The standard postcard size is 4 × 6 inches (102 × 152 mm) in the United States and A6, 105 × 148 mm, in most other countries. Both qualify for the cheaper US postcard rate, which covers cards from 3½ × 5 up to 4¼ × 6 inches, 0.007 to 0.016 inches thick. Bigger cards, such as 5 × 7 inches or A5, are posted at the letter rate.

The rest of this guide gives every common size in millimetres and inches, the size rules of the US, UK and international post, which envelope each card fits, and how much bleed a printer wants.

What is the standard postcard size?

There are two. In the US it is 4 × 6 inches, the largest card USPS carries at the postcard rate. Everywhere that uses metric paper it is A6: 105 × 148 mm, or about 4.1 × 5.8 inches, which papersizes.org lists as the international standard postcard. The two are only a few millimetres apart, so a card in either size posts as a postcard almost anywhere.

Here are the sizes people mean when they say "postcard", and how each one is treated by the post. "US postcard rate" is the USPS rule; "UPU postcard" is the international size range set by the Universal Postal Union; "Royal Mail letter" means it fits the UK's cheapest Letter format.

Size Millimetres Inches US postcard rate UPU postcard Royal Mail letter
A6 105 × 148 4.1 × 5.8 Yes Yes Yes
4 × 6 102 × 152 4 × 6 Yes, at the maximum Yes Yes
5 × 7 127 × 178 5 × 7 No, letter rate No, too tall Yes
A5 148 × 210 5.8 × 8.3 No, letter rate No, too tall Yes
DL 99 × 210 3.9 × 8.3 No, letter rate Yes Yes

A5 is A6 doubled: each A size is the one above it cut in half, which keeps the same 1 : 1.41 shape (papersizes.org). DL is a third of an A4 sheet, 99 × 210 mm (The Print Blog), and is the long, narrow card you see on racks and in direct mail.

What are the USPS postcard size limits?

To be posted at the US postcard rate, a card must be rectangular, at least 3½ inches high, 5 inches long and 0.007 inches thick, and no more than 4¼ inches high, 6 inches long and 0.016 inches thick. Its length divided by its height must fall between 1.3 and 2.5. All of this is set out in the USPS Domestic Mail Manual, section 201.

Three to-scale panels of the smallest and largest card each post allows: USPS postcard, UPU postcard, Royal Mail letter
The three rule books side by side, drawn to one scale, so you can see why a 5 × 7 card is a postcard in none of them.

In metric, the USPS window runs from 89 × 127 mm to 108 × 152 mm. Thickness is the one people forget: 0.016 inches is 0.41 mm. Papersizes.org puts the allowed range at roughly 145 to 390 gsm card stock (papersizes.org), so most printed postcards sit comfortably inside it, while a thick, laminated or layered card can tip over.

What happens if a postcard is bigger than the limit?

It still goes, but at the letter price. USPS says a card larger than 4¼ × 6 inches can be mailed as a letter-size piece at the higher rate (DMM 201). On the USPS price list that is the difference between a postcard stamp from $0.65 and a letter stamp from $0.82, so check the size before you stamp a 5 × 7 card.

Letter size has its own ceiling: up to 6⅛ inches high, 11½ inches long and ¼ inch thick, with the same 1.3 to 2.5 shape rule (DMM 201). A 5 × 7, A5 or DL card is well inside that. Any card over 4¼ × 6 inches must also be at least 0.009 inches thick, a little stiffer than the postcard minimum.

What size is a postcard in the UK and internationally?

The UK has no separate postcard size or price. Royal Mail's size and weight guide lists postcards under Letter: up to 24 × 16.5 cm, 5 mm thick and 100 g, and no smaller than 9 × 14 cm. Every size in the table above, including A5, fits as a Letter, and a stamp for a Letter starts at 91p for 2nd Class (Royal Mail).

Between countries, the rules come from the Universal Postal Union. Article 17-104 of its Convention Manual sets a postcard at no smaller than 90 × 140 mm and no larger than 120 × 235 mm, each with a 2 mm tolerance, and the card must be at least 1.41 times as long as it is high. Article 17-107 adds that a postcard travels without an envelope and that at least the right-hand half of the address side is kept for the address and stamp. A card that breaks these rules is treated as a letter.

Two things follow from that. A 5 × 7 card is 127 mm high, past the 120 mm ceiling, so it is never an international postcard. A DL card, at 99 × 210 mm, is. The same manual also lets each post office drop postcards as a separate price, which is why Royal Mail charges them as letters.

