I know my

54

EU size

Calibrate for a true-size circle

Hold a bank card against the screen and move the slider until the outline matches its width (85.6 mm).

Measure at home

Wrap a thin strip of paper around the base of the finger, snug but not tight.
Mark where the paper overlaps and measure the length in millimetres.
Choose “inside circumference” above and enter the number.
Measure in the evening at room temperature, when fingers are at their usual size; for wide bands go up half a size.