Envelope · US standard
US #10 envelope
The US #10 envelope is the North American business standard: 4.125 × 9.5 inches (104.8 × 241.3 mm). It is the equivalent of DL in role — built to hold a US Letter sheet folded into thirds — though the underlying dimensions are quite different.
Exact dimensions
US #10 is defined in inches. The millimetre values are derived. Inside the envelope, an 8.5 × 3.67 inch tri-folded letter has about 1 inch of horizontal clearance and 0.45 inch vertical clearance — comfortable but tight.
| Unit | Width | Height |
|---|---|---|
| Millimetres (mm) | 104.8 | 241.3 |
| Centimetres (cm) | 10.48 | 24.13 |
| Inches (in) | 4.126 | 9.5 |
US #10 — 9.5 × 4.125 in
Typical contents
- · US Letter business correspondence
- · Invoices and statements
- · Cover letters with a small enclosure
- · Direct marketing and mailshots
- · Cheques and remittance slips
Window envelopes for #10
Most US #10 envelopes are produced in three variants: plain, single window (address only), and double window (address plus return). The standard window position aligns with the address block on a tri-folded Letter sheet — roughly 4.125 inches from the top of the letter and 0.875 inches from the left edge.
Printing & mailing notes
USPS automation reads the lower band of the front — keep roughly the bottom ⅝ in clear of design elements so the barcode has a home. The tri-fold that fills a #10: Letter folded at 3⅔ in intervals gives 8.5 × 3.67 in panels, dropping into the 9.5 × 4.125 in pocket with comfortable clearance. Home printers take #10 through the manual slot; feed singly and verify flap orientation with one test envelope before a batch.
Frequently asked questions
What does "#10" mean?
The number is a traditional US envelope-size code. "#10" specifically refers to a 4.125 × 9.5 inch envelope — long enough to hold a Letter-size sheet folded into thirds. The numbering does not follow any dimensional logic; #9, #10 and #11 are sequential codes, not sizes.
What fits inside a US #10 envelope?
A US Letter sheet (8.5 × 11 in) folded into thirds gives a strip about 8.5 × 3.67 inches, which slides comfortably into the 9.5 × 4.125 inch envelope opening.
How do #10 and DL compare?
US #10 is longer (241.3 mm vs 220 mm for DL) and narrower (104.8 mm vs 110 mm). Despite the different dimensions, both serve the same purpose: business letters folded into thirds. A #10 envelope is too narrow for a folded A4 sheet (which is ~210 mm wide); a DL envelope is too short for a folded Letter strip.
Can I use a US #10 for international mail?
Yes, but be aware that postal sorting equipment in some countries is tuned for local envelope sizes. International rates may also be higher for non-standard dimensions in destination countries.
What paper weight is standard for US #10?
20–24 lb bond is standard for office US #10 envelopes (roughly 75–90 gsm). Premium business stationery sometimes uses 28 lb (~105 gsm) for a slightly more substantial feel.
Where is the window on a standard #10 envelope?
The common US convention is a 1⅛ × 4½ in window placed ⅞ in from the left edge and ½ in up from the bottom — sized so that a Letter sheet tri-folded with standard 1 in margins shows its address block through the glass. Manufacturers vary slightly, so confirm against your actual stock before a mail merge.
What is the difference between #10 and DL?
Different continents, same job. DL (220 × 110 mm) is shorter and taller; #10 (241.3 × 104.8 mm) is longer and slimmer. A US Letter tri-fold technically slips into a DL with about 2 mm of side clearance — too tight for practical stuffing — so each region’s standard fold pairs with its own envelope.
Does Legal paper fit in a #10?
Not tri-folded: 14 ÷ 3 gives 4.67 in panels, taller than the 4.125 in opening. Fold Legal in four (3.5 in panels) and it fits with room, or step up to a #14 envelope (5 × 11.5 in) which takes the Legal tri-fold.
What is a #9 envelope?
The classic reply envelope: 8.875 × 3.875 in, sized to nest inside a filled #10. Statement and donation mailings pair the two — the outgoing #10 carries the letter plus an unfolded #9 for the response.