A method for determining a composition of an object using a spectral x-ray system is provided. X-ray photons of at least two different energies are transmitted through the object. The energy of each detected x-ray photon using a detector in the x-ray system is estimated. A first weighted sum of the number of detected photons of each energy is found using a first weighting function, wherein the first weighting function is dependent on the attenuation coefficient function of a first material. In another embodiment, the photons are binned into two energy bins wherein there is a gap between the energy bins.