Direct Mail Formats
and USPS Size Rules.

Why size rules exist at all

USPS automated sorting equipment is built to handle mail within specific size, thickness and rigidity ranges. A piece outside those ranges cannot run through the machinery and has to be handled by a person instead, which costs more — so USPS prices pieces outside the standard ranges at a higher rate or adds a nonmachinable surcharge. The size rules are not arbitrary; they are a direct reflection of what the sorting equipment can physically process.

Letter-size mail

To qualify for letter-size pricing (the least expensive structural category), a mail piece generally needs to fall between 3.5″ × 5″ and 6.125″ × 11.5″, be between 0.007″ and 0.25″ thick, and — a rule that surprises a lot of first-time mailers — have a length-to-height ratio between roughly 1.3 and 2.5. A nearly square piece, even if it is well within the size limits, can fail that aspect-ratio test and get bumped into the more expensive flat category (pieces up to roughly 12″ × 15″, allowed to be thicker) or picked up as nonmachinable.

Postcards specifically

The First-Class postcard rate — cheaper than the First-Class letter rate — has its own, tighter size window: generally 3.5″ × 5″ minimum up to 4.25″ × 6″ maximum, with a thickness between 0.007″ and 0.016″. A postcard designed even slightly larger than 4.25″ × 6″ does not get a smaller postcard rate at a slight penalty — it moves to the full First-Class letter rate, a meaningfully bigger jump than the size difference suggests. See the USPS mail-piece size reference to check where a specific piece falls.

Folded, unenveloped mail

A self-mailer — a folded piece with no envelope — has to be sealed shut on its open edges before it can run through sorting equipment; an unsealed folded flap can catch and jam the machinery. That sealing is usually done with a small clear adhesive dot called a wafer seal, applied to the open edge. It is a structural requirement of mailing a folded piece, not an optional finishing touch.

What this means when choosing a format

• Decide the mail class and structural category (postcard, letter or flat) before finalizing the size — not after the design is done.
• Check the aspect ratio, not just the height and width against the maximum and minimum.
• If the piece folds, plan for it to be sealed before it goes into the mail stream.
• A piece slightly over a size threshold rarely costs slightly more — it usually jumps to the next pricing category entirely.

Getting the postage class right at the design stage is worth as much attention as the printing itself, since postage on any mailing of size is usually the larger line item on the job — the products that go out this way are printed here first. See direct mail and presorted mail explained for how the rate is set once the size question is settled.