There is no standard set of mailer box sizes: a custom mailer is sized from the packed product and then checked against the postal or courier band it has to ship in. The binding number is usually 25 millimetres, the maximum thickness of a Royal Mail Large Letter. A box that comes out at 26 mm moves into a more expensive parcel band on every order. Similar thresholds exist in the Netherlands and Germany, and couriers elsewhere price on volume. Sizing a mailer is therefore less about picking from a chart and more about designing to the price break, with the board, the product and the tolerances all accounted for.
This guide covers how mailer dimensions are stated, the letterbox formats most often designed for, and how to build a size range. It supports our main page on custom printed mailer boxes, which covers construction, board and print.
How are mailer box sizes measured?
Mailer box sizes are normally stated as internal dimensions in the order length x width x depth, measured in millimetres. Length is the longer side of the opening, width the shorter side, and depth the height from base to closed lid. Postal and courier limits, by contrast, are always external, so you need both sets of numbers.
The difference between internal and external is the board thickness, counted more than once in some directions. On a FEFCO 0427-style mailer the side walls are rolled double-thickness panels, and the front of the lid tucks inside the base. As a rough guide:
| Dimension | What adds to the external size | Rule of thumb |
|---|---|---|
| Length | Double-thickness rolled side walls on both ends | Add about 4 x board thickness |
| Width | Front tuck and back wall | Add about 2-3 x board thickness |
| Depth | Base panel plus lid panel | Add about 2 x board thickness, plus any lid bow |
These multiples are approximations; the exact allowance depends on the die design, crease positions and board caliper, and is fixed on the dieline. The important point is that a nominal 25 mm internal depth will not fit a 25 mm external limit.
How to measure the product for a mailer
Measure the product as it will be packed, including any wrapping, tissue, sleeve or insert, and add a small clearance so the box closes without bulging. For soft goods such as folded apparel, compress the product to the state the packer will actually achieve, not the ideal fold. A box that fits on the drawing but needs force at the packing station will bulge at the lid, and a bowed lid is often what pushes a letterbox box over its thickness limit.
Letterbox and small-parcel formats by market
Letterbox formats let the carrier deliver without the recipient being home, which reduces failed deliveries as well as postage. The figures below are those commonly cited by the carriers; postal products and limits change, so check current carrier rules before a dieline is approved.
| Market and format | Maximum external size | Maximum weight | Design note |
|---|---|---|---|
| UK - Royal Mail Large Letter | 353 x 250 x 25 mm | 750 g | Thickness is the binding constraint for boxes |
| UK - Royal Mail Small Parcel | 450 x 350 x 160 mm | 2 kg | Next band up; much more room |
| NL - PostNL brievenbuspakje (letterbox parcel) | 380 x 265 x 32 mm | 2 kg | 7 mm more depth than a UK Large Letter |
| DE - Deutsche Post Großbrief (to 31 Dec 2026) | 353 x 250 x 20 mm | 500 g | Very shallow; suits flat items only |
| DE - Deutsche Post Maxibrief (to 31 Dec 2026) | 353 x 250 x 50 mm | 1 kg | Deeper letter format; parcel products cover heavier items |
Deutsche Post has announced new national letter formats from 1 January 2027: the Großbrief stays at 353 x 250 mm but allows up to 30 mm height and 1,000 g, while the Maxibrief becomes up to 381 x 305 x 30 mm and 2,000 g (the 50 mm Maxibrief height and the surcharged Maxibrief variant are dropped; Warensendung keeps a 50 mm height). Check deutschepost.de for the format that applies on your ship date.
Two points apply across all of these. First, the gauge carriers use is a slot or template: a box that is within size on paper but bowed at the lid can still fail the gauge. Second, weight limits include the box. An E-flute mailer of letterbox size typically weighs several tens of grams, which matters when the product is close to a weight threshold.
The United States has no real letterbox format for boxes
USPS "flats" (large envelopes) are limited to about 15 x 12 x 0.75 in and must be flexible and uniformly thick, so a rigid corrugated mailer is generally treated as a parcel, not a flat. For US shipments, the sizing lever is dimensional weight rather than a letterbox slot.
Designing a mailer under a thickness limit
To fit a 25 mm or 32 mm external limit, start from the limit and work inwards: subtract the board allowance for lid and base, subtract a safety margin for lid bow and production tolerance, and what remains is the maximum product height.
