CRL calculator (crown-rump length to gestational age)
Convert a crown-rump length measurement from an ultrasound into gestational age — and, with the scan date, an estimated due date. Runs in your browser; nothing is saved.
Reliable for about 15–95 mm (7–15 weeks).
Add this to also get an estimated due date.
How to use it
How CRL dating works (gestational age from CRL)
In early pregnancy, the embryo grows at a very predictable rate, so its length maps closely to gestational age. This crown rump length calculator uses the widely cited Robinson–Fleming (1975) equation, which converts crown-rump length in millimetres to gestational age in days — in other words, it works out the gestational age from CRL. Because that relationship only holds while growth is uniform, the formula is used mainly between about 7 and 14 weeks — the first-trimester dating window discussed in crown-rump length dating.
Worked example
A CRL of 45 mm corresponds to a gestational age of about 11 weeks 2 days. If that scan was done on a known date, the tool adds the remaining days to 40 weeks to estimate the due date.
Accuracy and limitations
First-trimester CRL is the single most accurate dating method, which is why an early scan often confirms or revises last-period dating (see how accurate is your due date). Outside roughly 15–95 mm the formula becomes unreliable and clinicians use other measurements, so this tool flags out-of-range values. It’s an educational estimate — your clinician’s report is definitive.
Frequently asked questions
What does CRL mean on an ultrasound?
CRL is the crown-rump length — the measurement from the top of the baby’s head to the bottom of the torso, taken on ultrasound. In the first trimester it’s the most accurate way to date a pregnancy.
Can CRL estimate gestational age?
Yes. Between roughly 7 and 14 weeks, CRL closely tracks gestational age, so a measurement in millimetres converts to weeks and days. This tool uses the Robinson–Fleming formula to do that.
Why does the measurement range matter?
The CRL dating formula is validated for about 15–95 mm. Below or above that, CRL is a less reliable dating measure and clinicians switch to other biometry, so results outside the range should be treated with caution.
Is CRL dating more accurate than my last period?
Usually, yes, in the first trimester. Because early growth is very consistent, a CRL-based date is often used to confirm or replace last-period dating. Your clinician’s report is the definitive record.
How do I get a due date from CRL?
Enter the CRL and the scan date. The tool converts CRL to gestational age, then counts forward to an estimated due date. Without the scan date, it still gives the gestational age at the time of the scan.
Last meaningfully updated: 2026-08-25. Method: Robinson & Fleming (1975).