Crown–Rump Length (CRL) to Gestational Age Calculator
Turn an early-scan crown–rump length in mm into gestational age and an ultrasound due date using the Robinson–Fleming formula.
How it works
GA (days) = 8.052 × √(CRL × 1.037) + 23.73 (Robinson & Fleming, CRL in mm); EDD = scan date + (280 − GA)
Between about 6 and 13 weeks the embryo grows so predictably that its length dates the pregnancy to within a week — more accurately than the last period, which assumes ovulation on day 14. Most maternity services adopt the scan date if it differs from the LMP date by more than 5–7 days. Beyond a CRL of 84 mm (around 14 weeks) the fetus curls and stretches too much, and head circumference takes over for dating.
Worked example
CRL 32 mm: 8.052 × √33.2 + 23.73 = 70.1 days → 10 weeks 0 days. A scan today gives a due date 210 days from now.
Frequently asked questions
Should the scan date replace my LMP date?
Guidelines (ACOG, NICE) say yes if the two differ by more than about 5 days before 9 weeks, or 7 days between 9 and 14 weeks.
Why is my CRL smaller than expected for my dates?
Most often ovulation was later than day 14. A follow-up scan a week or two later checks the growth rate.
Related calculators
Educational estimate only — your midwife, doctor or paediatrician will confirm dates and advice for your pregnancy or baby.