Yearly variation in maximum nut size dates of Carya illinoensis (Wang.) K. Koch., cultivars ‘Western’ and ‘Wichita’, was related to air temperature expressed in heat units over five growing seasons. Heat units accumulated from the last date of exposure to 0°C or lower (last spring frost) to maximum size calculated with a base temperature of 10.0°C provided the lowest year-to-year heat unit variability. Coefficients of variation for 5 yearly heat unit sums were 1.48 and 1.60% for ‘Western’ and ‘Wichita’, respectively. Average absolute deviation between observed and predicted dates was 2.0 days for both cultivars. An interactive computer program was developed to process the temperature data, expediting the use of different combinations of base temperatures and starting dates. Possible use and limitations of the heat unit model for improvement of pecan cultural management in the south-western irrigated U.S. is discussed.