The Aeron chair, recognized for its design by the Museum of Modern Art, is a 94 percent recyclable ergonomic office chair by Herman Miller. It features mesh fabric seat and seat back support rather than foam and upholstery, as well as completely customizable chair functions. Instead of an Aeron chair design serving in a one-size-fits-all capacity, it is sold in three sizes – "A" small, "B" medium and "C" large – to fit different body types. If you don't know your Aeron chair's size and it doesn't have a label, Herman Miller provides two ways for you to tell which size you have.
Turn the Aeron office chair around so you're looking at the seat back.
Place your fingertips under the frame edge near the top center of the seatback near the Herman Miller stamp. Press under the edge until you feel one or more bumps. Count the number of bumps with your fingertips to determine the chair size – one bump is an "A" size chair, two bumps a "B" size chair and three bumps organized in a triangle is a "C" size chair.
Face the front of the chair and run a measuring tape from one side of the seat frame to the other to measure the seat as another method of determining the chair size: 19 inches wide is an "A" chair, 20.25 inches wide a "B" chair and 21.5 inches wide a "C" chair.