It was probably only last week that a package arrived at your address in a box that seemed far too big for the product it was housing. One small item rattling around in a void, with a box showing signs of a rough trip through the delivery network. It happens all too often, and it costs the sender on two fronts: they pay for shipping, and the product arrives looking worse for wear.
Choosing the correct box size is one of the least glamorous decisions in packaging and yet one of the most expensive to get wrong. Too big and you pay for space you are not using. Not to mention a product that rattles around and potentially arrives damaged. Too small and you are crushing the product or scrimping on the protection it needs. The right size sits in a narrow band in the middle, and finding it means real savings and happy clients.
This is a practical guide to choosing that size: the common size bands, how to measure and choose, how couriers actually price your parcel, and when to opt for a custom size rather than a standard box.
Two ways the box size can go wrong
"Most people are using a box that's too big or too small, and both cause their own set of problems." – Roisin Donnellan, Boxes To Go
Most sizing problems fall into one of two camps.
Too big is the more common one and it’s an easy mistake to make. A product sitting loose in an oversized box moves in transit, and movement is what causes damage. You end up spending more not only on the main materials but also on void fill. Also, couriers charge you for the space the box takes up, not just what it weighs, so a light product in a big box can cost as much to send as something far heavier.
Too small is less common but more obvious when it happens. The product is packed so tightly against the box walls that there's no room for a protective layer. That puts more pressure on the box itself than it was designed to handle. The corners get crushed, the lid starts to bulge, and the parcel looks, well, ugly!
There is a version of this that most people have experienced as a buyer. A large online retailer sends one small thing in a much bigger box on a low grade of board, because at their volume, the priority is packing speed and unit cost, not how the box looks when it lands. For a business where the box is part of the brand, there’s no such trade-off. A parcel that turns up tattered forms an opinion before the customer has even touched the product.
There is no single ‘standard’ size
It helps to know up front that there’s no national standard size when it comes to shipping boxes. Dimensions vary between manufacturers, and what one supplier calls a medium mailer, another might call a large. What does exist is a set of rough size bands that most e-commerce packaging falls into:
|
Band |
Rough internal size (mm) |
Typical use |
|---|---|---|
|
Flat / satchel-style mailer |
Low profile, up to ~20 mm deep |
Apparel, print, flat goods |
|
Small mailer box |
~150 × 100 × 50 mm |
Cosmetics, small electronics, single items |
|
Medium mailer box |
~220 × 160 × 80 mm |
Multi-item orders, homewares |
|
Large mailer box |
~310 × 230 × 100 mm |
Bulkier or multi-product orders |
|
Shipping carton (RSC) |
300mm+ in multiple sizes |
Heavier, higher-volume freight |
Treat these as a guide, rather than as a rule of thumb. The measurement that actually matters is the one specific to your product, and that is where the choice becomes important.
How to measure and choose
Work in internal dimensions, not external. The box needs to fit around your product, so measure the space inside it.
Start with the product itself: length, width and height at its widest points. Then decide how much protective allowance it needs. A rigid, self-supporting product might need almost none. A fragile or awkward item needs enough room for a protective layer or an insert on every face that matters. Add that allowance to the product dimensions and you have your target internal size.
A few things to weigh up as you make your choice:
- Weight and fragility: A heavier or breakable product may need a stronger board grade or a double wall, not just a different size. Size and strength are separate decisions that often get muddled.
- Whether the product is self-supporting: Some products hold their own shape and protect themselves. Others rely entirely on the box and its fill. Knowing what you have changes how much room and structure you need.
- How it ships: A box that is fine on a pallet may not survive a parcel network where it gets sorted, stacked and handled dozens of times.
You don't have to get all of this right on your own. Once we've looked at the product and how it moves through the supply chain, we adjust roughly a third of the briefs we receive from what the customer originally requested. Sometimes that is sizing down to cut freight; sometimes it is stepping up a board grade because the product needs more protection than the original spec allowed. That is the part worth talking through with a team that specialises in this area every day.
How couriers actually price your box
This is what surprises many people: with most carriers, the size of your box can matter more than its weight.
Australia Post, like most carriers, charges based on whichever is higher: a parcel's actual weight or its cubic weight. Cubic weight is worked out by multiplying length, width and height in metres, then multiplying by a conversion factor of 250 (Australia Post). So a light product in a large box gets charged as if it were much heavier, because it is taking up space in a van that could have carried something else.
The practical upshot is that reducing the box size can move you into a cheaper bracket even when the product inside has not changed at all.
Two real examples from our own work show how much that can be worth
Banjo Valley, a camping equipment brand, came to us needing their boxes to fall under a specific Australia Post size threshold because dropping below it moved them into a cheaper shipping bracket. They had assumed one of our stock boxes would do the job, and it was close, but still slightly too big. Rather than send them off to design and tool up a fully custom box, we made a custom size on our table. That meant no die-form or tooling setup cost, so they could test it in the real world at low volume before committing. It worked. They came in under the threshold, and their shipping cost dropped from that point on.
A pathology testing business that sends out swab kits that customers return by post needed a packaging solution that was good value for money. The goal was to get the return mailer under 20mm thick, because at that thickness Australia Post treats it as a large letter rather than a parcel. A large letter starts at a few dollars (Australia Post), where the equivalent small parcel is closer to ten. For a business sending these constantly, a few dollars saved per delivery add up fast. (Australia Post letter and parcel prices are set to rise in the second half of 2026, so it is worth confirming the current figures when you plan.)
Neither of those was a dramatic redesign. Both were millimetre-level sizing decisions that changed what the client paid on every single order.
When custom beats standard, and when it doesn’t
Custom sizing is not automatically the right answer, and it would be dishonest to pretend otherwise.
A stock box is the right call when it genuinely fits your product with sensible protection and lands you in a good freight bracket. If it works, use it. You are not paying for air and you are not over-engineering the solution.
A custom size earns its place when a stock box leaves you paying to ship empty space, buying void fill to stop movement, or sitting just above a pricing threshold you could easily slip under with minimal effort. That was the Banjo Valley situation exactly: the stock box was close, but close was costing money on every order.
The old objection to custom was setup cost. Traditional custom boxes need a cutting die made first, which is a real upfront expense that only makes sense at volume. For short runs and testing, a box cut on a table skips that tooling step, so a smaller brand can trial the right size without the big financial commitment. That changes the maths for start-ups and anyone launching a new product who does not yet know their volumes.
The right size is also the greener choice
Right-sizing is one of the few packaging decisions where the cost savings and the environmental savings point in the same direction.
A box matched to its product uses less board to make and needs less void fill to protect. Less material in, less material for your customer to deal with, and more parcels fitting into the same delivery run. You don't have to reach for a sustainability angle to justify it. Getting the size right reduces waste as a matter of course, creating a genuine, measurable benefit rather than a vague green claim.
Getting it right for your product
If you are choosing a box size, the short version is this: measure your product internally, add only the protective allowance it actually needs, check where that lands you on cubic weight, and be honest about whether a stock size fits or whether you are paying for space and fill you don’t need.
The band in the middle, the size that protects the product, presents it well and keeps freight costs down, is narrower than most people expect. It’s also where the savings lie. If you are not sure your current box is sitting within this band, that is exactly the kind of thing worth discussing with someone who sizes boxes for a living.
Not sure your box is the right size?
This is exactly the kind of thing we solve daily. Send us your product or your current box and we will tell you honestly whether a stock size fits or a custom one will save you money, whether that is a short run to test the market or full production when you are ready. You hand it over, we make it easy, and it turns up done properly, made right here in Melbourne.