PrintMagic FAQ › #9 Envelope Printing FAQs › What size is a #9 envelope?What size is a #9 envelope?A #9 envelope measures 3 7/8" x 8 7/8" (3.875" x 8.875"). Shop Custom #9 Envelopes (3 7/8″ x 8 7/8″)Other questions in #9 Envelope Printing FAQsDoes a #9 envelope fit inside a #10 envelope?What is the difference between #9 and #10 envelopes?What is a #9 envelope used for?Will a folded 8.5" x 11" letter fit in a #9 envelope?Can I add a window to #9 envelopes?Can you print business reply (BRE) envelopes?How fast can I get custom #9 envelopes?Can you print on both sides of a #9 envelope?