Folds and layers
What may fit after a named fold
A folded reply piece may fit dimensionally, but window position, return flap, inserted payment documents and machine handling can matter more than face size alone.
US commercial · United States
#9 envelope size
3 7/8 × 8 7/8 in · converted: 98.42 × 225.42 mm
Exact decimal notation: 3.875 × 8.875 in decimal.
#9 is a US commercial envelope commonly associated with reply use and nesting inside a #10 outer envelope. Actual remittance and window constructions need their own product checks.
- Native dimensions
- 3 7/8 × 8 7/8 in
- Decimal inches
- 3.875 × 8.875 in decimal
- Converted display
- 98.42 × 225.42 mm
- Opening
- open side
- Aliases
- No. 9, Number 9, Commercial #9
What commonly fits flat
Start with the finished insert
Its nominal 3 7/8 × 8 7/8 inch face is smaller than a #10 in both dimensions. It is not a flat Letter-paper envelope.
- Reply envelope commonly nested inside a #10
Nominal exterior size ≠ usable interior space
Seams, lining, stock thickness, contents and manufacturing tolerance can reduce practical room. Confirm the manufactured envelope and sample before bulk use.
Decision
When to choose #9
Choose #9 when a compact reply envelope is specified and verified to nest within the selected outer. Choose #10 for the primary outbound Letter tri-fold.
Nearest useful alternative
#9 compared with #10
#10 adds 1/4 inch in width and 5/8 inch in length and is the familiar outbound commercial format.
The overlay is proportional to the canonical nominal dimensions. It compares the face rectangles only; it does not claim equal construction, internal capacity or postal treatment.
Clearance and tolerance
Why a positive number can still be too tight
Matcher clearance is applied per edge. Its “remaining” value is the total unused width and height after that allowance. A technically passing result with less than 2 mm remaining on either final dimension is labelled borderline and is never ranked as an ordinary recommendation.
Standards and rights note
US trade size name; not an ISO A-series paper designation and not a postal-eligibility claim.
Minimum factual dimensions independently recorded from trade and manufacturer material. Source presentation, prose, diagrams, marks and dielines are not reproduced.