Business envelopes are most commonly referred to by their numbers, such as #9 or #10 envelopes. Below are the most common envelope sizes:

Size   Width x Length
#6 1/4 3 1/2″ x 6″
#6 3/4 3 5/8″ x 6 1/2″
#7 3 3/4″ x 6 3/4″
#7 3/4 3 7/8″ x 7 1/2″
#8 5/8 3 5/8″ x 8 5/8″
#9 3 7/8″ x 8 7/8″
#10 4 1/8″ x 9 1/2″
#11 4 1/2″ x 10 3/8″
#12 4 3/4″ x 11″
#14 5″ x 11 1/2″

For more information about envelope sizing, check out “PIP Metro Indy’s Ultimate Envelope Guide” here.