. In this guide, you’ll learn how to measure both, how to work out one from the other, and how to choose the right box size for shipping UK orders without paying to post air. If you’re still picking a style, our shipping cardboard boxes range has plenty of sizes to compare as you read.

Inside Size vs Outside Size: Why Every Box Has Two Numbers

Internal box dimensions are the space you can actually use. You measure from the inner face of one wall to the inner face of the opposite wall. Your product, padding, and any inserts all have to fit within it.

External box dimensions are measured across the outside of the walls. They include the board, any tape or labels, and any bulge where a lid is packed tight. Couriers and warehouse teams work from this number.

So use the internal size to fit a product, and the external size to post a parcel. When you order custom boxes, give your supplier the internal size and let them work out the outside.

Most sizing mistakes happen when someone gives a supplier an external size, or checks an internal size against a courier limit. With single-wall boxes, the gap between the two is small. It still matters when you’re right on a limit.

Measure the Inside First and Get the Fit Right Every Time

Open the box and lay it flat in front of you. Use a steel tape or a rigid ruler – a soft tailor’s tape sags and throws the reading off by a few millimetres.

1. Push the flaps out of the way.
2. Start with the length – the longer side of the opening, taken from one inside wall to the other.
3. Next comes the width, the shorter side of the opening.
4. Then the height, from the inside floor up to the top edge of the walls.
5. Jot the three numbers down in that order before you move on.

Suppliers read your numbers in the length, width, height order, so stick to it. Swap them, and you can end up with the right volume in the wrong shape.

Measure the Outside Like a Courier Would and Skip the Surprise Charges

Measure the outside as the box will travel: closed, sealed, and packed.

1. Pack the box with the real product and padding, then tape it shut the way you normally would.
2. Stand it on a flat surface. Run the tape along the longest side, from one outer edge to the far one.
3. Repeat on the second side. For the height, find the highest point – the middle of the lid is often not it.
4. Add in anything that sticks out, like a raised label, a tape overlap or a bulging seam.
5. Round up to the next whole centimetre, never down.

Two or three millimetres sounds trivial. But a parcel 3mm over a limit is still over it, and that can mean a courier surcharge. The size limits apply to the parcel as it arrives, tape included.

How Much Bigger Is the Outside? A Board Thickness Cheat Sheet

Each wall has its own thickness, so the outside ends up about twice that board thickness bigger than the inside. How much depends on the flute type – the wavy layer sandwiched inside corrugated board. Typical figures look like this:

Thickness varies a little between mills and with liner weight, so treat the numbers as a guide. Our page on corrugated board shows how the layers build up.

How a 4mm Mistake Cost a Candle Seller a Price Band, and the Fix That Won It Back

Back to our candle maker. Her call to the box supplier went something like this: “Because the courier measures the outside – your board’s about 3mm, so that’s 6mm on top. Then there’s the tape across the lid and a bit of a bulge from the tissue.” “So I should’ve ordered a 14cm internal height?” “The jar’s 11cm, and you want 2cm of padding on top, so 13 or 14 would do it. You’d be well under 16 outside.” She changed the internal height to 13.5cm, and the parcel dropped back into the Small Parcel band. The jars stayed the same. So did the padding. Only the internal height changed.

Stay Under the Limit: UK Courier Sizes and Volumetric Weight Made Simple

Once you have an external size, check it against what carriers allow. The table below shows the common formats:

Limits change from time to time, so look at the carrier’s website before you place a big order. Go one centimetre too deep, and you don’t pay a small extra – you move into a different price band.

Couriers also check volumetric weight. Many bill on whichever is greater: the real weight, or the space the parcel takes up. That’s your volumetric weight in kg.

Most UK couriers divide by 5,000, known as the dimensional weight divisor. A few services use a different figure, so check yours. Our guide to corrugated vs solid board for retail display shows how to hold up your shelf.

Try it on a 40 x 30 x 20cm box. That works out to 24,000 cm³, or 4.8kg once you divide by 5,000. Even if the product only weighs 600g, you’ll be billed for 4.8kg. Now compare a snug box at 24.6 x 16.6 x 12.6cm – that’s about 5,145 cm³, or roughly 1.03kg. The product is the same. The bill isn’t.

From Product to Perfect Box: A Step-by-Step Worked Example

Add the board. With 3mm single-wall board, that’s about 6mm on each dimension. Then test that number against the courier limit and the volumetric sum.

Over a limit? Try trimming the padding, or switch to a thinner board. You can also change the shape – a flat, wider box often does better than a tall, narrow one, because height is where Small Parcel limits bite hardest.

A corrugated mailer box is worth considering for flat or lightweight items like our candle maker’s jars. It sits low, and the folding lid keeps you under a depth limit without losing protection.

Padding, Inserts and Weight: The Hidden Things That Change Your Numbers

Three things change your measurements once the box is packed.

The first is void fill and inserts. A foam or card insert takes up room, so measure with it fitted.

The second is weight. Heavy products need thicker board, and thicker board adds to the outside. Double wall boxes add around 14mm to each dimension, so recheck your external numbers. Very heavy loads may need tri-wall boxes, which add more again.

The third is bulging. Clothing and bedding compress, then spring back. Measure after the box has been sealed for a few minutes.

Don’t pick a box that fits the product with no spare room. A tight fit can crush the corners during packing. A small packing margin, usually 5 to 10mm, saves a lot of returns.

Mailers, Cartons and Die Cut Boxes: Measure Each Style the Right Way

Flat-pack box measurements catch a lot of people out. A flat sheet tells you almost nothing about the finished size, so measure once the box is assembled.

Slotted cartons, the classic shipping box, are measured at the opening, as above. Corrugated mailer boxes are measured inside the tray, with the height taken from the base and not the lid. Folded and glued styles can change the inside slightly, so check the supplier’s dieline as well as your tape measure.

If your product has an odd shape or needs a cut insert, our custom die-cut boxes let you set the internal size to the millimetre.

Seven Measuring Mistakes That Quietly Drain Your Shipping Budget

We see the same slips again and again. Most come from measuring at the wrong stage, or measuring the wrong side:

Branded boxes follow the same rules. Check the final spec when you move to printed logo shipping cardboard boxes, because board and coating choices can shift the numbers slightly. If you’re shipping into the US as well as the UK, the same sizing discipline applies to our custom printed corrugated boxes range on the US site, since American carriers price on a similar inside-vs-outside logic.

Before You Order…

These are the questions brands ask us most when they’re sorting out box sizes.

Should I give my supplier the internal or external size? Internal. It’s the number your product has to fit, and the supplier works out the outside from the board they use.

What if my product isn’t a neat rectangle? Measure its furthest points in each direction. That gives you the smallest rectangular box it could sit in. Then add your padding.

Does printing change the box size? Barely – the board does. Moving from B flute to C flute adds about 2mm in each dimension, so your last order’s numbers may no longer apply. Our printing and finishing page covers what each option changes.

Can I test a size before a big order? Yes. Ask for a sample, and check the minimum order quantity first. Our guide to custom packaging MOQ explained shows how run size affects price.

Conclusion: Measure Twice, Ship Smarter

Use the internal size to fit the product. Use the external size to price the parcel. Measure the box the way it travels, add twice the board thickness to get from one number to the other, and check the result against the courier limit and the volumetric formula before you order.

If you’d like a second pair of eyes on your numbers, the team at Hale Path Packaging helps UK brands work out the right size, board and style before production starts. Send us your product dimensions and tell us which courier you use. We’ll check the fit and the weight, and point out where you can save.

Written by the Hale Path Packaging team