Crown Rump Length Calculator
Use our free Crown rump length Calculator to get personalized health results. Based on validated medical formulas and clinical guidelines.
Reviewed by Rahul Singh, Health & Wellness Specialist
Formula
GA (days) = 8.052 x sqrt(CRL mm) + 23.73 | CRL (mm) = ((GA days - 23.73) / 8.052)^2
The Robinson and Fleming formula calculates gestational age in days from CRL in millimeters using the square root relationship. The inverse formula predicts expected CRL from a known gestational age. These formulas are accurate to plus or minus 3-5 days between 6-14 weeks gestation.
Worked Examples
Example 1: CRL to Gestational Age Calculation
Problem:An ultrasound measures a CRL of 25mm. What is the estimated gestational age?
Solution:Using Robinson formula:\nGA (days) = 8.052 x sqrt(25) + 23.73\nGA (days) = 8.052 x 5.0 + 23.73\nGA (days) = 40.26 + 23.73 = 63.99 days\nGA = 64 days = 9 weeks, 1 day\nExpected growth rate: ~1.0 mm/day
Result:Gestational Age: 9 weeks, 1 day | CRL: 25mm | Estimated weight: 2-4 grams
Example 2: Expected CRL from Known Gestational Age
Problem:A patient is confirmed to be 11 weeks, 0 days pregnant. What CRL should be expected?
Solution:Total gestational days = 11 x 7 + 0 = 77 days\nUsing inverse Robinson formula:\nCRL (mm) = ((77 - 23.73) / 8.052)^2\nCRL (mm) = (53.27 / 8.052)^2\nCRL (mm) = (6.615)^2 = 43.8mm\n5th percentile: 35.0mm\n95th percentile: 52.5mm
Result:Expected CRL: 43.8mm (range: 35.0-52.5mm) | Weight: ~7-14 grams
Frequently Asked Questions
What is crown-rump length and how is it measured?
Crown-rump length (CRL) is the measurement of the embryo or fetus from the top of the head (crown) to the bottom of the buttocks (rump) in its natural flexed position. It is measured during a transvaginal or transabdominal ultrasound examination, typically between 6 and 14 weeks of gestation. The sonographer identifies the longest straight-line measurement of the embryo or fetus in a true midsagittal plane, excluding the limbs and yolk sac. CRL is the single most accurate biometric measurement for determining gestational age in early pregnancy, with an accuracy of plus or minus 3-5 days in the first trimester. The measurement increases from approximately 4mm at 6 weeks to about 85mm at 14 weeks.
References
Reviewed by Rahul Singh, Health & Wellness Specialist · Editorial policy