Crown-Rump Length Gestational Age Calculator
Crown-rump length (CRL) is the gold-standard first-trimester ultrasound measurement for estimating gestational age. Because early fetal growth is highly uniform, a single CRL reading dates a pregnancy to within a few days. This calculator applies the Robinson and Fleming equation, the most widely used CRL formula, to convert a CRL in millimetres into gestational age in completed weeks and days. It is most reliable for CRL values of about 7 to 84 mm. Results are for education only; clinical dating must come from a qualified sonographer or clinician.
Robinson-Fleming CRL formula
GA (days) = 8.052 * sqrt(CRL * 1.037) + 23.73
CRL is in millimetres
Weeks = GA days / 7
Weeks + days = floor(weeks) w, remainder d
This equation, published by Robinson and Fleming in 1975, remains a standard for first-trimester dating. The square-root form reflects the decelerating relationship between length and age as the fetus develops.
CRL dating context
- CRL is the single most accurate measurement for first-trimester gestational age.
- Reliable range is roughly 7 to 84 mm (about 6 to 14 weeks).
- Above about 84 mm, head, abdomen, and femur biometry are used instead.
- Early CRL dating is generally preferred over last-menstrual-period dating.
- Measurements should be taken in a true mid-sagittal plane with a neutral fetal position.
Crown-rump length: frequently asked questions
What is crown-rump length?
Crown-rump length (CRL) is the ultrasound measurement from the top of the fetal head (crown) to the bottom of the buttocks (rump), in millimetres. In the first trimester it is the most accurate single measurement for estimating gestational age.
Which equation does this calculator use?
It uses the widely cited Robinson and Fleming equation: gestational age in days = 8.052 times the square root of (CRL times 1.037), plus 23.73, where CRL is in millimetres. The result is then expressed in completed weeks and days.
What CRL range is this valid for?
First-trimester CRL dating is most reliable from roughly 7 to 84 mm (about 6 to 14 weeks). Beyond about 84 mm, biometry such as biparietal diameter, head circumference, abdominal circumference, and femur length is used instead, because CRL accuracy falls as the fetus curls.
Why is early CRL dating preferred?
Early in pregnancy, fetuses grow at a very consistent rate, so a single CRL measurement closely tracks true gestational age. Dating accuracy is within a few days. Later in pregnancy, normal growth variation widens, so estimates become less precise.
Does this replace medical assessment?
No. This tool provides a general estimate for education. Gestational age and due dates should be determined by a qualified sonographer or clinician using calibrated equipment and clinical context.
Official sources
- U.S. National Library of Medicine, StatPearls: Gestational Age Assessment.
- American College of Obstetricians and Gynecologists: Clinical guidance.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.