The standard measurement for a business-sized envelope is 4 ⅛ inches by 9 ? inches. It is designed to hold a tri-folded standard 8 ?-inch by 11-inch letterhead or typing paper. Business-sized envelopes are also referred to as number 10 envelopes.
A standard business-sized envelope is wide enough to accommodate up to five tri-folded sheets of standard letter-sized paper while remaining relatively flat for easy mailing. While it can generally fit a few more sheets or small inserts, overstuffed envelopes risk damage in modern postal sorting machines. For larger mail inserts, it is better to use large, flat side-flap envelopes that accommodate half-folded or non-folded letterhead in large quantities.