Which Royal Mail size band is your box in?
Royal Mail sells by band, not by the centimetre. Your box either falls inside a band or it does not, and there is nothing in between - a millimetre over and the whole consignment moves up a price step. So the question worth answering before ordering packaging is not "how big is my box", it is which band will it sit in, and what can I actually put inside it.
Below are the four bands, the trap that catches most sellers, and three worked examples with the real counts.
The four bands
Two of them are letter formats and two are parcels. The jump that matters most is between Large Letter and Small Parcel, because that is where a few millimetres of thickness change the price bracket entirely.
| Band | Maximum size | Maximum weight |
|---|---|---|
| Letter | 24 × 16.5 × 0.5 cm | 100 g |
| Large Letter | 35.3 × 25 × 2.5 cm | 750 g |
| Small Parcel | 45 × 35 × 16 cm | 2 kg |
| Medium Parcel | 61 × 46 × 46 cm | 20 kg |
Checked on 17 August 2026 on the carrier's own site: royalmail.com. We print the date because carriers revise their limits - if you are reading this much later, go and confirm at the source.
The weight cap is what really ends the parcel
Every band has its own ceiling, and for anything denser than packaging foam that ceiling arrives long before the box is full. This is the part that spreadsheets miss, because a spreadsheet answers "how many fit" and the carrier answers "how many you may send".
Take a candle in a retail carton, 75 × 75 × 95 mm, weighing 340 g:
- Small Parcel, interior 444 × 344 × 154 mm: 32 candles fit by volume, at 76% fill. The 2 kg cap stops you at 5.
- Medium Parcel, interior 604 × 454 × 454 mm: 192 candles fit, at 93% fill. The 20 kg cap stops you at 58.
In both cases the box is nowhere near full when the weight runs out. That is worth knowing before you buy a pallet of oversized cartons: for this product the extra space is simply not purchasable at that band.
Large Letter: 2.5 cm on the outside, 1.9 cm inside
The band limits describe the parcel as posted - the box from the outside, tape included. The space you actually pack into is smaller by two wall thicknesses on every axis. Ordinary single-wall board is around 3 mm, so each axis loses about 6 mm.
On a Medium Parcel that is a rounding error. On a Large Letter, where the whole allowance is 25 mm, it is the difference between one layer and two. A 5 mm greeting card in a Large Letter box with a 347 × 244 × 19 mm interior: 12 cards, weighing 240 g - comfortably inside the 750 g cap. Try the same sum against the 25 mm figure and you will plan for a layer that will not close.
Padding eats the interior from both sides of every axis: 5 mm of foam along the walls costs 10 mm of width, not 5. On flat goods in a Large Letter that single mistake removes the only layer you had.
Which band should you design the box for?
Work backwards. Start from the band you want to sell in, subtract the board, and only then ask how many units fit - rather than designing a box and discovering afterwards which band it landed in. Three questions settle it:
- Does the shortest dimension of your product fit the band's smallest axis after the board is subtracted? If not, no arrangement will save it.
- Does the weight cap arrive before the space does? If it does, a bigger box in the same band buys you nothing at all.
- Can you turn the product to lose a layer of height? On flat goods that is often the whole difference between Large Letter and Small Parcel.
Beyond the parcel: the box still has to travel
A box designed around a size band still has to arrive at your unit on a pallet, and those are two different optimisation problems. A carton tuned to 45 × 35 cm will not necessarily tile a 1200 × 800 mm euro pallet. If you are planning both, see how to stack boxes on a pallet and the guide to choosing a box size for an order.
How to work this out in Pallet3D
The Box tab answers exactly this question - one level below the pallet, arranging individual units inside a carton. In order:
- Enter the internal dimensions: the band limit minus two board thicknesses per axis.
- Add padding if the goods are wrapped - the field subtracts it from both sides of each axis for you.
- Draw the product shape on the grid. A plain box is one click; a bottle, a profile or an L shape is counted exactly rather than by its bounding box.
- Set the band's weight cap. The result states plainly whether space or weight ended the count.
Every figure above - 32, 5, 192, 58, 12 - came out of the same engine the tab uses. There is more on the box packing calculator page, and the demo lets you pack ready-made sample shapes without an account; drawing your own comes with the full version.
Frequently asked questions
What are the Royal Mail parcel size limits?
A Letter is up to 24 × 16.5 × 0.5 cm and 100 g. A Large Letter is up to 35.3 × 25 × 2.5 cm and 750 g. A Small Parcel is up to 45 × 35 × 16 cm and 2 kg. A Medium Parcel is up to 61 × 46 × 46 cm and 20 kg. Checked on 17 August 2026 on royalmail.com.
Is it the box size or the weight that decides the band?
Both, and you have to satisfy each of them. In practice the weight cap bites first for anything solid. A 75 × 75 × 95 mm candle weighing 340 g: a Small Parcel box holds 32 of them by volume, but 2 kg stops you at 5. A Medium Parcel box holds 192 by volume, and 20 kg stops you at 58.
Do the published dimensions describe the box or its contents?
They describe the parcel as posted - the outside of the box, including tape. Your usable interior is smaller by two wall thicknesses on every axis, roughly 3 mm per wall on ordinary single-wall board. On a Large Letter that leaves 19 mm of usable height inside the 25 mm limit, which is where most flat-goods calculations go wrong.
Check your box in 3D
Enter the interior of the band you are aiming at, pack ready-made sample shapes and see the arrangement inside along with a packing list and gross weight. Runs in the browser, no account needed; drawing your own product comes with the full version.
Run calculator →