A flat mailer has no standard size in the way paper has a standard size. What it has is a footprint driven by whatever goes inside it, and a depth shallow enough that the mailer still travels as a flat. If you are trying to find a number to put on a quote request, work from the contents and the paper standards below rather than from a chart.
Start from the flat thing you are shipping
Almost every flat mailer wraps a document, a print, a garment folded once, or a boxed accessory. Published paper and card standards give you a defensible starting footprint for the first three.
| Common flat contents | Published standard size | Footprint implication |
|---|---|---|
| US letter document or letterhead | 8.5 x 11 in | Internal length and width must clear 8.5 x 11 in plus clearance, so a 9 x 11.5 in internal is a sensible starting point |
| ISO A4 document | 210 x 297 mm (about 8.27 x 11.69 in) | Slightly narrower and longer than letter. If you ship to both markets, size to whichever is larger on each axis |
| ISO A5 booklet | 148 x 210 mm | Half the A4 footprint, usually paired with a heavier board because the panel is small and gets handled more |
| Standard US business card | 3.5 x 2 in | Card packs ship in small mailers where board caliper, not footprint, is the real decision |
These are the published paper standards, offered as starting points. They are not iBox stock sizes and you are not restricted to them.
How to measure, in the order a quote wants it
- Stack the contents exactly as they will ship, including any backing board, tissue or literature insert.
- Measure the stack at its widest point, not at a fold or a tapered edge.
- Add about 1/8 inch of clearance on length and width so the contents slide rather than jam. Add more if a void fill or a foam pad goes in with them.
- Measure the stack height for the depth. If it comes out over about 2 inches, you are specifying a shipper, not a flat.
- Write it as internal length x width x depth, and say the word internal. Most disputes on a first order come from one side quoting internal and the other reading external.
Board and thickness, once the size is settled
Flat mailers are built either from folding carton board, measured in points where one point is one thousandth of an inch, or from E flute corrugated at about 1/16 inch thick. Carton board gives a cleaner printed surface for photography and fine type. E flute adds a crush resistant layer for anything with a rigid edge that can punch through in transit. If the mailer carries a print or a garment and is going into a poly bag or a pouch downstream, carton board is usually enough. If it travels loose through a parcel network, ask for the corrugated version.
Ordering, minimums and turnaround
The minimum order is 100 boxes. Pricing starts from about $0.48 per unit at bulk volumes and is confirmed on your written quote, because size, board, print coverage and finish all move the number. The die, the plates, design support, US shipping and a digital proof are included, which is why a genuinely custom internal size costs nothing extra to tool.
Turnaround is quoted per product across three lanes: rush (expedited production plus air freight, at a premium of about 7 to 25% depending on the job), standard (air) and cost saver (sea). The floor is from about 7 business days, plus 2 to 5 days for delivery. Specialty imported items can run up to about 12 weeks. Production runs from two plants, West Chicago in the US and Guangzhou in China.
If you have the contents in front of you, send the internal length, width and depth and we will confirm the board and the lane on a written quote. Start at /quote/, or look at the stock format on mailer boxes first if you would rather see the style before you measure.
