Square envelope postage: why squares always cost more

The arithmetic, in one line

Aspect ratio is length divided by height. For a 5.5 x 5.5 in envelope that is 5.5 divided by 5.5, which is 1.0. For a 6 x 6 in envelope it is also 1.0. Change the size and the ratio does not move, because the definition of a square is that the two dimensions are equal.

The minimum for a machinable letter is 1.3. There is no square envelope that reaches it, at any size, on any paper, from any supplier.

Note the third row. A square larger than about 6.125 in on a side also breaks the letter height maximum, so it is not merely surcharged, it is no longer a letter at all.

Why the rule exists

Automated letter processing equipment feeds pieces on edge and needs to know which way round they are. A piece whose length and height are similar can enter the machinery in an orientation the equipment cannot resolve, which means the address may not be found and the piece may jam.

The aspect ratio window is a proxy for that. Outside 1.3 to 2.5 the piece is pulled out of the automated stream and handled differently, and the surcharge reflects the extra handling rather than the shape being disapproved of.

That also means the surcharge is not a penalty you can negotiate away with neat typography. It is a category, and the only way out of it is a different envelope shape.

The other triggers that usually travel with a square

Square envelopes are popular for invitations, and invitations tend to collect several nonmachinable characteristics at once. It is worth knowing that fixing the shape may not be enough on its own.

• Rigidity from a thick cotton or double thick card.
• Uneven thickness from a wax seal, a charm, a ribbon or confetti.
• Clasps, strings or ties.
• Total sealed thickness over 0.25 in from several enclosures.
• A hand addressed script that is hard for equipment to read.

If your suite has three of these, moving from a square to an A7 changes the shape problem and leaves the rest. Test the fully assembled, sealed piece rather than the envelope alone.

Rectangular alternatives that keep the feeling

The appeal of a square is that it feels ceremonial and unlike everyday mail. Two rectangular formats get most of the way there while staying inside the ratio window.

An A7 at 5.25 x 7.25 in has a ratio of 1.38, comfortably machinable, and holds a 5 x 7 in card. It is the standard invitation envelope for good reason: generous, formal, and it does not look like a bill.

An A9 at 5.75 x 8.75 in has a ratio of about 1.52 and holds a 5.5 x 8.5 in card. It is larger and more imposing than an A7 while staying a letter.

If the square proportion is genuinely the design, consider a square card inside a rectangular envelope. A 5 x 5 in card in an A7 envelope keeps the square object and mails at the machinable rate, with the card sitting slightly loose. Alternatively use a square flat card as an enclosure inside a rectangular outer envelope.

When to choose the square anyway

Sometimes the square is worth it, and that is a legitimate decision. For 120 wedding invitations the extra handling cost is small against the impression the envelope makes, and nobody remembers the postage.

The rule is to decide rather than discover. Price the surcharge before you order, include it in the budget line for the mailing, and assemble one complete sealed sample and take it to a Post Office so the actual rate is confirmed for your actual piece. Rates and surcharge amounts change, so confirm the current figure rather than relying on a number you read somewhere.

For a large commercial mailing the arithmetic is different. At several thousand pieces, a surcharge per piece becomes a meaningful line, and the rectangular alternative usually wins.

Do not forget the return path

If your mailing includes a reply envelope, the same rules apply to it. A square RSVP envelope inside a square outer envelope is surcharged twice, once on the way out and once on the way back, and your guests pay the second one if you have not stamped it.

A No. 9 envelope at 3.875 x 8.875 in is the conventional reply envelope because it fits inside a No. 10 and has a ratio of about 2.29. For invitation suites, an A2 at 5.75 x 4.375 in fits inside an A7 and has a ratio of 1.31, which is machinable with almost nothing to spare.

Square envelope checklist

• Accept that every square envelope is nonmachinable, at every size.
• Check the height too: a square over 6.125 in on a side is not a letter at all.
• Test the fully assembled sealed piece, because rigidity and lumps are separate triggers.
• Consider an A7 at 1.38 or an A9 at about 1.52 for a formal rectangular alternative.
• Consider a square card inside a rectangular envelope.
• Apply the same check to any reply envelope in the suite.
• Confirm the current rate with a sealed sample at a Post Office before ordering the run.