Short answer: measure the item's length, width and height at its widest points, add space for the cushioning you will use on every side (often a few centimetres for robust items and more for fragile ones), and choose the smallest box whose internal dimensions are equal to or slightly larger than the result in all three directions. Then check the packed parcel's outside dimensions and weight against your carrier's limits before you buy postage.
Getting this right matters more than it looks. A box that is too small crushes the item or bursts at the seams. A box that is too large wastes void fill, lets the item move, collapses under weight because the top is unsupported, and can push the parcel into a more expensive postage format. This guide walks through measuring, allowances, rounding to stock sizes and carrier checks, with worked examples.
Contents
- Internal and external dimensions
- Step 1: measure the item properly
- Step 2: add a cushioning allowance
- Step 3: match to a stock box size
- Step 4: check carrier size and weight limits
- Worked examples
- Orientation, shape and awkward items
- Sizing multi-item orders
- Filling the gaps
- Building a sensible range of sizes
- Questions buyers ask
Internal and external dimensions
Every cardboard box has two sets of measurements. The internal dimensions are the space inside, which is what your item and its cushioning must fit into. The external dimensions are the outside of the made-up box, which is what a carrier measures. The difference comes from the thickness of the board and the way the flaps fold over each other, so it is larger for double wall boxes than for single wall boxes.
When you buy a box, check which set the supplier quotes. Our single wall box listing gives internal sizes (length x width x height). If you are working close to a carrier limit, make up one box and measure the outside yourself before standardising on it. Do not assume a nominal size in inches converts to an exact millimetre measurement on the finished box; boxes are made to tolerances, and the inch and millimetre figures on a listing are rounded equivalents of each other.
Box dimensions are normally written length x width x height. Length and width describe the opening at the top; height is the depth from the opening to the base. Keeping to that order avoids ordering a box that is the right volume but the wrong shape.
Step 1: measure the item properly
Measure the item as it will be packed, which is not always the same as the product's listed size.
- Measure at the widest points. Handles, spouts, lids, plugs and feet all count. A mug is wider across the handle than across the body.
- Include retail packaging if you keep it. If the item ships in its own printed box, measure that box, not the product.
- Measure folded or assembled state. For textiles, measure the folded bundle. For flat-pack items, measure the pack.
- Group multi-item orders. Arrange the items as they will sit in the box, ideally in a compact block with heavier items at the bottom, then measure the block.
- Weigh it too. Weight affects which board you choose and which postage format the parcel falls into.
Use a rigid tape measure or ruler and round up to the nearest half centimetre. Rounding down is how items end up a few millimetres too big for the box.
Step 2: add a cushioning allowance
Cushioning needs space. The amount depends on how fragile the item is and what you use to protect it. There is no single correct figure; these are practical starting points to test against your own products:
| Item type | Suggested space on each side | Typical protection |
|---|---|---|
| Robust, not breakable (books, soft goods in a box, tins) | About 1 to 2cm | Paper void fill to stop movement |
| Moderately fragile (boxed electronics, cosmetics, small homeware) | About 3 to 5cm | Wrapping plus void fill |
| Very fragile (glass, ceramics, framed items) | About 5cm or more | Several layers of wrap, plus cushioning on the base and top |
"Each side" means you add the allowance twice to each dimension: once for each opposite face. So for a fragile item with a 5cm allowance, add 10cm to the length, 10cm to the width and 10cm to the height.
The allowance should reflect the thickness of the protection after it has been packed. Bubble wrap compresses when the box is closed, and different materials take up different amounts of space. Our small bubble wrap has 10mm bubbles and our large bubble wrap has 25mm bubbles, so two turns of each add quite different thicknesses. The only reliable way to know is to wrap one item and measure it.
Step 3: match to a stock box size
You now have a required internal size. Compare it with the boxes available and choose the smallest box that is at least that large in every direction. A few practical rules help:
- Every dimension must fit. A box that is generous in length but 1cm short in height will not work, however much total space it has.
- You can turn the item. If the item fits better lying on its side, use that orientation, unless it must stay upright (liquids, plants, items marked this way up).
- A little extra is fine; a lot is not. Up to a few centimetres of extra space can be filled with void fill. Much more than that and you are paying to ship air, and the top panel loses support.
- Consider the board. Heavy or breakable items may need double wall boxes, which have thicker walls. Our separate guide to single and double wall boxes explains that choice.
Our current single wall sizes run from 5 x 5 x 5" (127mm cube) to 24 x 18 x 18" (610 x 457 x 457mm), and our double wall sizes from 9 x 6 x 6" (229 x 152 x 152mm) to 24 x 24 x 24" (610mm cube). For small products, crash lock boxes run from CL-A (100 x 100 x 70mm) to CL-K (426 x 300 x 300mm), and fold up with a self-locking base.
