Several of the more powerful approximate methods for calculating on the first derivative of the power‐law exponent are appreciably more shear rates for non‐Newtonian fluids in coaxial viscometers are compared. Each is applied to model fluids representing the two most common types of non‐Newtonian behavior, in a wide‐gap viscometer. Results with the power law are moderately accurate (ca. 1%) for fluids which do not display yield stresses, but much less accurate for those which do. Methods based on the first derivative of the power‐law exponent are appreciably more accurate, but a second‐derivative term may be needed for fluids which show yield stresses.