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This page is an editorial and informational resource about Through the Letterbox: Designing Postal Packaging for Royal Mail Large Letter and Small Parcel Sizes.
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How the Large Letter and Small Parcel sizes are defined and how the slotted gauge checks them.
Choosing flutes, liners and boxboard so that material thickness does not consume the depth allowance.
Shallow box styles, envelopes, closures and inserts that keep packs flat, tidy and protected.
Prototyping, gauge checks, weight margins and drop tests before committing to a print run.
A package that slips through a letterbox never waits on a doorstep, never prompts a missed-delivery card and never sits in a sorting office waiting for collection. For anyone posting small, flat goods across the UK, that one quality shapes almost every packaging decision, from the choice of board to the position of a tear strip. This guide sets out how Royal Mail defines its Large Letter and Small Parcel formats, how the official gauge measures an item, and how to engineer envelopes, flat mailers and shallow boxes that land in the intended format every time. It is written for makers and small dispatch teams who design their own packaging and want figures they can work to: millimetres, grams and calipers rather than slogans. Where a figure depends on a supplier or a specific material, the guide gives a typical range and explains how to confirm it on a sample before committing to a full print run.
Royal Mail prices and handles items by size first and weight second, a system introduced in 2006 under the name Pricing in Proportion. Before that change, cost was driven almost entirely by weight. Afterwards, a thin, light item and a bulky, light item stopped being treated the same way, and packaging design became a direct lever on how a piece travels through the network.
The consequence for designers is simple to state and harder to achieve: every millimetre of depth matters. A mailer that measures 24 mm deep sorts as a Large Letter, rides through letter-sorting machinery and is pushed through the recipient's door. The same contents in a box 26 mm deep become a Small Parcel, move through a different stream and may need a signature, a safe place or a trip to the enquiry office.
There is also the household side. Recipients increasingly expect small orders to arrive without anyone needing to be at home, and a parcel left on a step is exposed to rain and to passers-by. Designing to the letterbox is therefore a question of reliability as much as of postal format.
Most dispatch ranges need two families of packaging. The first is engineered tightly around the Large Letter limits for thin goods. The second is designed for the Small Parcel format, where the generous 160 mm depth allows proper boxes but where weight, rigidity and handling become the dominant concerns. Treating these as separate design problems, with separate specifications, avoids the common mistake of a single compromise box that suits neither format well.
The table below summarises the domestic formats most relevant to small dispatch. All dimensions are maximums and apply to the finished item, including the packaging, any label and any bulge in the contents. Royal Mail revises its terms from time to time, so treat these as the working reference and confirm against the current published size guide before tooling up a new design.
| Format | Maximum length | Maximum width | Maximum depth | Maximum weight | Typical handling |
|---|---|---|---|---|---|
| Letter | 240 mm | 165 mm | 5 mm | 100 g | Fully automated letter sorting |
| Large Letter | 353 mm | 250 mm | 25 mm | 750 g | Flats sorting, usually fits a standard letterbox |
| Small Parcel | 450 mm | 350 mm | 160 mm | 2 kg | Parcel stream, may need a safe place or collection |
| Medium Parcel | 610 mm | 460 mm | 460 mm | 20 kg | Parcel stream, handled as a box |
Within the Large Letter format, weight is banded, traditionally at 100 g, 250 g, 500 g and 750 g. A design that sits comfortably below a band boundary, with a margin for variation in the contents, is worth far more than one that occasionally tips over by a few grams.
Note that A4 paper measures 297 × 210 mm, which fits inside the Large Letter footprint with 56 mm and 40 mm to spare. That margin is the space a designer has for board thickness, side walls, overlapping flaps and the padding of any envelope. It disappears faster than most people expect.
Counter staff and sorting offices check formats with a slotted gauge: a rigid template with an aperture cut to 353 × 250 mm and a slot 25 mm deep. The test is practical rather than theoretical. If the item passes through the slot freely, without being forced, it is a Large Letter. If it has to be pushed, squashed or wiggled, it is not.
