Side view of a person sitting on a sofa
A person sits on a sofa. The seat height, knee height and cushion softness can be changed, and the figure shows whether the feet rest flat and the thighs sit level.
Your knee height
Sofa seat height
Cushion sinks by
Try a typical sofa
Low lounger
Typical
Higher
Rule of thumb, not a medical measure: after the cushion has sunk, a seat that is within a few centimetres of your knee height keeps your feet flat and your thighs roughly level.