How to Choose the Right Box Size for Your Ecommerce Orders

If you're sending parcels every day, box size is one of those things that seems simple until it starts costing you money. Too big, and you're paying to ship air — and using excess void fill to stop your product rattling around. Too small, and you risk damage, returns, and unhappy customers.

This guide covers everything you need to know about choosing the right box size for your ecommerce orders: how to measure correctly, how Royal Mail and couriers think about size, and which box types work best for different kinds of products.

Why box size matters more than most sellers realise

The obvious reason to get box size right is protection. A product packed in a box that's too large will shift in transit, and no amount of void fill is a complete substitute for a well-fitting box.

But there's a less obvious reason: postage costs. Royal Mail and most couriers now use volumetric (dimensional) weight to calculate shipping charges. This means a large, light parcel can cost as much to send as a much heavier one, because carriers charge based on the space a parcel takes up in their vans and sorting facilities — not just its weight on a scale.

In short: a box that's bigger than it needs to be isn't just wasteful, it's expensive.

How to measure your products correctly

Before you can choose a box, you need accurate dimensions for what you're packing. Measure the product at its widest points — length, width, and height — and add clearance for any protective packaging you plan to use inside (bubble wrap, void fill, foam inserts).

A common rule of thumb is to allow 10–20mm of clearance on each side for standard products. Fragile items may need more. Flat, rigid products like books or prints may need very little.

Write these dimensions down. You'll want them when you're comparing box sizes.

Single wall vs double wall: which do you need?

The most common question we get asked at Datec is whether to use single wall or double wall boxes. Here's the short answer:

Single wall boxes are made from one layer of corrugated board. They're lightweight, cost-effective, and perfectly adequate for most ecommerce orders — clothing, accessories, homewares, books, non-fragile gifts. If your products aren't particularly heavy or fragile, single wall is the right starting point.

Double wall boxes have two layers of corrugated board and offer significantly more protection. They're the right choice for heavier products, fragile items, anything being shipped over long distances or via multiple handling points, and products that might be palletised during transit.

You can browse our full range of single wall boxes and double wall boxes to compare sizes and find the right fit.

Ecommerce boxes vs standard cardboard boxes

Standard RSC (Regular Slotted Container) boxes are fine for many applications, but if you're running an ecommerce operation, it's worth looking at boxes designed specifically for the job.

Our ecommerce box range includes:

• Crash lock base boxes — the base locks into place automatically, saving time on the packing bench. No taping the base needed.
• Size-adjustable boxes — these can be scored and folded down to fit smaller products, reducing the need to stock multiple box sizes.
• White inside boxes — ideal if presentation matters. The white interior gives a cleaner unboxing experience without the need for tissue or inner packaging to hide the brown board.

If you're shipping high volumes, the time saved by crash lock bases or size-adjustable boxes adds up fast.

What about Royal Mail size limits?

If you're sending through Royal Mail, size limits determine which service your parcel qualifies for — and the cost difference between a large letter and a small parcel is significant.

Here's a quick overview of the key formats:

If your product can fit within large letter dimensions — flat items, cards, thin books, jewellery — it's worth choosing a box or envelope that keeps you in that category. Our PiP postal boxes are designed exactly for this: they fit within Royal Mail's Parcels in Packet size limits so you get tracked postage at a lower price point.

For anything that won't fit in a box, cardboard envelopes are a lightweight, rigid alternative for flat items like prints, documents, and card-based products.

Common mistakes to avoid

Stocking too many box sizes. It sounds counterintuitive, but having 10 different box sizes often leads to more packaging waste and slower packing, not less. Most ecommerce businesses can cover the majority of their orders with 3–4 well-chosen sizes. If you're not sure what sizes to stock, we're happy to work through this with you.

Measuring the box instead of the product. Box dimensions are always listed as internal dimensions (length × width × height). Make sure you're comparing your product measurements to the internal size of the box, not the external.

Ignoring the depth. It's easy to find a box with the right footprint and overlook the height. A box that's too tall will need extra void fill and risks your product shifting during transit. A box that's too shallow will leave a lid that doesn't close properly.

Forgetting to account for void fill. If you're using bubble wrap, paper void fill, or air cushions inside the box, that takes up space. Add the thickness of your protective packaging to your product dimensions before selecting a box.

Not sure which size to go with?

We've been helping businesses pack and ship products from our base in Coventry for over 20 years. Whether you're setting up your packing operation from scratch or trying to reduce packaging costs on an existing setup, we're always happy to give practical advice — not a sales pitch.

Call us on 02476 611234 or get in touch online and we'll help you find the right box for what you're shipping.

Related products: eCommerce Boxes | Single Wall Boxes | Double Wall Boxes | PiP Postal Boxes | Cardboard Envelopes