Step 4: check carrier size and weight limits
Once the box is chosen, check the packed parcel against the service you plan to use. Carriers measure the outside of the parcel and weigh it, including all packaging.
For Royal Mail, the size and weight guide sets these maximums at the time of writing:
| Royal Mail format | Maximum size | Maximum weight |
|---|---|---|
| Large Letter | 35.3 x 25 x 2.5cm | 750g (UK Tracked bought online has a 1kg limit) |
| Small Parcel | 45 x 35 x 16cm | 2kg |
| Medium Parcel | 61 x 46 x 46cm | 20kg |
Royal Mail updates its guidance from time to time, so recheck the live page if you are designing your whole packing range around a format. Other carriers set their own limits and some price parcels using a calculated "volumetric" weight based on the parcel's dimensions as well as its actual weight. If you use those carriers, ask for their current size limits and any volumetric formula, because a box that is too large can cost more to send even if it is light.
Two of our boxes are designed with Royal Mail formats in mind:
- Our single wall 17.7 x 13.7 x 6" box (450 x 350 x 160mm) is listed as made to the Royal Mail Small Parcel maximum size.
- Our PIP large letter boxes are 20mm deep, for slim items in the Large Letter format.
In both cases the parcel must also be within the weight limit, and anything that makes the parcel thicker, such as a label wrapped over an edge or a bulging lid, can take it over the size limit.
Worked examples
These examples are illustrative. The item sizes are assumptions chosen to show the method, and you should measure your own items. Box sizes are from our current listings.
Example 1: a boxed candle (moderately fragile)
The candle's own gift box measures 9 x 9 x 11cm. With a 3cm allowance on each side, the required internal size is:
- Length: 9 + (2 x 3) = 15cm
- Width: 9 + (2 x 3) = 15cm
- Height: 11 + (2 x 3) = 17cm
A 6 x 6 x 6" box (152 x 152 x 152mm) is 15.2cm in each direction, which is fine for length and width but short on height (15.2cm against 17cm needed). Turning the candle on its side does not help because all three directions need at least 15cm and one needs 17cm. The 9 x 6 x 6" box (229 x 152 x 152mm) gives 22.9cm in one direction, so the candle can lie on its side with the 17cm requirement along the 22.9cm length, leaving about 6cm to fill. That works, provided the candle may travel lying down.
Example 2: two hardback books (robust)
Stacked, the books measure 24 x 16 x 6cm. With a 1.5cm allowance on each side for paper void fill:
- Length: 24 + 3 = 27cm
- Width: 16 + 3 = 19cm
- Height: 6 + 3 = 9cm
The 12 x 9 x 4" box (305 x 229 x 102mm) gives 30.5 x 22.9 x 10.2cm, which fits all three. Books are dense, so check the packed weight: if it is near or over the 2kg Small Parcel limit, the parcel moves into a heavier format. A double wall equivalent may be worth it for heavy books that will travel in a busy network.
Example 3: a framed print (very fragile)
The frame measures 40 x 30 x 3cm. With a 5cm allowance on each side:
- Length: 40 + 10 = 50cm
- Width: 30 + 10 = 40cm
- Height: 3 + 10 = 13cm
The 18 x 10 x 5.5" box (457 x 252 x 143mm) is too narrow. The 24 x 18 x 12" double wall box (610 x 457 x 305mm) fits but leaves a lot of height to fill. In practice a flat item like this suits a box close to its footprint; if no stock size fits well, the usual answer is to add corner protection and fill carefully, or use a larger box and build up protection in layers. This is a case where measuring before ordering saves money.
Orientation, shape and awkward items
The three measurements tell you whether an item fits, but the way it sits in the box affects how well it travels. A few principles help with less straightforward products.
Keep the heaviest face down
Where an item can travel in more than one orientation, put its largest, flattest and heaviest face on the base of the box. That keeps the centre of gravity low, makes the parcel less likely to tip on a conveyor, and spreads the load across the box floor rather than concentrating it on a small area. A tall, narrow item standing upright in a tall box is more likely to topple and to hit the box wall when the parcel is tilted.
Respect "this way up"
Liquids, plants, items with loose parts and anything with a lid that could leak should travel upright, so choose a box tall enough for that orientation even if a lower box would fit on its side. Mark the box clearly, but remember that orientation arrows are a request to handlers, not a guarantee. Packing must still protect the item if the parcel is turned over, which usually means sealing liquids in a leak-proof inner container as well.
