United States reference
The #10 is the familiar US business envelope at 4 1/8 × 9 1/2 inches. A #9 is smaller and is often used as a reply envelope inside a #10.
Nominal size reference
Every table uses nominal exterior width × height. International sizes lead with millimetres; US sizes lead with conventional fractional inches.
| Format | Native size (width × height) | Converted size (width × height) | Source checked | Context |
|---|---|---|---|---|
| #6 3/4US commercial | 3 5/8 × 6 1/2 in3.625 × 6.5 in decimal | 92.07 × 165.1 mm | Envelope Manufacturers Association ↗27 August 2026 | open side |
| #9US commercial | 3 7/8 × 8 7/8 in3.875 × 8.875 in decimal | 98.42 × 225.42 mm | Neenah Paper ↗27 August 2026 | Reply envelope commonly nested inside a #10 |
| #10US commercial | 4 1/8 × 9 1/2 in4.125 × 9.5 in decimal | 104.77 × 241.3 mm | Neenah Paper ↗27 August 2026 | US Letter paper folded into thirds |
| #11US commercial | 4 1/2 × 10 3/8 in4.5 × 10.375 in decimal | 114.3 × 263.52 mm | Envelope Manufacturers Association ↗27 August 2026 | open side |
| #12US commercial | 4 3/4 × 11 in4.75 × 11 in decimal | 120.65 × 279.4 mm | Envelope Manufacturers Association ↗27 August 2026 | open side |
| #14US commercial | 5 × 11 1/2 in5 × 11.5 in decimal | 127 × 292.1 mm | Envelope Manufacturers Association ↗27 August 2026 | open side |
The numbers do not convert into dimensions and should be paired with the measured size. Commercial envelopes are usually open side, with the opening along the longer edge.
Nominal envelope dimensions describe the exterior format, not guaranteed usable internal space. Seams, lining, stock thickness, multiple pieces and manufacturing tolerances reduce practical room; sample before a bulk order or print run.
Check the manufacturer specification when windows, machine insertion, multiple sheets or close tolerances matter.
EnvelopeLogic does not infer postage price or eligibility from the envelope name. Use the current postal operator guidance for the destination and mailing method.