We propose a method under the effective mass approximation with an original formulation that applies to quantum wells, circular quantum wires, and spherical quantum dots of arbitrary materials with sizes as small as 1 nm. Hundreds of structures are resolved on the second scale on a laptop, allowing for optimization procedures. We demonstrate its capability by confronting bandgap calculations with exhaustive literature data for CdS, CdSe, PbS, and PbSe nanoparticles. Our approach includes a correction of the mass to address the nonparabolicity of the band structure. The correction gives an accuracy comparable to more demanding calculation methods, such as eight-band k·p, tight-binding, or even semiempirical pseudopotential methods. The effect of the correction is shown on the intrasubband optical properties of InGaAs/AlGaAs coupled quantum wells.