A new volume-explicit equation of state for the compressibility factor of square-well monomers is developed based on a hard-sphere equation of state, a coordination number model and available computer simulations data. The equation is simple and it gives the correct second virial coefficient of square-well monomers. Without applying any adjustable parameter, an accurate and simple representation of mixtures of square-well monomers is obtained by extending the pure equation of state using conventional mixing rules. The mixture equation of state was found to give predictions of compressibility factors of square-well mixtures that are in good agreement with available computer simulation data. We also derived a volume explicit equation of state for square-well chain molecules by using Wertheim's first order thermodynamic pertubation theory. Comparison with simulation data shows that the chain equation gives satisfactory representation of compressibility factor of square-well chain molecules.