Calculate gestational age in weeks and days from crown-rump length, or estimate CRL from gestational age, in millimeters and centimeters with an optional due date.

Educational Robinson–Fleming estimate, not a diagnosis or a replacement for clinician-assigned pregnancy dating. Use a measured first-trimester ultrasound CRL.

Operating range: about 5.1484–84 mm, corresponding to at least 6 weeks by this equation.

Optional scan date

CRL to Weeks Formula

The following equation is commonly used to estimate gestational age (GA) from Crown-Rump Length (CRL) measured on ultrasound in early pregnancy. It is intended for first-trimester dating (from 6 weeks through a CRL of 84 mm (about 13.93 unrounded weeks)).

W = (8.052√(Crl) + 23.73) / (7)

Variables:

  • W is gestational age in weeks
  • Crl is the Crown-Rump Length in millimeters (mm)

This comes from the Robinson & Fleming regression equation for gestational age in days: GA(days) = 8.052 × √CRL + 23.73, then converted to weeks by dividing by 7.

Source: Robinson HP, Fleming JE. A critical evaluation of sonar “crown-rump length” measurements. Br J Obstet Gynaecol. 1975. (See PubMed: https://pubmed.ncbi.nlm.nih.gov/1182090/)

CRL to Gestational Age Conversion Table (Formula: weeks = (8.052 × √CRL + 23.73) ÷ 7)
Crown-Rump Length (mm) Gestational Age (weeks)
66.21
76.43
86.64
96.84
107.03
127.37
147.69
167.99
188.27
208.53
259.14
309.69
3510.20
4010.67
5011.52
6012.30
Derived from Robinson & Fleming (1975): GA(days) = 8.052 × √CRL + 23.73; GA(weeks) = GA(days) ÷ 7.

What Is CRL to Weeks?

CRL to weeks is a way to estimate gestational age during pregnancy using the fetus’s crown-rump length (CRL). CRL is the measurement from the top of the baby’s head (crown) to the bottom (rump) in an appropriate standard ultrasound view. This measurement is taken during an ultrasound scan and is commonly used for pregnancy dating in the first trimester.

How to Calculate CRL to Weeks?

The following steps outline how to estimate weeks of gestation from CRL using the equation: W = (8.052 × √Crl + 23.73) / 7

  1. First, determine the Crown-Rump Length (Crl) in millimeters (mm).
  2. Next, take the square root of the Crl value (√Crl).
  3. Next, multiply √Crl by 8.052, then add 23.73 to get gestational age in days.
  4. Finally, divide by 7 to convert days to weeks (W).
  5. After calculating the result, you can check your answer with the calculator above.

Example Problem:

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

Crown-Rump Length (Crl) = 40 mm