CRL (Crown-Rump Length) Calculator
Estimate gestational age from first-trimester ultrasound CRL and optionally calculate an estimated due date (EDD).
What is a CRL calculator?
A CRL calculator uses the fetal crown-rump length measurement from an early pregnancy ultrasound to estimate gestational age. Crown-rump length is the distance from the top of the embryo/fetus (crown) to the bottom of the torso (rump), and it is one of the most accurate first-trimester dating measurements.
Because early growth is fairly consistent, CRL is commonly used to improve pregnancy dating, especially when the last menstrual period date is uncertain.
How this calculator works
This page converts your CRL value into millimeters, then uses a validated first-trimester dating equation to estimate gestational age in days and weeks. If you provide the scan date, it also estimates:
- Estimated Due Date (EDD)
- Estimated Last Menstrual Period (LMP) date
- Approximate conception date
How to use the CRL calculator
Step-by-step
- Enter the CRL measurement from your ultrasound report.
- Select the correct unit (mm, cm, or inches).
- Optionally add the scan date if you want an EDD estimate.
- Click Calculate.
Quick CRL reference (approximate)
| CRL (mm) | Approximate Gestational Age |
|---|---|
| 10 mm | ~7 weeks 1 day |
| 20 mm | ~8 weeks 4 days |
| 30 mm | ~9 weeks 6 days |
| 40 mm | ~10 weeks 6 days |
| 50 mm | ~11 weeks 5 days |
| 60 mm | ~12 weeks 4 days |
| 70 mm | ~13 weeks 2 days |
| 80 mm | ~13 weeks 6 days |
Why CRL is important in early pregnancy dating
During the first trimester, CRL is often more reliable than cycle-based estimates alone. It can help in:
- Confirming pregnancy viability and timeline
- Aligning prenatal screening windows
- Reducing error in due date estimation
- Supporting consistent follow-up planning
Common reasons dates can differ
1) Late ovulation or implantation
If ovulation occurred later than expected, ultrasound dating may appear “behind” menstrual dating.
2) Measurement variability
Even with high-quality scans, tiny differences in caliper placement can shift estimated age by a few days.
3) Growth variation
Natural biological variation exists. A single estimate should always be interpreted in clinical context.
FAQ
Is CRL dating accurate?
In early pregnancy, CRL dating is generally considered highly accurate compared with later biometric methods.
Can I use this after the first trimester?
CRL is primarily a first-trimester measurement. Later dating typically relies on head, abdominal, and femur measurements.
Does this confirm fetal health?
No. This calculator only estimates dates from CRL. It does not diagnose any condition.
Bottom line
A CRL calculator is a practical way to estimate gestational age and due date from early ultrasound data. For medical decisions, always rely on your ultrasound report and your clinician’s guidance.