CRL to Gestational Age Calculator
Use this tool to estimate gestational age from crown-rump length (CRL) measured during first-trimester ultrasound.
Gestational Age to Expected CRL
Already know gestational age? Estimate expected CRL range point value.
Educational tool only. Clinical decisions should always be made with your obstetric provider.
The crown-rump length calculator helps estimate early pregnancy dating using a fetus’s measured length from the top of the head (crown) to the bottom of the torso (rump). In first-trimester ultrasound, this is one of the most reliable ways to estimate gestational age when menstrual dates are uncertain.
What is crown-rump length (CRL)?
CRL is the straight-line measurement of the embryo or fetus from crown to rump, usually reported in millimeters. It is most commonly used between roughly 6 and 14 weeks of pregnancy. During this period, CRL growth is relatively predictable, making it useful for pregnancy dating.
Why CRL matters
- Improves estimated gestational age accuracy in early pregnancy.
- Helps establish an estimated due date (EDD).
- Supports consistent prenatal follow-up timing.
- Can reduce confusion when last menstrual period is unknown or irregular.
How this calculator works
This page uses a commonly referenced first-trimester dating equation (Robinson-Fleming style model) to convert CRL in millimeters into gestational age in days. The result is then shown as weeks + days. If you provide a scan date, the calculator estimates an EDD by adding the remaining days to a 280-day pregnancy framework.
Step-by-step use
- Enter CRL from the ultrasound report.
- Select the correct unit (mm or cm).
- Optionally add scan date to estimate due date.
- Click Calculate.
Typical CRL values by gestational age (approximate)
Reference ranges can vary by source and population. The values below are quick orientation points, not strict cutoffs:
| Gestational Age | Approximate CRL |
|---|---|
| 6 weeks | 2-5 mm |
| 7 weeks | 7-12 mm |
| 8 weeks | 14-20 mm |
| 9 weeks | 22-30 mm |
| 10 weeks | 31-40 mm |
| 11 weeks | 41-51 mm |
| 12 weeks | 52-63 mm |
| 13 weeks | 64-74 mm |
| 14 weeks | 75-84 mm |
How accurate is CRL dating?
In early pregnancy, CRL dating is generally considered more accurate than later biometric measures for setting the due date. Small differences can still happen because of measurement angle, fetal position, ultrasound equipment, and operator technique. Your clinician may prioritize the earliest reliable ultrasound for final dating decisions.
Factors that can affect measurement
- Suboptimal fetal positioning during scan
- Image quality and probe settings
- Differences between sonographers and machines
- Very early or late first-trimester scans outside ideal CRL range
Frequently asked questions
Can this replace my ultrasound report?
No. This calculator is a practical estimate tool. Your official report and obstetric clinician provide the authoritative interpretation.
What if my CRL is outside 2-84 mm?
The model is most useful in standard first-trimester ranges. If your value is outside this interval, interpret results cautiously and confirm with your provider.
Why might due date estimates differ from other apps?
Different apps may use different equations, rounding rules, or assumptions. Your care team’s dating protocol should always take priority.