  We bring resolution to the longstanding problem relating Fermi surface
reconstruction to the number of holes contained within the Fermi surface volume
in underdoped high Tc superconductors. On considering uniaxial and biaxial
charge-density wave order, we show that there exists a relationship between the
ordering wave vector, the hole doping and the cross-sectional area of the
reconstructed Fermi surface whose precise form depends on the volume of the
starting Fermi surface. We consider a `large' starting Fermi surface comprising
1+p hole carriers, as predicted by band structure calculations, and a `small'
starting Fermi surface comprising p hole carriers, as proposed in models in
which the Coulomb repulsion remains the dominant energy. Using the
reconstructed Fermi surface cross-sectional area obtained in quantum
oscillation experiments in YBa2Cu3O6+x and HgBa2CuO4+x and the established
methods for estimating the chemical hole doping, we find the ordering vectors
obtained from x-ray scattering measurements to show a close correspondence with
those expected for the small starting Fermi surface. We therefore show the
Coulomb repulsion to remain largely unscreened throughout the entire underdoped
regime where the pseudogap exists and further show that the quantum oscillation
frequency and charge-density wave vectors provide accurate estimates for the
number of holes contributing to the Fermi surface volume.
