If you are choosing fibre volume fraction (Vf) and fibre areal weight, then you essentially need fibre density information *and* a final desired thickness.
For a unit area and a total thickness t (total volume is 1*1*t), with average fibre density rho_f (see below for calculating rho_f):
Finding the weight of type of fibre and the average density depends on how the amounts of the types of fibre are defined. I'll assume just two types of fibre for simplicity.
If you have two fibre components of *mass* percentages:
75%/25%, say, and densities rho_f1 (the 75%) and rho_f2:
And in fact you don't actually need the average fibre density in this case, but for completeness:
rho_f = 0.75*rho_f1 + 0.25*rho_f2
You can simplify the formulas a bit, but it can be dangerous, as it's particularly easy to lose track of what you're doing with percentages by weight and volume knocking around. I've done a few checks on the above, but it's quite possible I've let an error (or two).
Also, do remember to use consistent units for weight, density and volume:
Metric areal weights are usally in grammes per square metre (gsm), and thicknesses tend to be in mm. It's safest to make all weights kg per square metre, all densities kg per cubic metre and all thicknesses in m. Then convert back at the end to gsm or whatever.
In Imperial/US customary, try to use areas in square inches, thicknesses in inches and densities in lb per cubic inch. If you want final areal weights in lb per square yard then convert afterwards.
