The absolute increase in weight over a period of time is attributed to the weight of the bird at the start of the period or to the average weight of the bird at the start and end of the period (relative growth rate). .The aim of this research is to use a function in a Microsoft Excel spreadsheet to calculate the growth rate percentage of poultry in one step instead of calculated it manually. The method depends upon entering the following function in the function bar of the Microsoft Excel spreadsheet ((=((B4-C4)/(0.5*(C4+B4))*100)) after we enter the values of the second bodyweight in column B and the first bodyweight in column C and enter the following function (=((B4-C4)/(0.5*(C4+B4))*100)) in the Cell E4 of an Excel worksheet, the result of Relative Growth Rate will appear automatically in Cell E4 in one step which will shorten the time needed for calculation by more than  1/100Key words: Excel ,Function ,  Relative growth rate