A worked example for a UK Large Letter box in E flute:
| Step | Example value |
|---|---|
| External thickness limit | 25.0 mm |
| Safety margin for bow and tolerance | about 2.0 mm |
| Lid and base board (2 x E flute at about 1.5 mm) | about 3.0 mm |
| Maximum internal depth available | about 20 mm |
The same calculation in F flute (around 0.8-1.0 mm) frees roughly another millimetre of internal depth, which is why F flute is popular for letterbox boxes containing slightly thicker items. The trade-off is lower crush resistance, discussed in our guide to corrugated board grades for mailer boxes.
Other details that help letterbox boxes stay within the limit:
- Avoid double-thickness lid panels on letterbox styles; a single lid panel with a tuck is usually enough at this depth.
- Keep tear strips and adhesive closures flat; thick tape layers add depth.
- Specify how the box is closed. A tucked lid can spring open slightly; a peel-and-seal strip holds it flat.
- Check the box closed, with the heaviest and thickest product variant inside, against a physical gauge on pre-production samples.
Dimensional weight: why a few millimetres cost money
Most couriers bill the greater of actual weight and dimensional (volumetric) weight. Dimensional weight is calculated as external length x width x height divided by a divisor. In international express, 5000 cm³ per kg is common; US domestic carriers mostly use 139 cubic inches per lb (USPS moved from 166 to 139 on 12 July 2026 for parcels over 1 cubic foot; UPS retail rates have used 166), depending on service and account terms. Some carriers also round dimensions up to the next whole centimetre or inch, which makes small reductions more valuable.
An illustration with a 5000 divisor: a box of 400 x 300 x 150 mm has a volumetric weight of 3.6 kg. Reduce it to 380 x 280 x 120 mm and it drops to about 2.55 kg. If the product weighs 1 kg, the customer was paying for air either way, but the second box costs a full weight band less on many tariffs.
Reducing box volume also helps with future EU rules. From 2030 at the earliest, the PPWR will cap empty space in e-commerce packaging at 50 % for whoever fills the box; see e-commerce packaging and the PPWR.
Mailing and shipping box size chart
There is no standard set of mailing box sizes; carriers' shipping box size charts list the boxes they sell, not a norm for custom mailers. The ladder below is a practical starting point built around the postal bands above. Internal sizes allow for E flute board and a small safety margin.
| Size band | Indicative internal size (mm) | Approx. inches | Typical contents | Usually ships as |
|---|---|---|---|---|
| UK letterbox | up to about 340 x 240 x 20 | 13.4 x 9.4 x 0.8 | Folded apparel, books, flat cosmetics sets | Royal Mail Large Letter |
| NL letterbox (brievenbusdoos) | up to about 365 x 250 x 25 | 14.4 x 9.8 x 1.0 | As above, slightly thicker items | PostNL brievenbuspakje |
| Small | about 200 x 150 x 60 | 7.9 x 5.9 x 2.4 | Single accessory, skincare, small electronics | Small parcel |
| Medium | about 300 x 220 x 100 | 11.8 x 8.7 x 3.9 | Multi-item orders, gift sets, candles | Small parcel or courier |
| Large | about 430 x 330 x 150 | 16.9 x 13.0 x 5.9 | Bulky apparel, bundles, subscription boxes | Royal Mail Small Parcel or courier band |
Treat these as placeholders until your own order data confirms where the bands fall. The method below replaces the generic chart with sizes that fit your products.
How many mailer sizes should a brand stock?
Most brands with a varied catalogue can cover the large majority of orders with three to five mailer sizes. Fewer sizes lengthen print runs and lower unit cost; more sizes reduce empty space and postage. The right number is found from order data, not from a standard chart.
A practical method:
- Export a few months of order lines with the products in each order.
- Group orders by the combined packed dimensions of their contents.
- Plot those against the postal and courier bands you use.
- Set one size per band where most orders fall, and test whether a mid size closes a big empty-space gap.
- Check the largest order profiles: an occasional oversized order may be better shipped in a standard brown carton than given its own printed mailer.
Every additional size needs its own cutting die and usually its own plates or print set-up. Where sizes share a footprint and differ only in depth, some artwork can be shared, which reduces artwork and approval work. The commercial effect of SKU count is covered in packaging cost drivers.
Size checklist before requesting a quotation
- Products and combinations per box, packed dimensions and weights, including wrapping and inserts.
- Internal dimensions (L x W x D) per size, or a request for a size proposal from product data.
- Target carrier format per size and its maximum external dimensions and weight.
- Board preference, noting where thickness is constrained.
- Closure: tuck only, tear strip, number of adhesive strips.
- Annual quantities per size, so a size range can be priced together.
Send this with your artwork brief through the custom printed mailer boxes page or our packaging RFQ guide, and the dimensions will be checked on the dieline before quotation.