Mailer Box Sizes and Postal Bands
Inside sizes in millimetres, matched to the postal band each one fits once the board is added.
Royal Mail bands and the mailers that fit them
The band limits below are outside sizes. Inside sizes in the last column already allow for the board on each face. Check the current band limits before you order, and see our postal band guide for mailer boxes for worked examples.
| Postal band | Maximum outside size | Smallest mailer we usually make | Largest inside size that stays in band |
|---|---|---|---|
| Large Letter | 353 x 250 x 25 mm | 160 x 110 x 20 mm | 340 x 240 x 22 mm |
| Small Parcel | 450 x 350 x 160 mm | 200 x 150 x 50 mm | 440 x 340 x 150 mm |
| Medium Parcel | 610 x 460 x 460 mm | 300 x 220 x 100 mm | 600 x 450 x 450 mm |
Mailer box size chart
Common inside sizes and the items they suit. Every box is cut to order, so any of these can move up or down by a few millimetres to fit your product.
| Inside size (L x W x H) | What it holds | Postal band |
|---|---|---|
| 160 x 110 x 20 mm | Earrings on a card, a lip balm pair or a set of badges | Large Letter |
| 220 x 160 x 20 mm | A5 notebook, greeting cards or a pair of socks | Large Letter |
| 320 x 230 x 20 mm | A4 prints, brochures and certificates laid flat | Large Letter |
| 200 x 150 x 50 mm | A candle jar, a serum and cleanser, or a mug | Small Parcel |
| 250 x 180 x 80 mm | Two folded T-shirts or a three-piece skincare set | Small Parcel |
| 300 x 220 x 100 mm | A hoodie, a boxed homeware gift or a hamper for one | Small Parcel |
| 350 x 250 x 120 mm | A pair of trainers or a subscription box with several items | Small Parcel |
| 400 x 300 x 150 mm | A folded throw, a jacket or a larger gift set | Small Parcel |
| 440 x 340 x 150 mm | The largest box that stays in Small Parcel, such as a bulky jumper bundle | Small Parcel |
| 450 x 350 x 200 mm | Boots, a cushion or a set of storage jars with inserts | Medium Parcel |
| 500 x 400 x 300 mm | Bedding, a small lamp or a large hamper | Medium Parcel |
| 600 x 450 x 450 mm | The largest mailer we make, for bulky homeware | Medium Parcel |
How do you turn a product into an inside size?
Measure the product as it will be packed, wrapping included. Take the longest side, the widest side and the tallest point, then add 3 to 5 mm to each so it slides in without scraping.
Write the result as length x width x height, for example 250 x 180 x 80 mm. Our guide on how to measure for a mailer box covers stacks, sets and awkward shapes.
Why does board thickness change the outside size?
Every wall adds its own thickness to the outside. A box in E-flute grows by about 3 mm on each measurement, and one in B-flute by about 6 mm.
That is why the Large Letter rows above stop at 20 mm deep. The band allows 25 mm outside, and the board on the lid and base takes up most of the gap.
| Inside size | Outside in E-flute 1.5 mm | Outside in B-flute 3 mm |
|---|---|---|
| 320 x 230 x 20 mm | about 323 x 233 x 23 mm | about 326 x 236 x 26 mm |
| 250 x 180 x 80 mm | about 253 x 183 x 83 mm | about 256 x 186 x 86 mm |
| 440 x 340 x 150 mm | about 443 x 343 x 153 mm | about 446 x 346 x 156 mm |
Notice that the slim box only stays in Large Letter in E-flute. In B-flute it would be just over the depth limit.
Which size suits a range of different products?
If you sell many items, you do not need a box for each. Group products by size, then pick two or three boxes that cover the groups with the least empty space.
A small, a medium and one Large Letter size covers most small shops. Spare depth can be taken up with paper void fill or a simple divider.
Can you make a size that is not on the chart?
Yes. The chart shows common starting points, not stock. We cut any inside size from about 100 x 80 x 30 mm up to 600 x 450 x 450 mm.
Send your measurements through the written quote request page and we check them against your band before pricing.
Questions
What is the deepest mailer box that still posts as a Large Letter?
In E-flute, an inside depth of about 20 mm keeps the outside under the 25 mm limit. In B-flute the walls are too thick to make a useful Large Letter box.
Is the size chart showing inside or outside measurements?
Inside. The first column is the space your product sits in. Board adds about 3 mm in E-flute or 6 mm in B-flute to each outside measurement.
Which size mailer box fits an A4 print?
An A4 sheet is 297 x 210 mm, so an inside size of about 305 x 218 mm with a shallow depth gives room to slide it in. A depth of 20 mm keeps it in Large Letter.
What size mailer box fits a pair of trainers?
Most adult trainers fit in about 330 x 220 x 120 mm inside, or 350 x 250 x 120 mm with tissue. Measure your largest pair before deciding.
What is the biggest box that stays in Small Parcel?
About 440 x 340 x 150 mm inside in E-flute, which keeps the outside under 450 x 350 x 160 mm. Check the current band limits before you order.
Can I order a size between two rows on the chart?
Yes. Every box is cut to order, so any millimetre measurement in our range is possible.
Why do the chart sizes leave space around the product?
The items listed are guides only. Your own measurements, plus 3 to 5 mm of clearance a side, are what set the final inside size.
Does the postal band change if I add an insert?
Not the outside, but an insert takes up inside space. Measure the product in the insert, or tell us and we allow for the insert board in the size.
Which size is the most common for clothing?
Around 250 x 180 x 80 mm suits one or two folded garments. Thinner single items like a T-shirt often fit a 30 to 40 mm deep box.
Can a mailer box be taller than it is wide?
Yes, within our range. Tall, narrow boxes suit bottles stood upright, though a die-cut insert is usually needed to keep them from tipping.
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Tell us what you post
Send the product, a rough inside size and a quantity. We reply in writing within one working day with a price and a free design proof. Prefer to talk? Call +44 7578 382054 or message us on WhatsApp.