Complete reference index
Envelope sizes and dimensions
Compare common international and US formats in millimetres and inches, then open the family or individual reference for naming, folds, practical clearance and sources.
InternationalC-series and DL
Paper relationships and withdrawn-standard context.
United StatesCommercial envelopes#6 3/4, #9, #10 and larger numbered formats.
United StatesAnnouncement envelopesA1/4-bar, A2, A6, A7 and A9 with the regional warning.
Opening sideCatalog and bookletCommon 9 × 12 and 10 × 13 inch choices.
ShapeSquare envelopesCommon face sizes plus current postal-policy link-out.
International
International formats
| Format | Native size (width × height) | Converted size (width × height) | Source checked | Common relationship / opening |
|---|---|---|---|---|
| C3Common internationally | 324 × 458 mm | 12.756 × 18.031 in | Poczta Polska ↗27 August 2026 | A3 paper flat |
| C4Common internationally | 229 × 324 mm | 9.016 × 12.756 in | Envelope Manufacturers Association ↗27 August 2026 | A4 paper flat |
| C5Common internationally | 162 × 229 mm | 6.378 × 9.016 in | Envelope Manufacturers Association ↗27 August 2026 | A5 paper flat |
| C6Common internationally | 114 × 162 mm | 4.488 × 6.378 in | Envelope Manufacturers Association ↗27 August 2026 | A6 paper flat |
| C7Common internationally | 81 × 114 mm | 3.189 × 4.488 in | Envelope Manufacturers Association ↗27 August 2026 | A7 paper flat |
| DLCommon in the UK and Europe | 110 × 220 mm | 4.331 × 8.661 in | Envelope Manufacturers Association ↗27 August 2026 | A4 paper folded into thirds |
US commercial
US commercial formats
| Format | Native size (width × height) | Converted size (width × height) | Source checked | Common relationship / opening |
|---|---|---|---|---|
| #6 3/4United States | 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 |
| #9United States | 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 |
| #10United States | 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 |
| #11United States | 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 |
| #12United States | 4 3/4 × 11 in4.75 × 11 in decimal | 120.65 × 279.4 mm | Envelope Manufacturers Association ↗27 August 2026 | open side |
| #14United States | 5 × 11 1/2 in5 × 11.5 in decimal | 127 × 292.1 mm | Envelope Manufacturers Association ↗27 August 2026 | open side |
US announcement
US announcement formats
| Format | Native size (width × height) | Converted size (width × height) | Source checked | Common relationship / opening |
|---|---|---|---|---|
| A1 / 4-barUnited States | 3 5/8 × 5 1/8 in3.625 × 5.125 in decimal | 92.07 × 130.17 mm | Neenah Paper ↗27 August 2026 | open side |
| A2 announcementUnited States | 4 3/8 × 5 3/4 in4.375 × 5.75 in decimal | 111.13 × 146.05 mm | Neenah Paper ↗27 August 2026 | open side |
| A6 announcementUnited States | 4 3/4 × 6 1/2 in4.75 × 6.5 in decimal | 120.65 × 165.1 mm | Neenah Paper ↗27 August 2026 | 4 1/2 × 6 1/4 inch card |
| A7 invitationUnited States | 5 1/4 × 7 1/4 in5.25 × 7.25 in decimal | 133.35 × 184.15 mm | Neenah Paper ↗27 August 2026 | 5 × 7 inch card or photo |
| A9 announcementUnited States | 5 3/4 × 8 3/4 in5.75 × 8.75 in decimal | 146.05 × 222.25 mm | Neenah Paper ↗27 August 2026 | open side |
US square
US square formats
| Format | Native size (width × height) | Converted size (width × height) | Source checked | Common relationship / opening |
|---|---|---|---|---|
| 5 × 5 squareUnited States | 5 × 5 in5 × 5 in decimal | 127 × 127 mm | Neenah Paper ↗27 August 2026 | open side |
| 5 1/2 × 5 1/2 squareUnited States | 5 1/2 × 5 1/2 in5.5 × 5.5 in decimal | 139.7 × 139.7 mm | Neenah Paper ↗27 August 2026 | open side |
| 6 × 6 squareUnited States | 6 × 6 in6 × 6 in decimal | 152.4 × 152.4 mm | Neenah Paper ↗27 August 2026 | open side |
| 6 1/2 × 6 1/2 squareUnited States | 6 1/2 × 6 1/2 in6.5 × 6.5 in decimal | 165.1 × 165.1 mm | Neenah Paper ↗27 August 2026 | open side |
| 7 × 7 squareUnited States | 7 × 7 in7 × 7 in decimal | 177.8 × 177.8 mm | Neenah Paper ↗27 August 2026 | open side |
US booklet
US booklet formats
| Format | Native size (width × height) | Converted size (width × height) | Source checked | Common relationship / opening |
|---|---|---|---|---|
| 9 × 12 bookletUnited States | 9 × 12 in9 × 12 in decimal | 228.6 × 304.8 mm | Neenah Paper ↗27 August 2026 | US Letter paper flat with generous nominal space |
| 10 × 13 bookletUnited States | 10 × 13 in10 × 13 in decimal | 254 × 330.2 mm | Neenah Paper ↗27 August 2026 | open side |
US catalog
US catalog formats
| Format | Native size (width × height) | Converted size (width × height) | Source checked | Common relationship / opening |
|---|---|---|---|---|
| 9 × 12 catalogUnited States | 9 × 12 in9 × 12 in decimal | 228.6 × 304.8 mm | Neenah Paper ↗27 August 2026 | US Letter paper flat with generous nominal space |
| 10 × 13 catalogUnited States | 10 × 13 in10 × 13 in decimal | 254 × 330.2 mm | Neenah Paper ↗27 August 2026 | open end |
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Use the matcher as a supporting check
It tests both orientations, applies clearance per edge and separates comfortable, borderline and failed candidates.