Ultrasound Due Date Calculator (CRL)
Estimate gestational age and due date from a first-trimester crown-rump length (CRL) using the Robinson–Fleming formula.
How it works
Robinson–Fleming: gestational age (days) = 8.052 × √(CRL × 1.037) + 23.73, with CRL in mm · due date = scan date − gestational age + 280 days.
In the first trimester the crown-rump length — the straight-line measurement from the top of the head to the bottom of the torso — dates a pregnancy very accurately, typically within about five days, because early growth is remarkably consistent. The Robinson–Fleming equation converts CRL in millimetres to a gestational age, from which the due date is 280 days after the implied last-period date. It is most reliable for a CRL between roughly 10 and 84 mm; later in pregnancy other biometry (head, abdomen, femur) is used instead.
Worked example
A CRL of 30 mm gives 8.052 × √(30 × 1.037) + 23.73 ≈ 68.6 days, about 9 weeks 6 days. Measured on August 1, 2026 the implied start is around May 25, 2026 and the due date is about March 1, 2027.
Sources: Robinson & Fleming CRL dating (Wikipedia: Crown-rump length)
Frequently asked questions
How accurate is CRL dating?
Very — a first-trimester CRL usually dates a pregnancy to within about five days, more accurately than the last-menstrual-period date, which is why providers often re-date to an early scan.
Why does CRL stop being used later?
As the fetus curls and grows, a single crown-rump line becomes unreliable beyond about 84 mm (roughly 14 weeks). After that, measurements of the head, abdomen and femur are combined instead.
Should I use this over my clinic’s date?
No. Your sonographer measures carefully and applies validated charts; this is an educational estimate. Always use the date your clinic gives you.
Related calculators
Educational estimates from standard formulas and population averages — every pregnancy and baby is different. This is not medical advice; your doctor, midwife or pediatrician, with an examination or ultrasound, gives the accurate guidance.