How to Measure a Cardboard Box: Internal vs External Dimensions

Need to measure a cardboard box but aren't sure whether to measure the inside or outside? You're not alone.

Length, width and depth sounds simple enough.

But when you're ordering a cardboard box, a few millimetres can make the difference between a product fitting perfectly and not fitting at all.

Should you measure the inside or outside?

Which side is the length?

Does cardboard thickness matter?

And if you're ordering a custom-sized box, should you enter the dimensions of your product or the dimensions you want the finished box to be?

Let's make it simple.

How are cardboard boxes measured?

Cardboard box dimensions are normally written in this order:

Length × Width × Depth

Sometimes you'll see depth described as height, so you may also see:

Length × Width × Height

These mean essentially the same thing when describing the three dimensions of a conventional cardboard box.

For example:

400mm × 300mm × 200mm

would mean:

Length: 400mm
Width: 300mm
Depth: 200mm

But there's another important question to answer:

Are those internal or external measurements?

What are internal box dimensions?

Internal dimensions measure the usable space inside the cardboard box.

In other words, they tell you how much room you actually have for your product and any protective packaging.

If a box has internal dimensions of:

300mm × 200mm × 100mm

then you have approximately that amount of usable space inside the box.

For packaging products, internal dimensions are usually the most useful measurement.

After all, you're normally trying to answer one question:

Will my product fit inside it?

What are external box dimensions?

External dimensions measure the outside of the finished cardboard box.

Because corrugated cardboard has thickness, the outside of a box will be larger than the usable space inside it.

This difference might only be a few millimetres on each dimension, but it can be important.

External dimensions are particularly relevant when you're considering:

• Courier parcel-size limits

• Royal Mail dimensions

• Evri parcel categories

• Pallet utilisation

• Shelf space

• Storage restrictions

• Vehicle space

• Fulfilment systems

So there's a simple way of thinking about it:

Internal dimensions tell you what fits inside the box.

External dimensions tell you how much space the finished box occupies.

Does ASC Direct use internal or external dimensions?

ASC Direct's Box Builder uses internal dimensions.

This is extremely important when ordering your custom boxes.

If you enter:

300mm × 200mm × 100mm

you're telling us that you need approximately 300mm × 200mm × 100mm of usable space inside the box.

You're not specifying the finished external dimensions.

We manufacture our custom boxes to an internal dimensional tolerance of ±2mm, so this should also be considered when deciding how closely you want your packaging to fit the product.

Why do we use internal dimensions?

Because that's usually what matters most when designing packaging around a product.

Imagine your product measures exactly:

300mm × 200mm × 100mm

If those were the external dimensions of the cardboard box, the usable space inside would be smaller because the cardboard itself takes up space.

Your product wouldn't fit.

By working from internal dimensions, you can design the available space around the product you're trying to package.

Which side is the length of a cardboard box?

For a conventional cardboard box, the length is generally the longest dimension of the opening.

The width is the shorter dimension across that opening.

The depth is measured from the opening down to the base of the box.

So:

Length = longest side of the opening

Width = shortest side of the opening

Depth = top to bottom

This gives you:

L × W × D

For example, a box measuring:

500 × 300 × 200mm

has:

500mm length
300mm width
200mm depth.

How do I measure an existing cardboard box?

If you already have a cardboard box and want to reproduce its size, we'd recommend measuring the inside.

Open the box and measure the usable internal space.

Step 1: Measure the internal length

Measure the longest side of the opening from one inside wall to the other.

Step 2: Measure the internal width

Measure the shorter side of the opening, again from the inside surfaces.

Step 3: Measure the internal depth

Measure from the inside base of the box to the top edge.

You now have:

Length × Width × Depth

For example:

420mm × 310mm × 180mm

These are the dimensions you can use as the starting point when recreating the box in the ASC Direct Box Builder.

Don't measure the outside if you want to recreate the internal space

This is an easy mistake to make.

Imagine you've got an existing box that fits your product perfectly.

You measure the outside and get:

400 × 300 × 200mm

You then enter those measurements into a Box Builder that asks for internal dimensions.

The new box will have approximately 400 × 300 × 200mm of space inside.

It will therefore be larger than the original box you measured.

That's why it's always important to check whether a supplier is quoting:

Internal dimensions or external dimensions.

Don't assume.

How do I measure a product for a custom cardboard box?

This is slightly different from measuring an existing box.

Start by measuring the maximum:

Length × Width × Depth of your product.

Make sure you measure the widest points.

This is particularly important with irregular-shaped products.

Handles, switches, lids, corners, fittings and protruding parts can all affect the space you need.

Once you have the product dimensions, the next question is:

How much additional space does the product need?

Should I order a box the exact same size as my product?

Usually, we'd recommend allowing some clearance rather than making the internal box dimensions exactly the same as the product dimensions.

For example, if your product measures:

300 × 200 × 100mm

we wouldn't automatically recommend ordering a box with internal dimensions of exactly:

300 × 200 × 100mm

Why?

Firstly, both products and packaging can have manufacturing tolerances.

Secondly, you need to be able to get the product into the box comfortably.

And thirdly, the product may require protective packaging.

Right-sized doesn't mean tight-sized.

How much bigger should my box be than my product?

There's no single answer that works for every product.

It depends on what you're sending.

A robust item that needs little additional protection may only require a relatively small amount of clearance.

A fragile item may require significantly more space for cushioning.

Ask yourself:

Does the product need wrapping?

Does it need cushioning around the sides?

Does it need protection above and below?

Are there vulnerable corners or edges?

Could it move around during transportation?

Measure the product with the protection it actually needs, rather than simply adding an arbitrary amount to every dimension.

That's the space your box needs to accommodate.

