πΆ Crown-Rump Length Calculator
CRL in millimeters to gestational age, Robinson formula.
Educational. Official dating comes from your ultrasound report and obstetrician.
In the first trimester, the embryo's length from crown to rump dates the pregnancy better than any calendar. The Robinson formula converts CRL in millimeters to gestational age in days, accurate within about a week, which is why first-trimester ultrasound is the gold standard for dating. Enter the CRL from your report for the gestational age and the estimated due date.
How to Use This Calculator
Enter CRL in millimeters from the ultrasound report, plus the scan date for the due date estimate.
Find CRL in mm on your first-trimester ultrasound report.
Enter the scan date to get the estimated due date (280 days minus the gestational age).
Read gestational age in weeks and days. Trust this over LMP dating when they disagree meaningfully.
The Robinson Formula
Robinson and Fleming derived the formula in 1975 from CRL measurements in known-date pregnancies, and it has survived half a century because early embryonic growth is remarkably uniform: before 14 weeks, genetics and nutrition have not yet diverged growth curves the way they do later. The square-root relationship reflects that growth slows relative to length as the embryo gets bigger. Beyond about 84 mm (14 weeks), head circumference, abdominal circumference, and femur length take over dating, combined in formulas like Hadlock's.
Why ultrasound beats LMP: ovulation does not always occur on day 14, cycles vary, implantation bleeding masquerades as periods, and many people misremember dates. CRL measures the embryo itself, bypassing all of it. Guidelines redating pregnancies when first-trimester CRL disagrees with LMP by more than 5-7 days, because the clinical consequences (timing of screening tests, induction decisions) are real.
Worked Example
CRL 55 mm: gestational age = 8.052 x sqrt(55) + 23.73 = 8.052 x 7.416 + 23.73 = 83.4 days = 11 weeks 6 days. If the scan was March 10, the due date is March 10 + (280 - 83) = March 10 + 197 days = September 23. A CRL of 20 mm: 8.052 x 4.472 + 23.73 = 59.7 days = 8 weeks 4 days. Notice the compression: 35 mm of growth between the two measurements represents only 24 days, because early growth is explosive.
What the Dating Changes
Accurate dating sets the calendar for everything: nuchal translucency screening (11-14 weeks), anatomy scan (18-22 weeks), glucose testing, and the 41-week induction conversation. A week of dating error at term can mean the difference between expectant management and induction, so the first-trimester scan's precision pays dividends for eight months. It also resolves the conception question definitively: with the due date from CRL, our conception date calculator works backward precisely.
Discrepancies need interpretation, not panic. A CRL measuring small can mean later ovulation (common), slow early growth (sometimes concerning), or simply measurement variation. A CRL measuring large can mean earlier ovulation. Serial scans distinguish: consistent growth along a curve reassures, while falling off the curve prompts evaluation. One measurement is a dot; two are a trend, and obstetrics runs on trends.
Beyond the First Trimester
After 14 weeks, dating shifts to the composite biometry: biparietal diameter, head circumference, abdominal circumference, femur length. These are less precise (Β±10-14 days in the second trimester, worse later) because genetics and placenta start individualizing growth. That is exactly why the first-trimester date, once established, is never replaced by later measurements: later scans assess growth against the established date, not the date itself. Get dated early, write it down, and let every later scan be judged against it.
Related Calculators
Keep exploring: try our conception date and child weight percentile for the next piece of the picture.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is crown-rump length?
The length of the embryo/fetus from the top of the head (crown) to the buttocks (rump), measured on first-trimester ultrasound in millimeters. It is the most accurate dating measurement in early pregnancy.
How do you convert CRL to gestational age?
With the Robinson formula: gestational age (days) = 8.052 x sqrt(CRL in mm) + 23.73. Example: 55 mm gives 83.4 days (11w6d). This calculator does it instantly, plus the estimated due date.
How accurate is CRL dating?
Within about 5-7 days in the first trimester, the best dating available. Accuracy falls later in pregnancy as genetics individualize growth, which is why the first-trimester date is never replaced by later scans.
When is CRL measured?
Roughly 6-14 weeks gestation (CRL ~10-84 mm). Before 6 weeks the embryo is often too small to measure reliably; after 14 weeks, head circumference and femur length take over dating.
What if CRL and LMP dating disagree?
Trust the CRL if the disagreement exceeds 5-7 days in the first trimester: guidelines redating to the ultrasound. LMP dating assumes day-14 ovulation, which is often wrong; CRL measures the embryo directly.
What is a normal CRL at 8 weeks?
About 16 mm on average (Robinson: 8.052 x 4 + 23.73 = ~56 days = 8w0d). Normal variation spans several millimeters; your report's gestational age already accounts for the expected range.
Can CRL predict the due date?
Yes: due date = scan date + (280 - CRL gestational age in days). Enter your scan date above for the estimate. Remember only ~5% of babies arrive on the due date; 37-42 weeks is full term.
What if CRL measures small?
Often just later ovulation than assumed. Serial scans distinguish: steady growth along a curve reassures, while slowing growth prompts evaluation. Discuss any concern with your obstetrician rather than interpreting alone.
Who developed the CRL dating formula?
Robinson and Fleming published it in 1975 from CRL measurements in known-date pregnancies. It remains the standard first-trimester dating formula half a century later.
Is CRL used after the first trimester?
No, for dating. After ~14 weeks, composite biometry (head circumference, abdominal circumference, femur length) assesses growth against the established first-trimester date. Later scans measure growth, not age.