Free Pregnancy Tool
Crown-Rump Length (CRL)
Enter the crown-rump length (CRL) from an early ultrasound and the scan date. We estimate gestational age and due date.
General information only, not medical advice. Ask your doctor or midwife.
How this works
Formula: Robinson & Fleming (1975): gestational age (days) = 8.052 × √CRL (mm) + 23.73.
- Crown-rump length is the baby's length from the top of the head to the bottom, measured in early pregnancy.
- The Robinson formula turns that length into gestational age in days.
- The due date is the scan date plus the days left to 280.
- The formula is meant for CRL up to about 84 mm, roughly 14 weeks. After that, other measurements are used.
Everything is worked out in your browser. Belly Weekly does not send or store your answers.
Questions
Why is CRL used to date a pregnancy?
In the first trimester babies grow at a very similar rate, so length is a good guide to age.
My result differs from my scan report. Why?
Clinics may use other CRL formulas or round differently. Use the date your care team gives you.
Can I enter centimetres?
Enter millimetres. 1 cm is 10 mm, so 4.5 cm is 45 mm.