A numerical method for calculation of molecular surface area, based on the counting of dots generated on the molecular surface, has been developed for personal computers. Our molecular surface areas conform with Hermann’s values (Hermann, R. B. J. Phys. Chem. 1972, 76, 2754) within 0.1%. Our method can calculate the molecular area rather fast and can draw the three-dimensional structure of the molecule simultaneously. Since the program is written in ASSEMBLER, it can deal with large molecules composed of about 1000 atoms. This method has been applied to understand critical micelle concentrations (cmc’s) of lecithins possessing saturated acyl chains. The atomic coordinates of the lecithins are calculated on the basis of the crystal structure of dimyristoylphosphatidylcholine (DMPC). The molecular surface areas of the lecithins based on these coordinates are linear to the logarithms of the cmc’s. This fact suggests that the conformation of the lecithin monomer in water is close to that of DMPC in crystals. That is, two acyl chains in the single lecithin molecule are in parallel with each other. The effects of the molecular water radius and the molecular conformation of lecithins on the surface area are also investigated. An appropriate water radius will be 1.4-1.5 Angstrom for accessible molecular surface area calculations.