Pick your aspect ratio, then type a diagonal OR width OR height. The other two fill in instantly, in both inches and centimeters. Area is also calculated so you can compare the visible surface of two TVs or monitors.
Tip: TV manufacturers advertise the diagonal in inches, but what actually matters for viewing distance is the width. A 55" 16:9 TV is 48" wide — very different from a 55" 21:9 ultrawide.