One of my favorite sites. A huge collection of units of measure.
The test I use for sites like this is dex - a unit used mostly in astrophysics. Hydrogen and helium accounts for most of the physical matter in the Universe (there is a larger amount of dark matter and dark energy, but that's another things entirely). Astrophysicists call any other elements 'metals' - the metallicity of a star is the fraction a star's mass made of of these metals. These are expressed in decimal exponents with the Sun as the reference of 0 (100 = 1 for the Sun) - so a dex of -1 means the star has a metallicity of 10-1, or a tenth that of the Sun.