Three things catch designers out at this stage.
Corrugated board production carries a dimensional tolerance of roughly plus or minus 1 mm on creased dimensions, and caliper itself varies from batch to batch. A sensible design target for a rigid Large Letter mailer is an external depth of 21 to 22 mm, leaving 3 to 4 mm of headroom against the 25 mm slot. For envelopes and flexible mailers, 23 mm filled is a reasonable ceiling.
It is worth making a gauge in-house. A strip of 3 mm acrylic or MDF with a 25 mm slot and a 353 × 250 mm outline, cut on a laser or by a local fabricator, lets the packing bench test items as they are packed. Some teams also keep a 22 mm feeler slot alongside it, so that anything failing the tighter slot is flagged before it becomes a problem.
Finally, remember the doors themselves. Letter plates vary widely: some period doors have narrow vertical slots, and internal draught brushes or letter cages reduce the usable opening. A 22 mm item clears far more letterboxes than a 25 mm one, which is another argument for keeping well inside the limit.
Every layer of material in a mailer steals depth from the contents. Choosing board is therefore a balance between protection, print quality and caliper, the industry term for the thickness of a sheet measured under a standard load.
Folding boxboard and solid bleached board are specified by grammage in gsm and by caliper in microns. Corrugated board is specified by flute profile and by the liners on each face, often written as a code such as 125K/B/125T, meaning a 125 gsm kraft outer liner, B-flute fluting and a 125 gsm test liner inside. Kraft liners use virgin fibre and give better burst and edge crush strength; test liners use recycled fibre and are cheaper and lighter in tone.
| Material | Typical caliper | Depth used by top and base | Large Letter suitability |
|---|---|---|---|
| Folding boxboard, 350 gsm | about 0.5 mm | about 1 mm | Excellent for very thin goods, limited crush resistance |
| Solid greyboard, 1000 micron | 1.0 mm | 2.0 mm | Good stiffener for envelopes and board-backed mailers |
| F-flute micro corrugated | about 0.8 mm | about 1.6 mm | Very good, fine print surface, light weight |
| E-flute single wall | 1.1 to 1.6 mm | 2.2 to 3.2 mm | The usual choice for shallow mailer boxes |
| B-flute single wall | about 3 mm | about 6 mm | Possible but leaves little internal depth |
| C-flute single wall | about 4 mm | about 8 mm | Better kept for Small Parcel boxes |
| E/B double wall | about 4.5 mm | about 9 mm | Small Parcel only, for fragile or heavy goods |
The arithmetic is unforgiving. A B-flute box built to a 22 mm external depth leaves about 16 mm inside before tolerances; the same box in E-flute leaves about 19 mm. That extra 3 mm is often the difference between a product fitting flat or not fitting at all.
Edge crush test values, quoted in kN per metre, and burst strength in kPa are useful for comparing grades from the same supplier. For Large Letter mailers, flat crush resistance matters more than stacking strength, since flats are rarely stacked under heavy load for long but are frequently pressed in sorting frames and mail bags.
The workhorse of letterbox packaging is the roll end tuck front mailer, catalogued in the FEFCO code as style 0427. It is cut and creased from a single blank, folds without glue and closes with a tuck flap that slides inside the front wall. Its rolled side walls give it surprising rigidity for its weight.
That rigidity comes from layering. In a 0427, the side walls fold over on themselves, so the board is doubled or tripled at the edges. This adds to the external length and width, not only the depth, and must be counted when designing close to the 353 × 250 mm footprint. As a rule of thumb, add four times the board caliper to each internal plan dimension to estimate the external size, then confirm with a sample.
For goods that do not need corrugated protection, a crash lock base carton in folding boxboard is another option, though it is less common for flats. A lighter alternative is the wallet or book wrap, a single scored sheet that wraps around a flat item and closes with an adhesive flap. Book wraps adjust to the thickness of the contents by means of multiple parallel creases, which makes them efficient for ranges with varying depth.