Don't forget protective packaging

Let's say your product measures:

300 × 200 × 100mm

But you're sending something fragile and want approximately 20mm of protective material around every side.

That protection adds to both sides of each dimension.

So, as a simple illustration, your protected product could require approximately:

340 × 240 × 140mm

rather than:

320 × 220 × 120mm

Why?

Because 20mm of protection on the left plus 20mm on the right adds 40mm to the width.

The same principle applies to the other dimensions.

This doesn't mean every fragile product needs exactly 20mm of cushioning.

The appropriate amount depends on the product, its weight, fragility and the protective material being used.

The important point is to measure the complete protected product, not just the product itself.

What if I'm packing several products in one box?

Treat the complete arrangement as your product.

Arrange the items as you intend to pack them.

Add any individual wrapping or dividers they require.

Then measure the overall:

Length × Width × Depth

Consider whether products could move or collide during transportation.

If you're regularly sending the same combination of products, creating a custom box around that arrangement can make the packing process much more consistent.

This can be particularly useful for:

• Product bundles

• Subscription boxes

• Gift sets

• Multipacks

• Ecommerce orders

• Promotional kits

Instead of finding a box and working out how to fill it every time, you can create packaging around the products you're actually sending.

Does cardboard thickness affect box dimensions?

Yes.

Corrugated cardboard has physical thickness.

A Single Wall corrugated board is thinner than a Double Wall board.

At ASC Direct, our Single Wall board is approximately 3mm thick, while our Double Wall and Heavy Duty boards are approximately 6mm thick.

This means the external dimensions of a box will be larger than its internal dimensions.

It's another reason why you shouldn't assume that a box with internal dimensions of:

450 × 350 × 160mm

will produce a finished parcel measuring exactly:

450 × 350 × 160mm externally.

It won't.

This is particularly important for courier size limits

Let's say you're trying to make a parcel fit within a courier's maximum dimensions.

You find that the courier allows a parcel measuring up to:

450 × 350 × 160mm

You then order a cardboard box with internal dimensions of:

450 × 350 × 160mm

There's a problem.

Once the thickness of the cardboard is taken into account, the finished external dimensions will be larger than those measurements.

Your parcel could therefore exceed the courier's limit.

If you're designing packaging around a strict external dimension, don't simply enter that maximum dimension into a Box Builder that works from internal measurements.

Allow for the cardboard construction and dimensional tolerances.

If you're unsure, speak to us before ordering.

Internal dimensions vs external dimensions: which should I use?

It depends on what you're trying to achieve.

If your question is:

"Will my product fit inside this box?"

Use internal dimensions.

If your question is:

"Will this finished parcel fit within a courier's size limit?"

Check the external dimensions.

If you're ordering from ASC Direct's Box Builder:

Enter the internal dimensions you require.

It's a small distinction that can make a very big difference.

What if my product is an unusual shape?

Not everything comes in a convenient rectangle.

You might be packaging:

A lamp.

A piece of machinery.

A musical instrument.

A long component.

An irregular piece of equipment.

Or something with handles, fittings or protrusions.

In these situations, identify the product's maximum dimensions.

Imagine an invisible rectangular box around the complete product.

Measure the longest, widest and deepest points.

Then consider how the item needs to be protected.

For long or unusually shaped products, the orientation of the box can also affect the most appropriate box style.

If you're unsure, contact our team and we can help determine the best approach.

What if my product is long and narrow?

This deserves particular attention.

A conventional 0201 box isn't always the most suitable or economical design for very long, narrow products.

Depending on the dimensions, an alternative box style such as an 0201 ALT or 0203 may provide a more appropriate solution.

That's another reason we recommend starting with the product rather than assuming every item should go into the same style of cardboard box.

Measure first. Choose the box style second.

Should I use millimetres or centimetres?

ASC Direct's Box Builder uses millimetres (mm).

So if you've measured your product in centimetres, simply multiply each measurement by 10.

For example:

30cm × 20cm × 10cm

becomes:

300mm × 200mm × 100mm

Using millimetres also makes it much easier to be precise when you're ordering made-to-measure packaging.

A simple box measuring checklist

Before ordering a custom cardboard box, check:

• Have you measured the maximum length, width and depth?

• Are your measurements in millimetres?

• Have you measured the product at its widest points?

• Have you allowed room for any protective packaging?

• Have you considered manufacturing tolerances?

• Are you entering internal rather than external dimensions?

• If you're working to a courier limit, have you considered the finished external box size?

• Is the cardboard grade appropriate for the product?

• Is a conventional 0201 box the best style for the dimensions?

Taking another minute to check your measurements before ordering can save considerably more time later.

Measure the product. Then build the box around it.

One of the main reasons businesses end up using oversized packaging is simple.

They start with the boxes available to them.

A product measures:

330 × 215 × 125mm

But the nearest stock box is:

400 × 300 × 200mm

So that's the box they use.

The remaining space gets filled with additional packaging.

Custom-sized packaging reverses the process.

Start with the product.

Measure it.

Work out how much protection it needs.

Calculate the internal space required.

Then manufacture the box around those dimensions.

At ASC Direct, you can order custom-sized corrugated cardboard boxes from just 25 boxes using our online Box Builder.

Enter your required internal:

Length × Width × Depth

Choose your box style and cardboard grade.

Select your quantity.

And get an instant online price.

Don't make your product fit the box. Make the box fit your product.

BUILD YOUR CUSTOM-SIZED BOX →

 

James Palfrey-Smith is the General Manager at ASC Direct with extensive experience in both operations and sales. James is passionate about solving packaging problems, delivering fast customer support, and building lasting relationships. Outside of work, he enjoys outdoor activities like hiking and skiing. A fan of real ale and business podcasts, he continuously seeks personal and professional growth. 

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