The short version
The postcard you buy in a souvenir rack is almost always one of two sizes: 4 by 6 inches in the United States, or A6 — 105 by 148 millimeters — in countries that use metric paper sizes. Both fit the postal rules that matter. A card that stays within its postal service's limits travels at the cheap postcard rate; a card that breaks them quietly becomes a letter and costs more.
That is the whole subject in one paragraph. The rest of this guide gives the exact numbers and the practical things to check.
Quick reference: the numbers
Keep this strip handy — it covers the three questions people actually ask:
What size is a postcard in inches? The US retail standard is 4 by 6 inches. For the cheaper USPS postcard rate a card must be at least 3-1/2 by 5 inches and no more than 6 by 4-1/4 inches, and no thicker than about 0.016 inches.
What size is a postcard in cm? 4 by 6 inches is 10.2 by 15.2 centimeters. The metric standard A6 is 10.5 by 14.8 centimeters — a hair taller and a little narrower, close enough that either works as a standard card.
What size is a postcard in mm? A6 is 105 by 148 millimeters. The US 4-by-6 is 102 by 152 millimeters. Both sit inside the size bands most postal services reserve for the postcard rate.
The USPS size limits
For the United States Postal Service, a card qualifies for the postcard rate only within set bounds. The minimum is 3-1/2 by 5 inches, and the maximum is 6 inches long by 4-1/4 inches high, with a thickness between 0.007 and 0.016 inches — thin enough to stay a card rather than a letter.
A standard 4-by-6 card sits comfortably inside those limits: its 4-inch height is below the 4-1/4-inch ceiling, and its 6-inch length meets the maximum exactly. Oversized, thicker, or heavier cards fall into letter pricing — the practical cost of getting the size wrong, explained further in our postage guide.
The sizes you actually buy
In the United States the retail standard is 4 by 6 inches — the size of nearly every card in this atlas and the one most racks are built around. The premium alternative is 5 by 7 inches, which travels as a letter because it exceeds the postcard maximum. Where metric sizing rules, the standard is A6 at 105 by 148 millimeters, a near-twin of 4 by 6; the two are close enough that either mails as a standard card in its home system.
The cards in this atlas — like those filed under the United States route — are a standard postcard size by design — sized to be mailed, not just framed.
Why size decides your postage
Postal services sort cards by machine, and the machines read size before anything else. A card within the postcard bounds takes the postcard rate because it runs through the card stream; anything larger must join the letter stream, which costs more. This is why the cost guide keeps repeating one piece of advice: check the size before you buy the stamp.
The good news is that the rules protect the classic card. The 4-by-6 format was not chosen at random — it was built to thread exactly between the minimum and the maximum, which is why it has been the default for generations.
Measuring your card in practice
A ruler settles every question in ten seconds. Lay the card flat and measure its longest side and its height; then check the thickness with your fingers — a single card is card-thin, while a folded or padded piece is not. If both dimensions sit between the minimum and maximum, and the card is a single flat piece, a postcard stamp is right.
Two edge cases trip people up. Odd shapes — round, die-cut, or folded cards — never qualify for the flat postcard rate, and cards with attached objects mail as parcels. When a card is unusual, the counter is the fastest answer.
International sizes
Beyond the United States, the practical standard is A6, at 105 by 148 millimeters, which is the size European souvenir racks and the universal postal union's guidelines revolve around. The United Kingdom mails postcards as standard letters within its own size bands, and Australia's postcard class accepts the common sizes without fuss.
Travelers rarely need the fine print. If the card came from a rack or an atlas like this one, it is a standard size by definition — and it will mail at the standard rate wherever the rules follow the international norms.
What is the standard postcard size?
The classic retail standard is 4 by 6 inches, with the metric standard A6 at 105 by 148 millimeters sitting close beside it. For the US postal postcard rate, a card must be between 3-1/2 by 5 inches minimum and 6 by 4-1/4 inches maximum.
What is the maximum size for postcard postage?
In the United States, a card may be at most 6 inches long and 4-1/4 inches high to travel at the postcard rate. Anything larger is processed and priced as a letter.
Is a 4-by-6 card a postcard?
Yes. A 4-by-6 card fits inside the USPS postcard limits (4 inches of height against a 4-1/4-inch maximum), which is exactly why 4 by 6 became the standard retail size.
What size is an A6 postcard?
A6 is 105 by 148 millimeters — about 4.1 by 5.8 inches — and it is the standard postcard size across most of the world outside the United States.