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The smallest letter USPS will accept is 5 inches long, 3-1/2 inches high, and 0.007 inches thick, rectangular with square corners. Anything smaller or thinner gets rejected or charged as a parcel. That is about the size of a standard index card: a #10 envelope is well above it, and tiny gift-card envelopes often are not.
Because the machinery demands it. Sorting equipment moves letters at speed, pinching them between belts and rollers. A piece smaller than the minimum can jam the line, flip sideways, or shoot through unread. The thickness rule has the same logic: a flimsy single sheet of tissue-thin paper tears in the machinery, which is why there is a floor of 0.007 inches. The minimums exist to keep the whole automated system from choking on odd pieces.
Per the USPS Domestic Mail Manual, a mailpiece a quarter-inch thick or less must be at least 3-1/2 inches high and at least 5 inches long, and at least 0.007 inches thick. It must be rectangular with four square corners and parallel opposite sides. Postcards share the same minimum size; a standard 4x6 postcard clears it comfortably. For automated processing there is also a shape expectation: the length should be between 1.3 and 2.5 times the height, so a very long skinny strip or a near-square piece can still be kicked out of the automated stream.
| Piece | Minimum (USPS) | Common example |
|---|---|---|
| Letter | 5 in long, 3-1/2 in high, 0.007 in thick | Index card; anything smaller fails |
| Postcard | Same minimum as a letter | A standard 4x6 postcard clears it easily |
| #10 envelope | Well above the minimum | 4-1/8 x 9-1/2 in: the normal business envelope |
| Large envelope (flat) | 11-1/2 in long, 6-1/8 in high | Bigger pieces start a new size category |
Tiny gift-card sleeves and miniature greeting-card envelopes are the classic failures: cute, and too small to mail. Coin envelopes, key tags with a stamp stuck on, and folded note cards under the size floor all run into the same problem. The fix is usually just a bigger envelope: put the small item in a regulation-size envelope and mail that instead.
Round, triangular, or die-cut envelopes count as nonmachinable. The post office will still take most of them, but they get a nonmachinable surcharge and travel through manual handling, which is slower. Square envelopes fall into the same bucket: a square envelope cannot be read reliably by the facer-canceler orientation logic, so squares pay the surcharge too. If you want the cheapest, fastest path, stay rectangular.
Canada Post also sets minimum dimensions for Lettermail, and the smallest mailable pieces are in the same ballpark as the US ones: roughly index-card sized. The principle is identical (the sorting machines need something to grip), so the practical advice carries across the border: when in doubt, use a standard envelope size. Anything genuinely tiny goes inside a bigger envelope, not through the mail stream on its own.
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