Enter the Femur Length and Gestational Age into the calculator to determine the Crown Rump Length; this calculator can also evaluate any of the variables given the others are known.

Crown Rump Length Formula

The following formula is used to calculate the Crown Rump Length (CRL) in a pregnancy:

CRL = (FL * GA) / 7 + 42


  • CRL is the Crown Rump Length
  • FL is the Femur Length
  • GA is the Gestational Age in weeks

To calculate the Crown Rump Length, multiply the Femur Length by the Gestational Age in weeks, then divide the result by 7 and add 42.

What is a Crown Rump Length?

Crown Rump Length (CRL) is a measurement of the length of a human fetus from the top of its head (crown) to the bottom of its buttocks (rump), taken when the fetus is curled up in the fetal position. This measurement is typically used in prenatal ultrasound imaging during the first trimester of pregnancy (between 7 to 13 weeks gestation) to estimate the age of the fetus and predict the expected date of delivery. The CRL is considered the most accurate method of dating a pregnancy in the first trimester, as there is very little variation in fetal size during this period. It is a crucial parameter in assessing the development and growth of the fetus and can help identify potential abnormalities or risks.

How to Calculate Crown Rump Length?

The following steps outline how to calculate the Crown Rump Length (CRL).

  1. First, gather the necessary measurements and data.
  2. Next, measure the length from the top of the baby’s head to its bottom (in centimeters).
  3. Next, record the measured length as the Crown Rump Length (CRL).
  4. Finally, calculate the CRL.
  5. After measuring and recording the CRL, compare it to the standard growth charts to determine the baby’s gestational age.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Crown Rump Length (CRL) = 7.5 cm

Based on the standard growth charts, the gestational age is determined to be 10 weeks.