Crown Rump Length Calculator

The Crown Rump Length Calculator estimates gestational age and due date based on embryo length and the measurement date. This tool helps obstetricians and expectant parents help track pregnancy progress and confirm dating. Whether you are verifying an early ultrasound, assessing growth percentiles, or comparing LMP dates, this calculator provides reference clinical insights.

Enter the crown-rump length measurement in millimeters (mm)
Date when the ultrasound was performed
First day of last menstrual period for comparison

Important: Ultrasound dating becomes better-informed than menstrual dates as pregnancy progresses. This tool is for educational purposes only and does not constitute medical advice, screening assessment, or treatment. Always consult with a qualified healthcare provider for professional medical advice regarding pregnancy or fetal health.

How Estimated Gestational Age Is Calculated

Estimated Gestational Age is calculated by measuring the length of the embryo from the top of the head to the bottom of the buttocks. This measurement uses the Robinson and Fleming formula to convert millimeters into weeks and days.

GA = 8.052 × √(CRL) + 23.73

Where:

  • GA = Gestational Age in days
  • CRL = Crown Rump Length in millimeters

First, the tool takes your Crown Rump Length input in millimeters. Then, it applies the mathematical equation to determine the estimated age of the pregnancy. This method is commonly used for dating pregnancies between 7 and 14 weeks because it relies on actual fetal growth rather than just calendar dates.

What Your Estimated Gestational Age Means

This result provides an estimate of the specific stage of pregnancy development. It provides information to help confirm if the baby is growing at the expected rate for that time frame.

Early Pregnancy (5 to 9 Weeks): At this stage, the embryo is very small, so a slight difference in measurement can change the dates by a few days. This is a good time to confirm the pregnancy viability.

Optimal Dating Window (10 to 14 Weeks): Measurements taken here are the standardized for setting a due date. If the ultrasound age differs from your last period by more than 5 days, doctors usually reset the due date based on this scan.

Growth Percentiles: A percentile between 10 and 90 is considered average. Scores below 10 may indicate the baby is measuring smaller than average, while scores above 90 suggest they are larger than average.

Use these results to discuss your specific timeline and next steps with your doctor or midwife.

Related Calculators

Calculation logic verified using publicly available standards.

View our Accuracy & Reliability Framework →