Why category matters
USPS prices by shape. The same artwork can price as a postcard, a letter, or a flat depending on its finished dimensions, and each step up costs more. A piece that busts a category maximum by a fraction of an inch jumps to the next, higher price. Confirm the finished size before printing.
What size can I mail as a postcard?
To qualify for the First-Class postcard (card) price, a piece must be rectangular and within:
- Minimum: 3.5 inches high, 5 inches long, 0.007 inch thick
- Maximum (retail / single-piece card price): 4.25 inches high, 6 inches long, 0.016 inch thick
- Maximum (commercial / presorted and automation card price, effective July 12, 2026): up to 6 inches high by 9 inches long, with the thickness requirements USPS sets for the larger card (thicker stock than the retail card minimum)
Outside the applicable limit a piece is charged as a letter, not a postcard. Retail: a 5 x 7 or 6 x 9 "postcard" exceeds the retail card maximum and prices as a letter. Commercial mailings after July 12, 2026 can run 5 x 7, 6 x 9 and other sizes up to 6 x 9 at the commercial card price when they meet the thickness rule. See DMM 101 and the July 2026 price change notice.
What size is a letter for USPS pricing?
- Minimum: 3.5 inches high, 5 inches long, 0.007 inch thick
- Maximum: 6.125 inches high, 11.5 inches long, 0.25 inch thick
A machinable First-Class letter weighs up to 3.3 ounces; a nonmachinable letter up to 3.5 ounces. Letters exceeding any maximum become flats. See DMM 201.
Why would a letter be nonmachinable?
A letter-size piece is nonmachinable, and pays a nonmachinable surcharge, if it has any of these (card-price pieces are exempt):
- Aspect ratio (length divided by height) less than 1.3 or more than 2.5 (a square piece has a ratio of 1 and is always nonmachinable)
- Rigid, or contains clasps, strings, buttons, or similar closures
- Too floppy: more than 4.25 inches high or more than 6 inches long and less than 0.009 inch thick
- Uneven thickness, or contents that shift
See DMM 201.5 for the full list. The surcharge amount is in Notice 123.
What size is a flat?
A flat exceeds a letter maximum in at least one dimension and is within:
- Maximum: 12 inches high, 15 inches long, 0.75 inch thick
A flat-size piece that is rigid, nonrectangular, or not uniformly thick (varies more than 0.25 inch) is charged as a parcel, based on weight. See DMM 201.
Parcel
A piece that exceeds flat maximums, or a flat-size piece that is rigid, nonrectangular, or unevenly thick. Priced by weight. See DMM 201 and 202.
Is my piece a postcard, letter, flat, or parcel?
Measure the finished piece. If it is within card limits and rectangular, it can price as a postcard. If it is within letter limits, it is a letter (watch the nonmachinable list). If it exceeds a letter maximum, it is a flat. If it is rigid, nonrectangular, or lumpy, expect parcel pricing. Final category and price are confirmed by USPS at acceptance; use the Postage Estimator for the figure.