Measure carefully around the fleshy part of the limb. This is where the seal will sit
The cover will fit comfortably & form approximately a 1cm wide seal around your limb which acts as the waterproof seal.
A little extra guidance:
For a limb circumferences up to 30cm, choose a 40mm seal size cover, i.e. ML/M/40
Up to 26cm limb circumference choose a 30mm seal size cover SL/30 – SHORT LEG reaches to below knee
Over 26cm limb circumference choose a 40mm seal size i.e. for an arm SA/M/40 – reaches to below elbow
Please note the seal contact area should be as oil & hair-free as possible