If you use armrests, look for a pair that are contoured and cushioned and have adjustable height. Otherwise you may end up with poor posture from leaning to one side, or with muscle tension from raising your shoulders.
When you sit, your soles should rest on the floor. The chair height should adjust for multiple users (or if you wear high heels and flats). If your feet dangle, it’s harder to maintain the S-shape of the spine and your posture could pay the price.
You should be able to adjust the backrest recline by at least 15 degrees for different sitting positions. Look for lumbar support to maintain your lower back’s curvature. (If the chair you want doesn’t offer lower-back support, you can use a pillow.)
Sit back on the seat: If you can slide your hand in between the front edge and the backs of your knees, it’s the correct size. Also make sure the edges are rounded so the seat doesn’t dig into your legs.
BaseCount five legs on the base; any fewer might not provide enough support (and the chair could tip over). Casters should move freely so you don’t have to bend or reach to move about.