Some countries set one exact size for the official card. India Post's postcard must be 140 × 90 mm and weigh no more than 5 grams; a card that breaks its conditions is charged as a letter, with double the shortfall collected on delivery (India Post).

What envelope fits a postcard?

A postcard needs no envelope; it is sent bare. When you post a card in an envelope, it is a letter, and the usual pairings are a C6 envelope for A6, an A6 envelope for 4 × 6, an A7 envelope for 5 × 7, C5 for A5 and DL for DL.

Three cards drawn inside their envelopes to scale: A6 in a C6 envelope, 5 x 7 in an A7 envelope, DL in a DL envelope
How much room each card has in its envelope. A few millimetres a side is what makes a card slide in without bending.

Card Envelope Envelope size
A6, 105 × 148 mm C6 114 × 162 mm
4 × 6 in A6 4¾ × 6½ in
5 × 7 in A7 5¼ × 7¼ in
A5, 148 × 210 mm C5 162 × 229 mm
DL, 99 × 210 mm DL 110 × 220 mm

The C-series sizes, and the fact that C6 holds an unfolded A6 sheet and C5 an unfolded A5, are from papersizes.org. The US A-style envelope sizes are from French Paper, which describes the A6 envelope as made for 4 × 6 inch photographs.

These are the card sizes our pen writes on, straight from the catalogue our editor uses, with the envelopes in our range that each one fits. We allow 2 mm of clearance on every side so the card goes in without scraping:

Our card Size Fits our envelopes
A6 card (the default) 105 × 148 mm DL, A7
4×6 postcard 101.6 × 152.4 mm DL, A7
Thank-you 4.25×5.5 108 × 139.7 mm DL, A7
Compact card 107 × 139 mm DL, A7, A2
Note 6.25×4.25 107 × 158 mm DL, A7
Invitation 5×7 127 × 177.8 mm A7

One thing the table shows: a 4¼ × 5½ inch card is usually sold for an A2 envelope (4⅜ × 5¾ in), and it does go in, but with only about 1.5 mm to spare on each long edge. If you are hand-filling a stack of them, an A2 is tight; the slightly smaller compact card is the one that slips in easily.

How much bleed does a postcard need?

Add 0.125 inches, about 3 mm, on every side. Bleed is the extra margin of background printed past the cut line, so that a slightly off-centre cut never leaves a white sliver at the edge (Vistaprint). The printer trims it away; you never see it on the finished card.

For a 4 × 6 card, that means building the file at 4.25 × 6.25 inches and keeping words and logos inside a safe area of 3.75 × 5.75 inches, 0.125 inches in from the trim (4over4). In metric, an A6 design is set up at 111 × 154 mm with 3 mm bleed. Three rules cover almost every print job:

  • Bleed: run photos and background colours 3 mm (⅛ inch) past the edge.
  • Trim: the finished size, for example 105 × 148 mm.
  • Safe area: keep text at least 3 mm (⅛ inch) inside the trim. More is kinder; a phone number right at the edge looks like a mistake even when it is not cut.

Handwriting does not need bleed, because nothing runs off the edge. It needs a margin instead. Our pen writes at least 7 mm in from every edge of every card, the space taken by the tape or clips that hold the card flat while it writes. On a 4 × 6 card that leaves a writing area of about 88 × 138 mm, and on the back of a postcard you also lose the right half to the address. What to put in the half that is left is in our postcard message ideas.

Which postcard size should you choose?

Choose 4 × 6 inches if most cards go to US addresses and A6 if most go anywhere else. Both are postcards under the USPS, UPU and Royal Mail rules, so they post at the lowest price wherever a postcard price exists. Choose 5 × 7 or A5 only when you want more room and are happy to pay the letter price.

A few practical points help decide:

  • Writing a lot? A 5 × 7 card has about half as much area again as a 4 × 6. In an envelope it costs the letter price anyway, so the extra room is free.
  • Sending abroad bare? Stick to A6 or 4 × 6. A 5 × 7 card is over the UPU postcard height and is charged as a letter.
  • Marketing mail? A DL card fits a DL envelope and a leaflet rack, and travels as an international postcard, but in the US it pays the letter rate.
  • Address side. Whatever the size, the right half of the back is for the address and stamp. The line-by-line format is in how to address a postcard, and the message side in how to write a postcard.

If you send cards from a business, the For business page covers volume and pricing, and the developer API takes the card size on each order, so a shop can send A6 to one customer and 5 × 7 to another.

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