For optimum back pain relief, a chair seat should be firm enough for you to sit on without more than about 1.3 cm (half an inch) of give. For well-padded individuals, a hard seat without any cushioning is fine. The depth of the seat - measured from back to front - should be about 45-60 cm (18-24 inches), depending on your size. For lower back pain relief, you want enough seat depth to support about three-quarters of your thighs. Less will create a pull on your lower back; more, extending to your knees or beyond, will also play havoc with your back.

When considering the depth of a chair, assume that you might be using a back support, which will take up some of this measurement. In fact, if the only problem with a chair is that it has too much depth, that’s easy to solve. Add a firm pillow, a back support or both.