Crown Rump Length Calculator - Calculator Academy

Enter the Crown Rump Length (CRL) or Gestational Age into the calculator to estimate the other (first trimester dating). CRL is typically measured directly by ultrasound; femur length is generally used later in pregnancy and is not used to calculate CRL.

Related Calculators

Crown Rump Length Formula

The following equations are commonly used to estimate gestational age from a first-trimester Crown Rump Length (CRL) measurement (and to estimate an expected CRL from gestational age). The formula uses CRL in millimeters and gestational age in days:

GA_{\text{days}} = 8.052\sqrt{CRL_{\text{mm}}} + 23.73\\ CRL_{\text{mm}} = \left(\frac{GA_{\text{days}} - 23.73}{8.052}\right)^2

Variables:

CRL is the Crown Rump Length (typically measured by ultrasound; in mm for the formula)
GA is the Gestational Age (in days for the formula; can be converted to weeks by dividing by 7)

To estimate gestational age from CRL, take the square root of CRL (in mm), multiply by 8.052, then add 23.73 to get gestational age in days. To estimate CRL from gestational age, subtract 23.73 days from GA (in days), divide by 8.052, then square the result to get CRL in mm.

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 (commonly about 7 weeks to 13 weeks 6 days gestation) to estimate the age of the fetus and help determine the estimated due date. When measured correctly in the first trimester, CRL-based dating is generally considered the most accurate method of estimating gestational age because there is relatively little variation in fetal size during this period. CRL is also used to assess early fetal development and can help identify potential concerns when interpreted alongside other clinical information.

How to Measure Crown Rump Length (CRL)?

The following steps outline how CRL is typically obtained and used.

Obtain an ultrasound image where the fetus is seen clearly in a mid-sagittal (side-on) view.
Measure from the top of the fetal head (crown) to the bottom of the buttocks (rump), excluding the yolk sac and limbs.
Record the measurement as the Crown Rump Length (CRL), typically in millimeters.
If needed, estimate gestational age using validated CRL-to-GA formulas or standard reference charts for the first trimester.

Example Problem:

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

Crown Rump Length (CRL) = 7.5 cm (75 mm)

Using the formula above, GA (days) = 8.052 × √(75) + 23.73 ≈ 93.46 days, which is about 13.35 weeks (approximately 13 weeks 2 days). Standard charts and clinical practice may round slightly differently.