What size envelope do I need?

It depends on what you're mailing. For an 8.5" x 11" letter folded in thirds, use a #10 envelope. To mail 8.5" x 11" documents unfolded, use a 9x12 envelope. For 5x7 invitations, use an A7. For 4x6 photos or cards, use an A6, and for 4.25" x 5.5" RSVP or thank-you cards, use an A2. See the size chart above for every option.