OBJECTIVE: Since the management of pregnancy is gestational age dependent, accurate knowledge of the dating of gestational age is essential. The gestational age calculation system(GACS) was made to get a precise informations of exact gestational age of pregnant mothers. METHODS: Using the personal computer and Microsoft Visual Basic soft ware, the GACS program was made to meet obstetrician's desire. This program is designed and embodied to calculate gestational age controlling many variables such as last menstrual period(LMP), expectant date of confinement(EDC), gestational age on the calculating date, ultrasonographical gestational age, and conceptional date. RESULTS: The accurate gestational age was displayed by GACS according to various input data. The work sheet of whole gestational age can be printed by GACS. CONCLUSION: The GACS is a tool to calculate gestational age of pregnant mothers precisely. This can be used very conveniently and informatively by obstetric clinicians. We recommend this program for the members of perinatologists and obstetricians.