In the calculation of belt drives, Eider’s formula is usually used. But it is significant only a formula for its approximate values; many factors, such as belt thicknesses, diameters of pulleys, and speeds of belts et al. are not accounted in this formula. For this reason, using theories of mathematics and mechanics, the author devised some new formulas for the calculation of belt drives. Although they are more difficult to handle than Euler’s formula in calculations, there formulas are helpful in the study of belt drives.