Mailer Size Checker
Find out whether your mail piece qualifies for the cheap postcard rate, the letter rate, or a flat — based on the USPS size rules.
Enter your mail piece width and height in inches.
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Mailer size FAQ
Need this done for you? We handle Mailing Services for businesses across Buffalo and the Twin Cities.
USPS postcard rate applies to pieces from 3.5 x 5 inches up to 4.25 x 6 inches (within thickness limits). Stay inside that range for the lowest postage — popular for EDDM and quick promos.
It moves up to the First-Class letter rate, allowed up to 11.5 x 6.125 inches. Larger than that mails as a flat (large envelope) at a higher rate.
Size sets the rate category, but final postage also depends on weight, quantity, and how you mail (First-Class, marketing mail, or EDDM). Our mailing team handles the details.
How to tell if your mailer is a postcard, letter, or flat
The rule of thumb: keep your piece at 4.25" × 6" or smaller and it mails at the cheapest postcard rate; up to 6.125" × 11.5" it mails as a First-Class letter; anything bigger counts as a flat (large envelope) at a higher rate. There’s a floor, too — USPS won’t accept anything smaller than 3.5" × 5".
The checker sorts your two measurements into a long side and a short side, so orientation doesn’t matter, then tests them against the USPS thresholds in order: at least 3.5" × 5" to mail at all; postcard rate when the long side is 6" or less and the short side 4.25" or less; letter rate up to 11.5" long and 6.125" tall; flat beyond that.
Worked example: a popular 6" × 9" jumbo postcard. The long side is 9" and the short side 6". It clears the minimum, misses the postcard window (9" is longer than the 6" limit), but fits inside 11.5" × 6.125" — so the tool calls it Letter rate at 9" × 6". Weight and mailing method set the final postage; our mailing services team handles that, or see postcards & EDDM.
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