Square envelope sizes and postage
Common US square envelope sizes run from Square 5 × 5 in to Square 12 × 12 in, and the usual size for cards and invitations is the Square 6 × 6 in at 152.4 × 152.4 mm (6 × 6 in). In the US a square letter can't go through the sorting machines, so it costs more to mail than a rectangular letter of the same weight.
- Size
- 152.4 × 152.4 mm (6 × 6 in)
- Printing
- Fits most home printers; the icon on the paper tray shows which way round to feed it.
- Also called
- square 6
What a square envelope is for and what fits inside
The Square 6 × 6 in envelope measures 152.4 × 152.4 mm (6 × 6 in), and it's the square size most greeting cards, invitations and photo cards are made for. A square envelope looks deliberate and stands out in a pile of rectangular mail, which is why people choose it for wedding invitations, holiday cards and art prints.
Square cards are usually cut about a quarter inch smaller than their envelope, so the card slides in without bending its corners. If you're sending a letter instead, US Letter folded in half twice (4¼ × 5½ in) fits the Square 6 × 6 in and larger, while A4 folded the same way (105 × 148.5 mm) only just fits it and sits better in the Square 6½ × 6½ in. A letter folded only once, 8½ × 5½ in for US Letter or 210 × 148.5 mm for A4, needs the Square 9 × 9 in or larger.
The common square sizes compared
Common square sizes include the ones below, from note cards to prints and albums that shouldn't be folded.
| Envelope | Size | Typical use |
|---|---|---|
| Square 5 × 5 in | 127 × 127 mm (5 × 5 in) | Small thank-you notes and gift cards |
| Square 5½ × 5½ in | 139.7 × 139.7 mm (5½ × 5½ in) | Compact greeting cards and note cards |
| Square 6 × 6 in | 152.4 × 152.4 mm (6 × 6 in) | Most square greeting cards, invitations and photo cards |
| Square 6½ × 6½ in | 165.1 × 165.1 mm (6½ × 6½ in) | Layered invitations, cards with a ribbon or insert, a 4 × 6 photo |
| Square 7 × 7 in | 177.8 × 177.8 mm (7 × 7 in) | Large cards, art prints, invitations with several inserts |
| Square 7½ × 7½ in | 190.5 × 190.5 mm (7½ × 7½ in) | Large invitations and photo cards with a mat or insert |
| Square 8 × 8 in | 203.2 × 203.2 mm (8 × 8 in) | Large greeting cards and small art prints |
| Square 8½ × 8½ in | 215.9 × 215.9 mm (8½ × 8½ in) | Art prints and photos |
| Square 9 × 9 in | 228.6 × 228.6 mm (9 × 9 in) | Brochures, booklets and letters folded once |
| Square 10 × 10 in | 254 × 254 mm (10 × 10 in) | Larger prints, brochures and booklets |
| Square 12 × 12 in | 304.8 × 304.8 mm (12 × 12 in) | Large art prints and albums |
When a card falls between two sizes, go up rather than down, because in a tight envelope the corners crease and the flap won't lie flat. If your card is a 5 × 7 rectangle, a square envelope only wastes space, and the A7 envelope for 5 × 7 cards is the better fit.
Why square envelopes cost more to mail
USPS sorting machines are built for envelopes clearly longer than they are tall, so a square letter is handled by hand and pays a nonmachinable surcharge on top of the letter price. That's why one Forever® stamp won't cover even a single light card in a Square 6 × 6 in envelope. USPS takes letters within these limits: Up to 11½ × 6⅛ in, ¼ in thick, 3.5 oz. The Square 5 × 5 in, Square 5½ × 5½ in and Square 6 × 6 in envelopes fit within the length and height limits, so as long as the filled envelope also stays within that thickness and weight, they go as letters with the surcharge added; a thicker or heavier one goes as a large envelope.
From the Square 6½ × 6½ in up, every square is taller than a letter may be, so it's mailed as a large envelope, also called a flat, which costs more. Every square up to the Square 12 × 12 in fits within the flat limits: Bigger than a letter, up to 15 × 12 in, ¾ in thick, 13 oz; must be flexible and evenly thick.
