A flat mailer is easy to get almost right and annoying to get exactly right. The length and width are usually obvious from the contents. The thickness is where the money is, because thickness drives both the board choice and the rate band the finished parcel lands in.
Step one: measure the contents flat
Lay everything that goes in the pack on a table in the arrangement it will ship: the garment folded as fulfilment folds it, the prints in their sleeve, the card, the tissue, the returns slip. Measure the footprint of the stack, then measure its height under light pressure. If your contents are paper based, these published sizes are the ones to design around:
| Content | Size |
|---|---|
| US letter | 8.5 x 11 inches |
| US legal | 8.5 x 14 inches |
| ISO A4 | 210 x 297 mm, about 8.27 x 11.69 inches |
| ISO A5 | 148 x 210 mm, about 5.83 x 8.27 inches |
Treat those as the contents, not the mailer. The mailer has to be larger, and how much larger depends on whether the contents need to slide in easily or sit snugly.
Step two: allow for thickness creep
Flat packs rarely stay flat. A single folded tee is thin. The same tee with a tissue wrap, a sticker, a card and a poly bag is meaningfully thicker, and a second tee roughly doubles it. Decide now whether the mailer has to handle a one item order or a three item order, because a mailer that only works at one unit will be replaced within a season. If you need both, it is usually cheaper to run two sizes than to run one oversized mailer on every order.
Step three: choose the board
- Folding carton board for documents, prints and anything light. Prints beautifully, folds crisply, offers little crush protection.
- E flute corrugated, about 1/16 inch caliper, for anything that needs to arrive square. This is the usual choice for a printed ecommerce mailer.
- B flute, about 1/8 inch, when the contents are heavier or the transit is rough. Slightly less crisp on fine print, noticeably stronger.
- Rigid mailer for framed or fragile flat goods, where the pack has to resist bending outright.
Step four: confirm internal against external
State your dimensions as internal, length x width x depth, and confirm on the quote that the number you have been given is the internal one. On a thin mailer the difference is small, but the carrier measures the outside, and a pack that is one increment over a thickness threshold costs more on every order forever.
Ordering
The minimum is 100 mailers. Pricing starts from about $0.48 per unit at bulk volumes, confirmed on your written quote. Turnaround is quoted per product in three lanes: rush, which is expedited production plus air freight, standard on air, and cost saver on sea, from about 7 business days in production plus 2 to 5 days for delivery. The cutting die, the plates, design support, US shipping and a digital proof come with the order, so a size cut to your product beats a stock size that nearly fits.
If you would rather start from the question than the method, what size flat mailer do I need works through common scenarios. To order, see mailer boxes, or send your stacked contents measurement with a quote request.
