Measuring maturity in Upland cotton (Gossypium hirsutum L.) cultivar trials is a simple calculation of percentage of first harvest to total harvest when most trials are harvested twice. This provides a rough estimate of maturity. Today, cotton trials are rarely harvested twice because of the use of synthetic boll-opening agents. Breeding programs in states such as Arkansas and North Carolina have estimated maturity by either visually estimating percentage bolls open or actually counting open and closed (green) bolls. This study was conducted to determine the optimum combination of replicates, years, and locations of data needed to show 1 d difference in maturity between cultivars. Data were used from the Arkansas testing program for years 2005 through 2012 and from North Carolina for years 2007 through 2012. Arkansas program estimates percentage bolls open visually in all replicates and North Carolina program counts number of open and closed bolls in a short section of each plot in two replicates. For the Arkansas method, we would need to collect data from four replicates, 2 yr, and five locations or four replicates, 3 yr, and three locations. For North Carolina we would need 3 yr, four replicates, and three locations; or 3 yr, three replicates, and five locations; or 3 yr, five replicates, and two locations to provide the same level of precision. Single-year data could detect a 2 d difference in maturity using the Arkansas method and a 4 d difference in maturity using the North Carolina method. The Arkansas method is quicker and provides fairly accurate data on maturity and would be the recommended method to follow.