Gestational Age from Ultrasound Calculator
In the first trimester, the crown-rump length (CRL) of the embryo is the most accurate way to date a pregnancy. This calculator uses the Robinson and Fleming equation, a standard CRL dating formula, to estimate gestational age in days and weeks from a measured CRL in millimetres. It is intended for information and does not replace dating performed by a sonographer or clinician.
Robinson CRL formula
GA (days) = 8.052 * sqrt(CRL * 1.037) + 23.73
CRL in millimetres
Weeks = days / 7
The Robinson and Fleming (1975) equation estimates gestational age in days from crown-rump length. It is most accurate between about 6 and 13 weeks, when CRL is the preferred dating measurement.
Worked example
For a CRL of 30 mm: GA = 8.052 * sqrt(30 * 1.037) + 23.73 = 8.052 * sqrt(31.11) + 23.73 = 8.052 * 5.5776 + 23.73 = 44.91 + 23.73 = 68.64 days = 9.81 weeks, about 9 weeks and 6 days.
Frequently asked questions
Why is CRL used for dating?
Early in pregnancy, embryos grow at a very consistent rate, so crown-rump length predicts gestational age accurately, usually within a few days. After about 13 to 14 weeks, growth varies more and other measurements such as head and femur are used instead.
How accurate is CRL dating?
Between roughly 6 and 13 weeks, CRL dating is typically accurate to within about 5 to 7 days. It is considered more reliable than dating from the last menstrual period, especially when cycles are irregular.
Does this replace my scan report?
No. This calculator reproduces a published formula for information only. Official dating must come from a trained sonographer using calibrated equipment, and your maternity provider will confirm your due date.
Sources
- U.S. National Library of Medicine (PubMed): Robinson and Fleming 1975, critical evaluation of crown-rump length dating.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.