Online Crown Rump Length Calculator to estimate gestational age. Crown-rump length (CRL) is the measurement of the length of human embryos and fetuses from the top of the head (crown) to the bottom of the buttocks (rump). The Crown rump length (CRL) is typically determined from ultrasound imagery. The CRL is used to estimate gestational age. Just input the number of weeks and the CRL calculator will estimate and update you with the crown-rump length. Gestational age (or menstrual age) is a measure of the age of a pregnancy.