Calculate gestational age from crown rump length or estimate CRL from gestational age in weeks or days for first-trimester dating.
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Crown Rump Length Formula
The calculator uses the historical Robinson and Fleming (1975) crown rump length dating relationship. Select gestational age or crown-rump length to calculate the other. The inverse estimates a point on this dating curve and does not assess fetal growth or viability.
- GA_days = gestational age in days
- CRL_mm = crown rump length in millimeters
- sqrt = square root
If you enter crown rump length, the calculator converts the CRL value to millimeters, applies the first formula, then converts the gestational age result to weeks or days.
If you enter gestational age, the calculator converts the age to days, applies the rearranged formula, then converts the CRL result to millimeters, centimeters, or inches.
This calculator restricts CRL to 5–84 mm and the inverse age to approximately 41.734819–97.527799 days. These are operational first-trimester limits, not diagnostic thresholds. Clinical ultrasound interpretation and the applicable dating protocol are required.
Crown Rump Length Values Computed from This Formula
These values are calculated from the inverse formula, rounded to one decimal. They are not fetal growth reference ranges or normality thresholds.
| Gestational age | Days | Approx. CRL |
|---|---|---|
| 6 weeks | 42 days | 5.1 mm |
| 7 weeks | 49 days | 9.8 mm |
| 8 weeks | 56 days | 16.1 mm |
| 9 weeks | 63 days | 23.8 mm |
| 10 weeks | 70 days | 33.0 mm |
| 11 weeks | 77 days | 43.8 mm |
| 12 weeks | 84 days | 56.0 mm |
| 13 weeks | 91 days | 69.8 mm |
Unit Conversions Used for CRL and Gestational Age
| Measurement | Conversion |
|---|---|
| 1 centimeter | 10 millimeters |
| 1 inch | 25.4 millimeters |
| 1 week | 7 days |
Crown Rump Length Examples
Example 1: Calculate gestational age from CRL
If the crown rump length is 20 mm:
That is about 8.53 weeks, or approximately 8 weeks and 4 days.
Example 2: Calculate CRL from gestational age
If the gestational age is 10 weeks, first convert weeks to days:
Then apply the CRL formula:
The estimated crown rump length is about 33.0 mm.
Crown Rump Length FAQ
What is crown rump length?
Crown rump length, or CRL, is the ultrasound measurement from the top of the embryo or fetus to the bottom of the torso. It does not include the legs. CRL is commonly used in the first trimester because it is one of the most reliable early measurements for estimating gestational age.
When is CRL most useful?
CRL is most useful in the first trimester, especially from about 6 to 14 weeks of pregnancy. After this period, other ultrasound measurements are usually used because fetal position and body proportions make CRL less practical.
Why might a CRL-based date differ from the last menstrual period date?
A CRL-based gestational age can differ from a date based on the last menstrual period if ovulation happened earlier or later than expected, if cycle length is irregular, or if the menstrual date is uncertain. Ultrasound dating from CRL is often used to clarify dating in early pregnancy, but any medical interpretation should come from a qualified healthcare professional.