Long or flat items
Long items such as tools, rolled prints or poles rarely suit a cube. Look for a box with one long dimension, such as our 18 x 10 x 5.5" single wall box (457 x 252 x 143mm), rather than a large square box with lots of empty space. Flat items such as frames, boards and panels need protection on the large faces and especially the corners, because corners take the impact when a flat parcel is dropped on its edge.
Irregular shapes
For products with protruding parts, such as a lamp with a fixed shade or a toy with arms, measure the overall envelope of the item and protect the protruding parts first. Then treat the protected shape as a block when you choose the box. It is common to need a box larger than the main body of the item suggests, and that is better than squeezing a fragile part against the wall.
Sizing multi-item orders
Orders with several items are where box sizing goes wrong most often, because people choose a box for the largest item and squeeze the rest in, or choose a box large enough for the total volume and leave everything loose.
A more reliable approach is to build the order into a compact block before choosing a box:
- Put the heaviest, most robust item at the bottom.
- Arrange lighter and more fragile items around and above it, with cushioning between any items that could knock together.
- Measure the block as a single object.
- Add your cushioning allowance and choose the box as normal.
If the block becomes too heavy to lift comfortably, or the box you need is much larger than your usual sizes, split the order into two parcels. Two well-packed medium parcels often arrive in better condition than one large, heavy parcel, and they may be easier to handle at both ends. Check the postage cost of both options, because the cheapest choice depends on your carrier's size and weight bands.
For businesses that often send the same combinations, such as a gift set or a bundle offer, it is worth measuring the standard combination once and adding it to your product-to-box list, exactly as you would for a single product.
Filling the gaps
Even a well-chosen box usually has some space left. Fill it so the contents cannot move when you shake the closed box gently. Movement is what causes most transit damage inside a box. Options include:
- Kraft paper, crumpled into gaps or used to wrap. We stock a 90gsm standard roll and two 70gsm centre-feed rolls in dispenser boxes.
- Packing peanuts, which flow around irregular shapes. Ours are EcoFlo loose fill in 5 cubic feet bags.
- Air cushion film, which needs an air cushion machine to inflate it, and suits higher-volume packing benches.
Put a layer of cushioning in the base before the item goes in and another on top before the lid closes. The top layer supports the lid, which helps the box resist crushing when other parcels are stacked on it.
Building a sensible range of sizes
Once you have measured your products, you will often find that a handful of box sizes covers most orders. A simple method:
- List your products and the box each one needs.
- Count how many orders of each product you sent last month.
- Group products that share a box size, and see which three to five sizes cover most orders.
- Stock those sizes in proportion to usage, with a small safety margin.
- For the occasional odd item, keep a few larger boxes and use more void fill, rather than stocking a size you rarely use.
Our boxes are sold per box with a minimum of 5 per size, so you can start small with a new size and scale up once you know how quickly you use it.
Common sizing mistakes
- Measuring the product, not the packed product. Wrapping adds centimetres.
- Confusing internal and external sizes. The outside is bigger, which matters at a carrier limit.
- Ignoring height. Length and width get checked; height is often forgotten.
- One box for everything. Convenient, but it costs more in void fill and sometimes postage.
- Forgetting weight. A box that fits the size limit can still be over the weight limit.
Questions buyers ask
Should I order the exact size of my item?
Not for anything that needs protection. Allow space for cushioning on every side. For robust items that do not need wrapping, a close fit with a little void fill is fine.
Are box sizes listed inside or outside measurements?
It varies by supplier. Our single wall box listing gives internal dimensions. Always check the listing, and measure a made-up box if you are close to a carrier limit.
What size box fits A4?
A4 paper is 210 x 297mm. Our 12 x 9" boxes (305 x 229mm base) take an A4 item lying flat, in 4, 6 and 9 inch heights in single wall, and 6 and 9 inch heights in double wall.
What is the largest box I can send as a Royal Mail Small Parcel?
Royal Mail's guide gives the Small Parcel maximum as 45 x 35 x 16cm and 2kg. Our single wall 450 x 350 x 160mm box is made to that size, and the packed parcel must also stay within the weight limit.
How much bigger should a box be than the item?
As a starting point, around 1 to 2cm on each side for robust items, 3 to 5cm for moderately fragile items, and 5cm or more for very fragile items. Test with your own products and adjust.
Is it cheaper to use one standard box?
It can simplify ordering, but it often costs more overall in void fill, packing time and sometimes postage. Most sellers do better with three to five well-chosen sizes.
Summary
Measure the item as packed at its widest points, add a cushioning allowance on every side, choose the smallest box whose internal size covers all three measurements, fill the remaining space so nothing moves, and check the outside size and weight against your carrier's current limits. Then stock the few sizes that cover most of your orders.
