What is the size and use of a number 10 envelope?

Number 10 envelopes are frequently used to enclose advertisements, business letters, or billing statements. It is a standard business-sized envelope that measures 4 1/8 inches by 9 1/2 inches.

Choices in paper weight, color, and flap style create diversity in the number 10 envelope. Flap styles include side seam flaps, square flaps, policy flaps, and commercial flaps. Some envelopes have open windows on the front whereas others are completely solid. Standard white is the most common color, although other colors are readily available. Pre-printed and stamped number 10 envelopes with coordinating printed seals are available from the United States Postal Service.