United States reference

US business and commercial envelope sizes

The standard #10 business envelope is 4 1/8 × 9 1/2 inches. A #9 envelope is 3 7/8 × 8 7/8 inches and is commonly used as a reply envelope inside a #10.

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
#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

Business envelope numbers are names, not measurements

The numbers do not convert into dimensions and should always be paired with the measured size. US commercial envelopes are usually open side, with the opening along the longer edge.

#6 3/4 is a compact commercial format.
#9 measures 3 7/8 × 8 7/8 inches and is commonly associated with reply use.
#10 measures 4 1/8 × 9 1/2 inches and commonly accepts US Letter paper tri-folded.
Larger #11, #12 and #14 formats provide progressively more nominal room.

A named size is not a product guarantee

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.

Postal questions remain live policy

EnvelopeLogic does not infer postage price or eligibility from the envelope name. Use the current postal operator guidance for the destination and mailing method.