What is the Crown-Rump Length (CRL) Calculator?
Crown-rump length (CRL) is the measurement from the top of the head (crown) to the bottom of the buttocks (rump) of an embryo or fetus. During the first trimester it is the most accurate ultrasound measurement for estimating gestational age. This calculator converts a CRL value in millimetres into an estimated gestational age expressed in weeks and days, using the widely cited Robinson-Fleming style regression.
How to use it
Enter the crown-rump length measured on your ultrasound scan in millimetres (mm) and the calculator returns the estimated gestational age. CRL-based dating is most reliable between roughly 6 and 14 weeks (about 5–84 mm). Outside this window other biometric measurements are preferred for dating.
The formula explained
The calculator uses:
$$\text{GA (days)} = 8.052 \times \sqrt{\text{CRL (mm)} \times 1.037} + 23.73$$
The square-root term reflects the non-linear, decelerating relationship between embryo growth and time. The result in days is then divided by 7 to express the age in weeks plus remaining days.
Worked example
Suppose the CRL is 20 mm. First compute \(20 \times 1.037 = 20.74\). The square root of \(20.74 \approx 4.5541\). Multiply by \(8.052 \approx 36.670\), then add \(23.73\) to get \(\approx 60.40\) days. Dividing by 7 gives about \(8.63\) weeks — roughly 8 weeks 4 days.
FAQ
How accurate is CRL dating? In the first trimester CRL dating is accurate to within about ±5 days, making it the gold standard for estimating a due date.
What if my CRL is very large? Beyond about 84 mm (around 14 weeks) CRL becomes less reliable and clinicians switch to head, abdomen and femur measurements.
Does this replace medical advice? No. This tool is for educational estimation only. Always rely on your healthcare provider's official dating.
