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A postcard needs less postage than a standard letter, but only if the card fits the size box: rectangular, between about three and a half by five inches and four and a quarter by six inches, and thin enough to run through sorting machines. Oversized cards are charged at the letter rate instead.
One note before the details: LetterAgent prints full-size 8.5 by 11 letters, not postcards, so postcard rates don't apply to our service. The rules below cover postcards you mail yourself.
| Card size | What the post office calls it | Postage tier |
|---|---|---|
| Up to 4.25 x 6 inches | Postcard | The postcard rate, cheapest |
| Over 4.25 x 6, up to 6.125 x 11.5 inches | Letter | The letter rate, higher |
| Larger than 6.125 x 11.5 inches | Flat (large envelope) | The flat rate, highest of the three |
The window is tighter than most people expect. A card qualifies for the postcard rate only if it is rectangular, at least 3.5 inches high by 5 inches long, no bigger than 4.25 inches high by 6 inches long, and between 0.007 and 0.016 inches thick. That thickness range is roughly index-card stock: sturdy, but not a rigid board. The classic 4 by 6 photo card sits comfortably inside the box, which is why it's the default size for cheap postcard mailing.
There is also a shape rule most people never hear about. The card's length divided by its height has to fall between 1.3 and 2.5. A square card fails this test even if its sides measure within the limits, because square pieces jam the sorting machines the discount depends on. If your card is square, it mails at the letter rate no matter what.
Yes, but you'll overpay. The post office sells dedicated postcard stamps at a lower denomination than a standard Forever stamp, precisely because postcards cost less to send. A Forever stamp covers a full one-ounce letter, so sticking one on a postcard works fine, it just spends more postage than the card needed.
The trap to avoid is the reverse: putting a postcard stamp on a card that doesn't qualify for the postcard rate. If your 5 by 7 "postcard" is really a letter in the post office's eyes, a postcard stamp underpays it, and the card comes back to you or arrives postage-due. When in doubt, measure the card before choosing the stamp.
Size is the main trigger. The moment a card grows past 4.25 by 6 inches, it stops being a postcard and mails at the letter rate, even if everyone involved still calls it a postcard. Marketers do this on purpose all the time: 5 by 7, 6 by 9, and jumbo 6 by 11 cards are all "postcards" in casual speech and letters on the postage meter. The bigger card costs more to send but is harder to ignore, so it's a deliberate trade.
Thickness and rigidity matter too. A card thicker than the 0.016-inch ceiling, or stiff enough that it can't flex through the sorters, gets bumped up. So does anything non-rectangular. And a card mailed in an envelope is no longer a postcard at all: it's a letter, charged as one, which also means it gains the privacy of an envelope.
The domestic postcard rate doesn't cross the border. A postcard sent from the US to Canada needs full international letter postage, the same as a letter to Canada would. Canada Post's own postcard rules apply to cards mailed within Canada, and those have their own size and weight bands. If you're sending cards both ways across the border, check each country's rules separately rather than assuming one rate covers the trip.
Ready to mail it? Send your letter with LetterAgent: describe it to your AI assistant, approve the exact quoted price, and it is printed and mailed.
LetterAgent is a working name. Prices include print, postage, and our service fee. US prices in USD, Canadian prices in CAD (Canadian prices exclude applicable sales tax, added at checkout). Currently in private testing.
Need help? Ask your assistant: it can check a job's status any time. If it ever can't handle something: hello@getletteragent.com