Plain stock mailers are widely available in Large Letter sizes, often quoted by internal dimensions such as 330 × 230 × 20 mm. Always check whether a catalogue size refers to internal or external measurements, because the difference in a 0427 can be 6 mm or more on each axis.
Not every item needs a box. Flat goods such as prints, textiles, paper goods and slim accessories often travel well in envelopes, which flex at the gauge and add very little depth.
Board-backed envelopes pair a paper front with a greyboard or solid board back, typically 500 to 1000 microns, to stop bending. They are the standard format for photographs and artwork. Fully rigid envelopes, made from solid board throughout, give more protection at the cost of weight and a little extra caliper.
Padded mailers deserve care. A traditional bubble-lined envelope adds around 3 to 4 mm of depth on each face before the contents are counted, so 7 to 8 mm in total. A paper-padded mailer, using a cellulose or honeycomb paper lining, is often similar in thickness and has the advantage that it can go in household paper recycling as a single material. Either way, a padded format leaves about 15 to 17 mm for the item in a Large Letter, and less if the padding is compressed unevenly.
Gusseted paper mailers have folded side panels that expand to accept thicker items. They are useful for soft goods but can easily exceed 25 mm when fully opened. If a gusseted mailer is used for Large Letters, decide on a maximum fill and mark it on the packing specification rather than relying on judgement at the bench.
Envelope sizes in the C series match the A series of paper: a C4 envelope at 324 × 229 mm holds an A4 sheet flat and sits inside the Large Letter footprint. C5, at 229 × 162 mm, holds A5. These standard sizes are cheap and easy to source, and they make a sensible starting point for designing around flat contents.
For any envelope format, think about how it behaves when wet. Kraft paper with a higher grammage, around 120 to 130 gsm, resists tearing and rain better than lightweight office stock at 80 to 90 gsm.
The closure is where many letterbox designs gain unwanted millimetres. Tape wrapped over a box edge adds its own thickness and often creases into a ridge. Designing the closure into the pack keeps the profile flat and the opening experience tidy.
A closure that cannot be lifted and reapplied without visible damage reassures the recipient that the contents have not been disturbed. Permanent adhesives on kraft board will tear fibres when lifted, which is itself a form of evidence. Perforated security labels across a seam offer a second layer, though they add a little depth and should be applied flat, away from the thickest point of the pack.
Whatever the method, keep adhesives away from the area where a postal label or franking will go, and avoid placing a closure exactly on a fold, where it will lift as the board relaxes.
Size gets an item into a format; weight decides where it sits within that format. For Large Letters the ceiling is 750 g, and the bands below it reward designs that shave off grams.
Packaging weight is easy to underestimate. A 0427 mailer measuring about 340 × 240 × 22 mm in E-flute typically weighs between 90 and 120 g, depending on the liners. The same footprint in F-flute may come in at 70 to 90 g, and a board-backed envelope at 40 to 70 g. Add a printed insert card, tissue and a label, and the packaging alone can reach 150 g before any product is inside.
Board weight varies with humidity. Corrugated board stored in a damp unit can gain several per cent in mass, which matters if a design sits close to a boundary. Storing packaging in a dry, ventilated space keeps both weight and stiffness consistent.
For Small Parcels the ceiling is 2 kg, which sounds generous until double wall board, protective fill and a dense product are combined. Couriers outside the Royal Mail network often apply volumetric weight, calculated by multiplying length, width and depth in centimetres and dividing by a factor such as 5000. A design that is light but oversized can be charged as though it were heavy, so compact geometry pays off whichever carrier is used.
Once an item grows beyond 25 mm deep, the design target changes. The Small Parcel format allows up to 450 × 350 × 160 mm and 2 kg, but the item will no longer go through most letterboxes. Design priorities shift from flatness towards protection, ease of handling and how the pack behaves when left in a safe place or a parcel postbox.
