Calf measurements are used for tall boots, tall spats, and jodhpurs and fitted crop pants.
Now, ignoring momentarily the fact that the crop pant itself is a sin against all that is sacred and holy, here’s the scoop on the measurement:
Measure around the calf at its widest point. If you’re blessed with very defined, dancer type legs, this will be easy to locate and high close to the knee. For those of us who were not so blessed, it’s often lower down. It’s helpful to have someone else take this measurement for you. Stand in a normal posture, with your weight evenly distributed on both feet.
If you are particularly athletic, take this measurement once with your muscles relaxed and once with them flexed: use the larger of the two. If your calves are imbalanced in size due to sports or another physical consideration, measure both calves.
In boots, this measurement is also called the shank. (Incidentally, the standard woman’s fashion boot uses something like a 14″ shank. This is why some of us have such a time finding boots that actually fit over our calves. Plus sized stores will often carry boots with a more realistic shank.)