Crown-Rump Length to Gestational Age Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: July 27, 2026
Disclaimer: This calculator is provided for informational and educational purposes only and does not constitute financial, medical, legal, or other professional advice. Always consult a qualified professional before making decisions based on these results.
The most accurate way to date early pregnancy
Crown-rump length grows very predictably in early pregnancy, which is why first-trimester ultrasound CRL measurement is considered the most accurate method available for establishing gestational age.
Worked example
For a CRL measurement of 30mm:
GA = 8.052 x sqrt(30) x 1.037 + 23.73 = 9w 6.5d (69.5 days)
The Robinson formula, published in 1975, remains the standard reference method for this calculation and is validated for CRL measurements between roughly 2mm and 84mm - outside this range, other dating methods (or later ultrasound biometry) become more appropriate.
This is an informational tool only, not a substitute for care from a qualified healthcare provider, who will verify and interpret dating directly against the actual ultrasound findings and clinical history.