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Envelope Sizes and Postage: A 2026 Size-to-Price Guide
Which envelope sizes mail for one Forever stamp, which need a flat rate, and which trigger the $0.49 nonmachinable surcharge. Sizes, prices, and a simple checklist.
Envelope size changes what you pay just as much as weight does. A standard #10 business envelope with one page mails for a single $0.82 Forever stamp, while the same page in a 9 x 12 envelope starts at $1.69 — and a square 5 x 5 invitation costs $1.31 even though it weighs almost nothing.
This guide maps the envelope sizes people actually use to the postage USPS charges in 2026.
The three price categories
| Category | Examples | Starting price (1 oz) |
| Letter | #10 business, A2, A6, A7, standard greeting card envelopes | $0.82 |
| Flat (large envelope) | 6 x 9, 9 x 12, manila envelopes, unfolded documents | $1.69 |
| Postcard | Standard 4 x 6 or 5 x 7 postcards | $0.65 |
A square, rigid, or lumpy envelope stays in the letter category for weight purposes but picks up a $0.49 nonmachinable surcharge, so a 1-ounce square letter costs $1.31.
Size limits that keep you at letter price
To qualify for the $0.82 letter rate, a mailpiece must be rectangular and stay within these dimensions:
| Dimension | Minimum | Maximum |
| Length | 5″ | 11-1/2″ |
| Height | 3-1/2″ | 6-1/8″ |
| Thickness | 0.007″ | 1/4″ |
Anything longer than 11-1/2″, taller than 6-1/8″, or thicker than 1/4″ moves to flat pricing. Flats can go up to 15″ long, 12″ high, and 3/4″ thick.
Common envelope sizes and what they cost
| Envelope | Typical size | Category | Typical postage |
| #10 business | 9-1/2″ x 4-1/8″ | Letter | $0.82 (1 oz) |
| #9 return envelope | 8-7/8″ x 3-7/8″ | Letter | $0.82 (1 oz) |
| A2 (response cards) | 4-3/8″ x 5-3/4″ | Letter | $0.82 (1 oz) |
| A6 (invitations) | 4-3/4″ x 6-1/2″ | Letter | $0.82 (1 oz) |
| A7 (5 x 7 photos) | 5-1/4″ x 7-1/4″ | Letter or nonmachinable | $0.82–$1.31 |
| A9 | 5-3/4″ x 8-3/4″ | Letter (check thickness) | $0.82–$1.31 |
| Square 5 x 5 or 6 x 6 | — | Nonmachinable letter | $1.31 (1 oz) |
| 6 x 9 booklet | 6″ x 9″ | Flat | $1.69 (1 oz) |
| 9 x 12 catalog | 9″ x 12″ | Flat | $1.69 (1 oz) |
Treat these as planning numbers, not guarantees. Two envelopes of the same nominal size can price differently depending on how thick and how flexible they are once sealed. USPS publishes its own size and shape requirements, and its guide to sending letters is worth skimming before a large mailing.
Why thickness matters as much as length
A 1/4″ thickness limit sounds generous until you put a folded card, a photo, and a response envelope inside. Once the envelope is lumpy or rigid — meaning it will not bend evenly through USPS sorting machinery — it becomes nonmachinable even if it is well under 1 ounce.
Wedding invitations are the classic case: the outer envelope may measure like a letter, but a thick inner card plus tissue plus a response card often pushes it past 1/4″ or makes it rigid. Weigh and bend-test one finished invitation before you buy postage for 150 of them. A single test envelope at the Post Office counter costs nothing and prevents 150 returned invitations.
A simple way to choose an envelope
- Can the contents fold? If yes, use a letter-size envelope and pay $0.82. If no, accept flat pricing at $1.69.
- Is the envelope square, rigid, or lumpy? Add $0.49 per piece, or switch to a rectangular envelope and skip the surcharge entirely.
- Is it thicker than 1/4″ once sealed? It is no longer a letter.
- Weigh one finished piece. Every size estimate in this guide is a shortcut for the scale.
Frequently asked questions
Do square envelopes need extra postage?
Yes. Square envelopes cannot run through USPS sorting equipment, so they carry the $0.49 nonmachinable surcharge. A 1-ounce square letter costs $1.31.
Is a 5 x 7 envelope a letter or a flat?
Usually a letter if it is rectangular and under 1/4″ thick — a 5 x 7 photo in an A7 envelope typically mails for one Forever stamp. Weigh it if it contains cardstock.
What size envelope needs two stamps?
Any letter over 1 ounce, or any flat (6 x 9 and larger). A 1-ounce 9 x 12 envelope costs $1.69, which is two Forever stamps ($1.64) plus a few cents, or exactly one flat-rate combination from the Post Office.
Does a padded mailer mail for one stamp?
No. Padded mailers are rigid and thick, so they are priced as packages, not letters, and ship under USPS Ground Advantage instead.
Match the envelope to the stamp you buy
Most households and small offices need mostly standard Forever stamps plus a smaller stack of additional-ounce stamps. For invitations and seasonal mail, browse wedding stamps — including the 2015 Yes, I Do issue — and holiday stamps such as Winter Berries. For size-specific guidance, read wedding invitation postage, Christmas card postage, and our explainer on the non-machinable surcharge.
Need the stamps? Browse our full shop of discount USPS Forever stamps.