CRL Gestational Age Calculator
Use crown-rump length (CRL) from early pregnancy ultrasound to estimate gestational age. You can also add the scan date to estimate LMP and due date.
Estimated Gestational Age:
Total Gestational Days:
Input CRL (converted):
What is a CRL gestational age calculator?
A CRL gestational age calculator estimates how far along a pregnancy is by using crown-rump length measured during ultrasound. Crown-rump length is the distance from the top of the embryo or fetus head (crown) to the bottom of the torso (rump), and it is one of the most reliable first-trimester biometric measurements for pregnancy dating.
In practical terms, this means if you have a CRL measurement from an ultrasound report, this calculator can estimate gestational age in weeks and days. If you add the scan date, it can also estimate an LMP date and an estimated due date (EDD).
Why CRL is commonly used in the first trimester
In early pregnancy, there is less size variation between embryos compared with later stages. That makes CRL a strong predictor for dating, especially when menstrual dates are uncertain or cycles are irregular.
- Useful when last menstrual period is unknown.
- Helpful for irregular cycle lengths.
- Commonly preferred for first-trimester dating consistency.
- Supports better timing for prenatal screening and follow-up scans.
How this calculator works
This calculator uses a well-known first-trimester CRL dating approximation (Robinson-Fleming style):
Gestational age (days) = 8.052 × √(CRL in mm) + 23.73
After calculating total gestational days, it converts that number into weeks + days. If you enter an ultrasound date, it also estimates:
- Estimated LMP by subtracting gestational days from scan date.
- Estimated due date (EDD) by adding 280 days to estimated LMP.
Typical interpretation examples
- A smaller CRL corresponds to earlier gestational age.
- A larger CRL corresponds to later first-trimester gestation.
- Dating is most reliable when measurement quality is high and imaging planes are appropriate.
How to use this CRL pregnancy calculator correctly
- Enter the CRL value exactly as listed on the ultrasound report.
- Select the correct unit (mm or cm).
- Optionally add the ultrasound date to estimate LMP and EDD.
- Click Calculate and review the result with your clinician.
Reference ranges and quality notes
CRL-based formulas are generally used in the first trimester, and many references emphasize best performance within a specific CRL interval (often around 5 to 84 mm). This tool will still produce a result outside that range, but interpretation should be cautious and clinically reviewed.
Measurement accuracy can vary based on:
- Sonographer technique and machine settings
- Fetal position during scan
- Single versus repeated measurements
- Presence of developmental or anatomical variation
Frequently asked questions
Is CRL more accurate than LMP dating?
In early pregnancy, CRL dating is often considered more reliable than menstrual dating when cycle history is uncertain, ovulation timing is unclear, or periods are irregular.
Can this calculator diagnose pregnancy problems?
No. It only estimates gestational age from one measurement. It cannot diagnose viability, growth restriction, anomalies, or miscarriage risk.
Why might my due date differ from another calculator?
Different tools use different equations, rounding rules, measurement ranges, and date standards. Your obstetric provider should determine the official clinical due date.
What if my CRL is in centimeters?
This calculator supports both mm and cm. If you select centimeters, it automatically converts to millimeters before calculation.
Important medical disclaimer
This CRL gestational age calculator is educational and informational. It is not a substitute for professional medical advice, diagnosis, or treatment. Always discuss ultrasound findings, gestational age, and estimated due date with your obstetrician, midwife, or maternal-fetal medicine specialist.