One extra millimetre can change the postage bill for an entire batch of ecommerce orders. A box measuring 24 mm thick may qualify as a Royal Mail Large Letter. A finished parcel measuring 26 mm thick does not. The product inside might be identical.
That is why I would settle the postal format before approving a box, rather than discovering the cost after the packaging has been printed. For brands ordering custom packaging from Custom Packly UK, this is a purchasing decision as much as a dimensional one.
Royal Mail Large Letter is generally the better-priced option for slim, lightweight orders that can be packed securely within its limits. Small Parcel gives you more depth, space and weight allowance. The mistake is assuming the cheapest postal format is automatically the cheapest fulfilment method.
Large Letter vs Small Parcel: The Size Limits
For standard Royal Mail UK 1st and 2nd Class services, the maximum dimensions are:
All three dimensions apply to the finished parcel, not the bare product or the inside of the box. The weight includes the box, insert, tissue, cards, label and everything enclosed.
There is an important service distinction: Royal Mail currently publishes a 1 kg Large Letter allowance for Tracked 24 and Tracked 48. The standard 750 g limit above applies to 1st and 2nd Class. Check the specific service before setting a production specification; do not mix one service's weight limit with another service's tariff.
A Royal Mail Large Letter box therefore has a stricter job than simply looking slim. It must stay within the correct external dimensions and packed weight when it is closed in the normal way.
How Much More Does Small Parcel Cost?
The price gap is real, but it depends on the mailing service, weight band and where postage is purchased. Compare like with like.
As at 26 October 2026, Royal Mail's published online 2nd Class prices include:
These are online 2nd Class retail rates, not contracted business rates, Tracked prices or universal tariffs for all Royal Mail products. Over-the-counter prices can differ. Check Royal Mail's current 2nd Class prices when dispatching, because rates can change.
At a packed weight of 200 g, the difference is £2.50 per order: £1.90 for a qualifying Large Letter versus £4.40 for a Small Parcel. At 450 g, the corresponding gap is £2.00.
The useful calculation is not merely postage saved. It is postage saved minus the extra cost of the slimmer box, its packing method, protective materials and any additional damage or handling cost. A postal saving that depends on poor protection is not a saving worth pursuing.
The 25 mm Limit Is Where Packaging Plans Often Fail
Twenty-five millimetres is the outside thickness limit for a Large Letter. It is not the available space inside the box.
Suppose a product measures 20 mm thick. If the proposed corrugated construction takes roughly 2 mm at the top and 2 mm at the bottom, the theoretical outside depth is already about 24 mm before any tissue, internal support or bulging. That calculation is illustrative, not a substitute for measuring a converted box.
A close-fitting drawing also hides differences between individual packs. Folded flaps may overlap. A leaflet can sit on top of a product rather than beside it. A self-adhesive closure may create a raised edge. Soft goods can spring back after packing.
My preference is to design below the maximum when a postal band affects repeat-order economics, with the margin determined by the actual material and packing variation. I would not promise that a nominal 25 mm box will pass because it says 25 mm on a dieline.
A letterbox box also needs a separate decision. Royal Mail's Large Letter classification does not guarantee that every customer's domestic letterbox aperture will accept the parcel. Household openings, brushes and internal flaps vary.
Illustrative Case Study: A Refill Brand Sending 400 Orders
Consider a fictional UK refill brand shipping a lightweight set of flat product sachets each month. Its complete packed order weighs 210 g. A shallow box with the sachets placed side by side measures 230 × 165 × 22 mm externally. The same contents in a deeper presentation pack measure 230 × 165 × 32 mm.
Both packs are assumed to offer acceptable protection for this example.
The first pack qualifies by size and weight for standard Large Letter. The second is too thick and moves into Small Parcel.
At the October 2026 online 2nd Class rates:
Now assume, purely for the calculation, that producing and packing the shallow option costs 30p more per order. That adds £120. The modelled saving is £880 before considering other costs.
Those are hypothetical order details and assumed packaging costs, not a Custom Packly client result or a supplier quotation. The point is to test whether the postage difference survives the cost of the packaging needed to achieve it.
I would approve the shallow pack only after verifying the finished dimensions and checking that the sachets do not bunch or overlap during normal packing.
When Small Parcel Is the Better Choice
A Large Letter box becomes a poor choice when depth is being saved at the expense of secure packing.
Imagine a fictional skincare seller dispatching a 30 mm-thick glass container. Its product already exceeds the Large Letter thickness ceiling before protective board is added. No clever dieline can make the finished rigid container 25 mm thick without changing the product or its orientation in a genuinely workable way.
For that shipment, a corrugated mailer box with the right strength and protective space is a more sensible starting point. A lightweight folded insert or divider can restrain movement where it actually helps, but inserts consume room and should be designed with the outer box.
The same decision applies to breakable gift items, awkward-shaped cosmetics, taller accessories and mixed sets. The relevant question is whether the product can survive the journey in the shallower package, not whether it can be forced inside one.
Shipping damage can be caused by movement inside an intact box as well as by crushing from outside. The choice between a stronger board and a fitted insert depends on that failure mechanism, as discussed in fragile product packaging: stronger box, insert or both.
For fragile goods, do not rely on postage savings alone. Test the packed item under handling conditions that reflect its route, and compare the true cost of damage, replacements and customer support.
Flat Products Need Corner Protection, Not Just a Shallow Box
Books, stationery, prints, flat accessories and printed samples are more natural Large Letter candidates, provided they meet the size and weight limits.
A common weak point is the corner. A thin carton may fit the postal band but still leave a book free to slide or bend. A close-fitting book mailer can wrap around the item and protect its edges without creating an unnecessarily deep cavity.
