Thermal field-flow fractionation (ThFFF) is used for the determination of molecular weight (MW) distributions of synthetic polymers. This is usually achieved utilizing a series of narrow MW polymer standards. A major drawback in this approach is the very limited number of polymers which have such standards available. This paper reports a method of calibration which utilizes one or more broad standards for which the average MW values are known. It was observed that 1) the use of number-average MW data tends to produce large variation in calibration constants and hence the MW generated, 2) the use of two polydisperse standards and their weight-average MW (M(w)) provides reasonable calibration in comparison with the conventional method using narrow MW standards, and 3) even better calibration is obtained using multiple broad standards and their M(w) values. The new method should expand the applicability of ThFFF to include a wider range of polymer types.