The regular slotted container, FEFCO 0201, is the classic choice: four flaps top and bottom, closed with tape. It is economical but plain to open. The 0427 mailer scales up well to depths of 50 to 100 mm and gives a neater presentation, while a crash lock base carton suits retail-style goods that will be opened at a table.
There is no benefit in using the full 160 mm of depth if the product does not need it. Smaller boxes use less board, need less void fill and fit more readily into parcel lockers, which have compartments of fixed dimensions. A common approach is to design a short family of standard boxes, for example three or four sizes, each sitting comfortably inside the format with 10 mm of margin on every axis.
It is also worth designing an intermediate size that sits just over the Large Letter limit, around 30 to 40 mm deep. Many orders that will not quite fit a letterbox still benefit from a compact, lightweight parcel rather than a deep standard box.
In a Large Letter there is almost no room for conventional void fill. Protection has to come from the structure of the pack and from inserts that hold the contents still rather than cushion them.
Die-cut board inserts are the main tool. A single sheet of E-flute or folding boxboard, cut with an aperture matching the product outline, locks the item in place and spreads any pressure across the whole lid. Because the insert sits beside the product rather than above it, it adds no depth. For items with an irregular profile, a two-layer insert, with a cut-out layer and a plain backing layer, holds the product while keeping the outer faces flat.
For Small Parcels, the aim is to stop movement with the least fill. A useful test is to close the box and shake it firmly: if anything moves or rattles, the fill is insufficient or poorly placed. Crumpled kraft paper, moulded pulp trays and paper cushioning all work, and single-material paper packs are easier for the recipient to recycle at the kerbside.
Wherever possible, design the insert and the outer pack together. A box that is a few millimetres larger than the product, with a fitted insert, will nearly always protect better and weigh less than a generic box packed with loose fill.
Postal handling depends on clear addressing. Royal Mail's machinery reads addresses optically, and its staff read them at speed, so the address area should be designed rather than left to chance.
Place the address panel on the largest flat face, parallel to the longer edge, with clear space around it. Leave the top right corner free for postage, whether a franking impression, a printed postage label or stamps. Avoid placing addresses across creases, seams or tear strips, where labels can wrinkle or lift. On dark or heavily printed packaging, a plain white panel or a white label gives the contrast that readers need.
Finishes affect more than looks. Gloss and soft-touch laminates resist scuffing in transit but can make labels peel and complicate recycling. Uncoated kraft takes labels and handwriting well and recycles easily. Interior printing, on the inside face of a mailer, gives a sense of occasion to the recipient without adding anything to the outer surfaces that the postal system must read.
Keep ink coverage light near folds on corrugated board, because heavy solids can crack along creases and expose the brown fibre beneath.
No specification survives contact with real contents unchanged. Before ordering a full run, request plain samples from the supplier, ideally a dozen or more, and build packs with the heaviest and bulkiest variants of the range. Run each one through an in-house gauge, weigh it, then post several to addresses with different types of door, including a narrow letter plate and a door with a draught brush.
Simple drop tests give useful information without a laboratory. Drop a packed parcel from about 760 mm, roughly bench height, onto a hard floor: once flat on each face, once on an edge and once on a corner. Then open it and inspect both product and pack. For Large Letters, also test resistance to pressure by placing a stack of books on a filled mailer overnight and measuring whether its depth recovers.
Revisit the specification whenever a product changes, a supplier changes board grade or Royal Mail updates its terms. Small drifts in caliper or content weight accumulate quietly, and a design that once passed the gauge with room to spare can begin to fail without anyone noticing until items are reclassified in the network.
Who publishes this resource, why it exists, and how to reach the editor with a correction or a question about the guide.
Originals publishes practical guides on the craft of packing and posting, written from the bench rather than the sales floor. We favour measurable figures, named techniques and honest ranges over claims, and we update guides when postal terms or materials change.