Enter a crown-rump length (CRL) to estimate gestational age and due date. You can also choose a target GA to see the expected CRL and published range where available.
How to use this calculator
- The calculator chooses the supported dating model automatically.
- Enter the CRL value (mm or cm). The tool auto-converts units.
- Optional: add the measurement date to calculate an EDD.
- Optional: choose a target GA to see the expected CRL and published percentile range where available.
- Read the outputs: gestational age at measurement, estimated due date, and expected CRL for the given GA.
Educational only. Ultrasound measurements vary with fetal position and equipment. Your clinician’s report takes precedence.
Last updated: 14 July 2026
Methodology
How the estimate works
- Automatic model choice: CRL 5-84 mm uses the unmodified Robinson-1975 equation:
GA (days) = 23.73 + 8.052 x square root of CRL (mm). - Outside the Robinson range: CRL 2 to under 5 mm and above 84 to 120 mm uses the Hadlock-1992 model. It is calculated from CRL in cm using the published exponential polynomial. Both models are rounded to a whole day.
- EDD from a measurement date: when a measurement date is entered, EDD is calculated as measurement date + (280 - GA days).
- Expected CRL for a given GA: for 58-105 gestational days, the tool uses the INTERGROWTH-21st CRL-by-GA average and standard-deviation equations. The displayed range contains the published 3rd-97th percentiles.
Units and inputs
- Units: input accepts mm or cm. The calculator converts cm→mm using
1 cm = 10 mmbefore applying the model. - Dates: measurement date is optional; without it, GA is shown at the time of scan only and no EDD is produced.
- Rounding: GA is rounded to whole days; expected CRL values are shown to 0.1 mm or 0.1 cm.
Assumptions & limitations
- Applies to first-trimester, mid-sagittal CRL measured with a neutral fetal position by ultrasound. Image quality and measurement technique affect accuracy.
- Ultrasound services may use Robinson, Hadlock, INTERGROWTH-21st, or another accepted chart. Small dating differences can result.
- After 14 weeks, Hadlock found CRL dating was comparable to, but not better than, measurements such as biparietal diameter and femur length.
- Values outside the combined 2-120 mm source range are not estimated or forced to the nearest supported age.
- Embryonic growth can vary; clinical dating should follow your sonographer’s report and local protocols.
- Robinson HP, Fleming JEE. “A critical evaluation of sonar ‘crown-rump length’ measurements.” Br J Obstet Gynaecol 1975;82:702–710. (Robinson CRL dating curve)..
- Hadlock FP, Shah YP, Kanon DJ, Lindsey JV. “Fetal crown-rump length: reevaluation of relation to menstrual age (5-18 weeks) with high-resolution real-time US.” Radiology 1992;182:501-505. (Extended 2-120 mm CRL dating model).
- Verburg BO, et al. “New charts for ultrasound dating of pregnancy and assessment of fetal growth.” Ultrasound Obstet Gynecol 2008;31:388–396. (Population-based CRL dating charts)..
- Papageorghiou AT, et al. International standards for fetal growth based on serial ultrasound measurements. Ultrasound Obstet Gynecol 2014. (CRL-by-GA mean and centiles).
- British Medical Ultrasound Society. Fetal measurements. (Corrected Robinson equation and model differences).
- NICE. Ectopic pregnancy and miscarriage: diagnosis and initial management. (Repeat-scan timing and use of mean gestational sac diameter).
- ISUOG Practice Guidelines: role of ultrasound in twin pregnancy. (Dating twin pregnancies).
- ACOG/AIUM guidance on ultrasound dating in early pregnancy: Methods for Estimating the Due Date.
Frequently asked questions
What is CRL and when is it used?
Crown-rump length (CRL) is the head-to-bottom length of the embryo/fetus on early ultrasound. It is the preferred way to date pregnancy in the first trimester.
How accurate is CRL dating?
CRL-based gestational age is typically more precise than LMP in early pregnancy, but there is still a normal margin of error. Small differences are expected between machines and sonographers.
Why does my GA not match my LMP dates?
Ovulation and implantation vary from cycle to cycle. If CRL and LMP disagree by more than common thresholds, clinicians usually use ultrasound dating. You can also compare with the Ultrasound Due Date Calculator.
Can I enter cm instead of mm?
Yes. The input accepts mm or cm and converts automatically for the calculation.
What if CRL is not visible yet?
If no fetal pole is visible, your sonographer may use the mean gestational sac diameter instead. When a repeat scan is needed, NICE advises waiting at least 7 days after a transvaginal scan or 14 days after a transabdominal scan in the situations covered by its guidance. Follow the timing given by your clinician. You can also use the Mean Sac Diameter Calculator.
Does this work for twins?
CRL is measured for each fetus. ISUOG advises using the larger CRL for spontaneous twins in the first trimester and embryo age or transfer date for IVF pregnancies. At or after 14 weeks, the larger twin's head circumference is used. For multiples planning, see the Twin & Triplet Due Date Calculator.
What about risk after a heartbeat?
CRL is a growth measure, not a risk estimate. For risk context after cardiac activity is seen, use the Miscarriage Risk After Heartbeat Calculator.