If you only know cap height and diameter, just remember this simple relationship:
Liquid Level = Diameter - Cap Height.

If you need a calculator that will accept other input units, then click here.

The total volume of a partially-filled spherical tank equals total sphere volume minus spherical cap volume.

To see other formulas for a partially-filled spherical tank, click here.

Of course you really don't need those formulas because this calculator does all the work for you.