The exact result depends on the book's trim size, spine thickness, cover rigidity and the wrap used. A hardback with vulnerable corners may need a different solution from a thin paperback. Measure both packed weight and actual thickness at the folded edges before approving Large Letter.
I would rather add necessary edge protection and use Small Parcel than strip it away merely to reach a cheaper label price.
One Box Size for Every SKU Can Be an Expensive Shortcut
A single standard mailer simplifies purchasing and storage. It can also push every small order into Small Parcel even when most items could be sent as Large Letters.
Consider a hypothetical shop with 500 monthly dispatches. Four hundred orders contain slim products that can be packed securely at 200 g within Large Letter. The other 100 contain thicker products that require Small Parcel.
Using one Small Parcel box for all 500 orders would cost £2,200 in online 2nd Class postage at £4.40 each. Sending 400 as Large Letters at £1.90 and 100 as Small Parcels at £4.40 would cost £1,200. The modeled postage difference is £1,000 a month, before subtracting any extra cost of owning and using two box specifications.
That saving only exists if the stock mix, size, weights and postal service remain as assumed. The additional box SKU brings its own tooling, purchasing, storage and picking demands.
I would separate box sizes when the recurring postage saving is material enough to justify that complexity. If only a handful of orders benefit, simplicity may win. The same cost discipline applies to board, finishes, stockholding and reorder quantities in common custom packaging cost mistakes.
Measure the Finished Mailing, Not the Empty Sample
A reliable Large Letter decision needs physical evidence from the packing bench.
Measure several closed, fully loaded units, including the heaviest and thickest product combinations. Check thickness at the centre, edges, corners and anywhere the lid bows or overlapping flaps create a raised area. Weigh the whole pack, including printed inserts and any protective wrap.
Keep the dimension language precise. Internal dimensions determine usable product space. External dimensions determine the parcel size. The distinction matters even more when the allowed thickness is only 25 mm; measuring a box for custom packaging deals with the dimensional conventions and product-clearance decisions in detail.
For repeat fulfilment, ask more than one packer to prepare the same order. If one packs the components side by side while another stacks them, an approved design can produce different postal classifications.
A sensible validation sequence is:
A 3D mockup is useful for checking shape and branding. It cannot certify a physical posting dimension. Where clearance is tight, evaluate a production-equivalent physical sample rather than trusting a visual proof.
Postage Cost Is Only One Part of the Delivery Decision
Royal Mail 1st Class, 2nd Class and Tracked services have different prices, service features and sometimes different allowable weights. A comparison becomes misleading if a Large Letter 2nd Class price is put against a Small Parcel Tracked price.
The service choice also depends on the value of the goods, customer expectations and the appropriate compensation cover. For example, Royal Mail states that standard 2nd Class compensation does not cover valuables such as jewellery; a suitable service must be selected for those shipments.
Postage cost can also change when the parcel exceeds an agreed size or the wrong weight is declared. Royal Mail warns that inaccurate measurements can result in additional charges or delivery problems.
A postal box is often the practical answer when the product naturally needs more depth or a stronger closure. It is better to specify that requirement clearly than redesign the same Large Letter pack repeatedly around an unsuitable product.
Do not confuse postal classification with successful home delivery, either. Meeting Large Letter dimensions does not guarantee that every item will pass through every letterbox, or that every order will be delivered without a handover.
What to Confirm Before Ordering Custom Postal Packaging
Before requesting a production quote, I would want the packed product rather than a guessed box size. The specification should include:
Custom packaging can start from a low quantity, but the commercial decision should account for repeated dispatches. A well-designed 100-box trial can reveal the actual packed profile and assembly consistency before a larger run locks in the same mistake.
Frequently Asked Questions
What are the Royal Mail Large Letter dimensions?
For standard UK 1st and 2nd Class services, a Large Letter can measure no more than 353 × 250 × 25 mm and weigh no more than 750 g. These are the limits for the complete packed mailing. Some other Royal Mail services, including Tracked 24 and Tracked 48, publish a 1 kg Large Letter weight allowance, so check the service you intend to purchase.
Is a 26 mm-thick box still a Large Letter?
No. A finished parcel 26 mm thick exceeds the standard 25 mm Large Letter thickness limit, even if its length, width and weight are otherwise acceptable. It may qualify as a Small Parcel if all that service's limits are met. Reducing thickness is worthwhile only if the product remains protected and the packing process is repeatable.
What is the maximum Royal Mail Small Parcel size?
A standard UK Small Parcel can measure up to 450 × 350 × 160 mm and weigh up to 2 kg. The outside of the packed parcel determines its physical size. Items exceeding those limits may require a different parcel format or service.
Are Large Letter boxes guaranteed to fit through letterboxes?
No. A Royal Mail Large Letter classification is not a guarantee of delivery through every household letterbox. Door apertures, brushes and internal flaps vary. If unattended through-door delivery matters, assess the actual finished pack against the intended delivery experience rather than treating the postal limit as a universal door opening.
Is a custom Large Letter box always cheaper overall?
No. Postage may be cheaper, but a slim structure can cost more to manufacture or pack. If it needs special inserts, slows fulfilment or increases damage, the total cost can be higher than that of a Small Parcel. Compare the same carrier service, actual packed weights, packaging quotes, handling time and expected damage costs.
Choose the Postal Format Before You Lock the Box
For flat, lightweight goods, staying within Large Letter limits can remove a meaningful recurring cost. For thicker or more vulnerable products, Small Parcel provides room that the product may genuinely need.
My approach is to treat the postal band as a design constraint from the outset, then test whether the finished packaging meets it reliably. If the only way to reach 25 mm is to compromise protection or make packing unpredictable, I would change the postal format rather than force the box.
