Crown-Rump Length Calculator
Use one mode to estimate gestational age from CRL, or reverse the equation to estimate expected CRL from gestational age.
Example Data Table
These examples show how averaged CRL values can map to gestational age and expected delivery date.
| Scan Date | CRL 1 (mm) | CRL 2 (mm) | CRL 3 (mm) | Mean CRL (mm) | Estimated GA | EDD |
|---|---|---|---|---|---|---|
| 2026-01-15 | 18.0 | 18.2 | 18.4 | 18.2 | 8 weeks 5 days | 2026-08-22 |
| 2026-01-22 | 34.1 | 34.6 | 35.1 | 34.6 | 10 weeks 2 days | 2026-08-18 |
| 2026-02-03 | 51.8 | 52.1 | 52.4 | 52.1 | 11 weeks 5 days | 2026-08-20 |
| 2026-02-11 | 67.3 | 67.8 | 68.3 | 67.8 | 13 weeks 0 days | 2026-08-19 |
Formula Used
- Mean CRL = average of all entered CRL readings.
- Gestational age in days = 40.9041 + 3.21585 × √CRL + 0.348956 × CRL.
- Dating SD in days = 2.39102 + 0.0193474 × CRL.
- Approximate 95% dating band = gestational age ± 1.96 × SD.
- Expected delivery date = scan date + (280 − gestational age in days).
- Reverse mode solves the same equation for CRL by using a quadratic in √CRL.
CRL is entered in millimeters. Delivery dating is shown only when a scan date is available.
How to Use This Calculator
- Choose whether you want to calculate from CRL or from gestational age.
- Enter the scan date if you want an expected delivery date.
- For CRL mode, enter one to three readings in millimeters.
- For gestational age mode, enter completed weeks and extra days.
- Select the decimal precision that suits your reporting style.
- Press Calculate to show the result above the form.
- Download the result as CSV or PDF after calculation.
Frequently Asked Questions
1. What is crown-rump length?
Crown-rump length is the straight-line fetal measurement from the top of the head to the rump. It is commonly used in early pregnancy to help estimate gestational age.
2. When is CRL most useful for pregnancy dating?
CRL is most useful during the first trimester. It is generally treated as strongest for early dating and becomes less reliable once the fetus is larger and positioning varies more.
3. Why does this calculator allow three CRL readings?
Multiple readings help smooth out small measurement differences. Averaging repeated values can produce a steadier estimate than relying on one reading alone, especially when fetal position changes slightly.
4. Does this tool replace a medical report?
No. This calculator is an educational and record-support tool. Clinical dating, scan interpretation, and care decisions should always be confirmed by a qualified prenatal care professional.
5. Why might the expected delivery date differ from another estimate?
Different clinics may use different dating charts, measurement methods, or scan dates. A small difference can also appear when one estimate uses menstrual dating and another uses ultrasound dating.
6. What happens if CRL is above 84 mm?
The calculator still returns a value, but it also warns that CRL dating is usually less reliable above that size. Later pregnancy dating often relies more on other fetal measurements.
7. Can I estimate CRL from gestational age?
Yes. Switch to the reverse mode, enter gestational weeks and extra days, and the calculator will estimate the CRL that matches the same dating equation.
8. Which unit should I enter for CRL?
Enter crown-rump length in millimeters. The calculator also displays centimeters in the result panel to make printed reporting and quick review easier.