Match a square insert with real clearance
A 5 × 5 inch insert cannot have positive per-edge clearance inside a nominal 5 × 5 inch envelope. Choose a larger format and account for thickness, seams and embellishments.
Shape reference
Square envelope sizes
Common US square formats include 5, 5 1/2, 6, 6 1/2 and 7 inch squares. A square name describes the nominal face, not usable space or postal treatment.
Nominal size reference
Compare the recorded formats
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 |
|---|---|---|---|---|
| 5 × 5 squareUS square | 5 × 5 in5 × 5 in decimal | 127 × 127 mm | Neenah Paper ↗27 August 2026 | open side |
| 5 1/2 × 5 1/2 squareUS square | 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 squareUS square | 6 × 6 in6 × 6 in decimal | 152.4 × 152.4 mm | Neenah Paper ↗27 August 2026 | open side |
| 6 1/2 × 6 1/2 squareUS square | 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 squareUS square | 7 × 7 in7 × 7 in decimal | 177.8 × 177.8 mm | Neenah Paper ↗27 August 2026 | open side |
Postal treatment can differ
USPS currently explains that square letter-size pieces can be nonmachinable. Rules and prices can change, so this page links to current official guidance rather than reproducing a permanent postal rule.
- Check the destination operator.
- Check the current service and thickness rules.
- Do not infer postage from size alone.
Reference boundary
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.
Current postal guidance
Check the live USPS rules
Postal dimensions, machinability and prices can change and depend on the complete mailpiece.