In the UK, Royal Mail's limits for a Letter are these: Up to 24 × 16.5 cm, 5 mm thick, 100 g. So a Square 6 × 6 in card can go as a Letter if the filled envelope also stays within that thickness and weight. A thicker or heavier one, and any larger square, is a Large Letter if it fits this limit: Up to 35.3 × 25 cm, 2.5 cm thick, 750 g. That covers squares up to the Square 9 × 9 in, but the Square 10 × 10 in and Square 12 × 12 in are too wide and need a parcel service.
Here is how that works out for a typical wedding invitation in a Square 6 × 6 in envelope, with an RSVP card and a small reply envelope inside. The filled envelope weighs about an ounce (around 30 g), often a little more, and stays well within both countries' thickness limits, so in the US it goes as a letter at the price for one or two ounces plus the nonmachinable surcharge, and in the UK it goes as a Letter. Put the same suite in a Square 6½ × 6½ in envelope and it becomes a flat in the US and a Large Letter in the UK, both of which cost more. Weigh one finished invitation and work out the stamps with the stamp calculator before you buy them for a whole batch.
Printing the address on a square envelope
Squares up to the Square 8½ × 8½ in fit the Letter width most home printers take. A printer that takes A3 or 11 × 17 paper also fits the Square 9 × 9 in and Square 10 × 10 in, but the Square 12 × 12 in is wider than either. The trap specific to square envelopes is orientation: the envelope fits the tray whichever edge goes in first, so a wrong guess prints the address sideways or upside down. To get it right, print the design once on plain paper cut to the same square, mark which edge went into the printer first and which side faced up, and then load the envelope the same way, keeping in mind that the flap edge is the top of the address side. Slide the paper guides in to the envelope's width so it feeds straight.
If your printer won't take your size, as with the Square 9 × 9 in and larger on a Letter-width printer, print address labels on a standard sheet instead. For a whole guest list, a free account lets you import addresses from Excel or a CSV file and print one envelope or label per address. For test prints and feeding a batch, see how to print envelopes.
Print the addresses instead of writing them
The free label editor opens at the size of your envelope (Square 6 × 6 in) with a sender and a recipient block. Type one address and print it, or import your address list with a free account and print one envelope per address. It runs in your browser: nothing to install.
Print on this envelope →FAQ
What size envelope do I need for a 6 × 6 card?
A card that measures a full 6 inches on each side is too big for the Square 6 × 6 in envelope, which is made for cards about a quarter inch smaller. Use the Square 6½ × 6½ in instead, which leaves room for the card and a thin insert.
Do square envelopes get hand-canceled?
Because square letters can't run through the sorting machines, USPS handles them by hand, and that often means a hand cancellation rather than a machine postmark. If the postmark matters to you, for wedding invitations for example, take the envelopes to the counter and ask for hand-canceling. Either way, the higher postage for a square envelope still applies.
Can I mail a square envelope to another country?
Usually, but check the size first. USPS sets a longer minimum length for international letters than for domestic ones, so the Square 5 × 5 in is too short to go abroad as a letter; use the Square 5½ × 5½ in or larger. The Square 5½ × 5½ in and Square 6 × 6 in pay the nonmachinable surcharge abroad too, and the Square 6½ × 6½ in and bigger go as international large envelopes. The US stamp prices page lists the prices abroad and the surcharge.
Does a wax seal on a square envelope cost extra?
On a letter-size square such as the Square 6 × 6 in, a thin seal doesn't add a second surcharge, because the shape already makes the envelope nonmachinable and USPS charges that surcharge once per envelope. A seal can still raise the price, though: if it pushes the envelope past the letter thickness limit, it goes as a large envelope, and on the Square 6½ × 6½ in and Square 7 × 7 in, which already go as large envelopes, a lumpy seal can mean the package price. Putting a wax-sealed envelope inside a plain outer envelope protects the seal on its way.
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Updated 7 November 2026