No O-ring at hand, or one too worn to measure? Measure the groove instead. Pick the type of gland, enter what the calliper shows, and the calculator tries every standard size in it. The ones that fit are listed, BS-1806 first, and a known diving seal is named if the dimensions match one.
The groove gives the size only. It says nothing about material or hardness, and O-rings made for a single product aren’t standard sizes at all. The calculator can’t know the exact dimensions of your parts either, and back-up rings change the picture, so treat the result as an aid, not a guaranteed fit.