A standard envelope is usually a #10 envelope, which measures 4 1/8 x 9 1/2 inches and is commonly used for business letters folded into thirds.

Common standard sizes

  • #10: 4 1/8 x 9 1/2 inches, for business correspondence.
  • #9: 3 7/8 x 8 7/8 inches, often used as a reply envelope.
  • 6 3/4: 3 5/8 x 6 1/2 inches, used for personal correspondence.
  • A2: 4 3/8 x 5 3/4 inches, often used for cards and invitations.

Practical note

If someone says “standard envelope” without more detail, they usually mean the #10 size in the U.S.. In other countries, common standard sizes can differ, such as DL or C-series formats.

TL;DR

For most U.S. mail, a standard envelope is #10: 4 1/8 x 9 1/2 inches.