This should be super easy, but I've found more than one equation in the literature:

What equation lets you calculate (estimate) stationary phase coverage (RP C8) by knowing the % carbon from an elemental analysis of the unpacked silica gel?

One obviously knows the # of C's on the chain, the surface area of the silica is reported by the mfg., and you've determined %C (assume all of it is from the chains).

My problem is that I've seen two eqns to calculate this, and of course, I've gotten 2 different answers :)

I came up with the following equation to check against your other two. Hopefully the math came out right:

coverage=83.2*[%C]*[#C/silane]/surface area

Where coverage is in micromoles/m^2 and surface area is in m^2. For a typical C8 there are ten carbons/silane including the two extra methyl groups on the silicon atom. The above equation assumes that all carbon is from the C8 silane, but if the packing material is endcapped there will be a significant amount of carbon which comes from the endcapping silane.
A more useful equation would be that which calculates the volume of phase per volume of void in a packed column from SA and %carbon. Does anyone have an equation or comments about that type of calculation?

I just attended an excellent Waters seminar given by Charles Phoebe, Understanding Column Selection for Reversed-Phase Chromatography. He included a general equation for calculation of surface coverage aka ligand density:
SC = umoles/m^2 = %C/(100*SA*[1-(%C/100*((MW-1)/(nC+12)))]
where %C = % carbon load
SA = specific surface area
MW = molecular weight of ligand
nC = # of carbon atoms in ligand

Good luck!

Chuck made a slight mistake. He used the formula in my book, which the typesetters mishandled. The following is the correct equation (for monofunctional silanes):
SC [micromoles/m^2] = %C/(100*SA*12*nC)*(1-(%C/100*(MW-1)/(nC*12)))
Best regards
Uwe Neue