Introduction
The Crown Rump Length (CRL) is the measurement from the top of a baby's head to its bottom, taken during an early ultrasound. Doctors use this measurement to figure out how far along a pregnancy is and to estimate a due date. ACOG puts the accuracy of CRL dating up to and including 13 weeks and 6 days at plus or minus 5 to 7 days, and calls first-trimester ultrasound the most accurate dating method.3
This Crown Rump Length Calculator works in two ways. In the first mode, you enter a CRL measurement in millimeters or centimeters, and it tells you the gestational age and estimated due date. In the second mode, you enter a gestational age in weeks and days, and it tells you the expected CRL for that stage of pregnancy.
The calculator uses two trusted medical formulas: Robinson & Fleming (1975) and Tan et al. (2023). Robinson and Fleming fitted 334 measurements at 6 to 14 weeks and found that a single measurement dates a pregnancy to within 4.7 days with 95 per cent probability.1 Tan and colleagues fitted a cubic curve to 1,436 IVF-dated scans, with a mean absolute error of 0.97 days.2 It shows results from both so you can compare them side by side. You also get a step-by-step breakdown of the math and a chart that plots CRL against gestational age. This tool is meant for educational purposes and does not replace advice from your doctor or midwife.
How to Use Our Crown Rump Length Calculator
Enter your ultrasound measurement or gestational age to get results. This calculator gives you the estimated gestational age, due date, or expected CRL using two trusted medical formulas.
Mode 1: CRL to Gestational Age & Due Date
Crown-Rump Length (CRL): Type in the CRL number from your ultrasound report. Use the mm or cm button to match the unit shown on your report. Values must be between 2 and 95 mm.
Date of Ultrasound Scan: Pick the date your ultrasound was done. This defaults to today. The calculator uses this date to figure out your estimated due date.
Press Calculate to see your gestational age in weeks and days, plus your estimated due date, from both the Robinson & Fleming and Tan et al. formulas.
Mode 2: Gestational Age to Expected CRL
Gestational Age (Weeks): Choose the number of weeks from the dropdown. You can pick any value from 6 to 14 weeks.
Additional Days: Choose the extra days (0 to 6) to add on top of the weeks you selected.
Press Calculate to see the expected CRL in both millimeters and centimeters for that gestational age. A step-by-step solution and growth chart are shown below the results.
What Is Crown Rump Length (CRL)?
Crown rump length is the measurement of a baby from the top of its head (crown) to the bottom of its buttocks (rump). Doctors take this measurement during an ultrasound scan in early pregnancy, usually between 6 and 14 weeks. It is one of the most accurate ways to figure out how far along a pregnancy is and when the baby is due.
Why Is CRL Important?
In the first trimester, all babies grow at nearly the same rate. This makes CRL a very reliable way to estimate gestational age, which is how many weeks and days pregnant someone is. Once the doctor knows the gestational age, they can calculate the estimated due date (EDD). An accurate due date helps track the baby's growth and plan for delivery. If you are trying to determine when conception occurred, CRL-based dating can also help you count backward from the gestational age to a likely conception window.
How Is CRL Measured?
A doctor or ultrasound technician measures the baby in a straight line from head to bottom. The baby is measured in millimeters. The best time to get this measurement is between 7 and 13 weeks of pregnancy. During this window the measurement is most accurate: ACOG gives plus or minus 5 to 7 days up to 13 weeks and 6 days, and beyond a CRL of 84 mm, about 14 weeks, it advises other measurements.3 After that point the baby starts to curl and bend, and measurements like head size and thigh bone length are used instead.
Formulas Used in This Calculator
This calculator uses two well-known formulas to estimate gestational age from a CRL measurement:
- Robinson & Fleming (1975): One of the oldest and most widely used formulas in clinical practice. It calculates gestational age using the square root of the CRL value. Their paper reports the curve's accuracy.1
- Tan et al. (2023): A 2023 cubic formula fitted to 1,436 first-trimester scans of IVF pregnancies with known transfer dates.2
Both formulas give results in days, which are then converted into weeks and days. The calculator also works in reverse. You can enter a gestational age to find out the expected CRL for that stage of pregnancy.
What Is the Estimated Due Date?
By convention the due date is 280 days from the first day of the last menstrual period.3 The calculator takes the gestational age from the CRL, subtracts it from 280 days, and adds the remaining days to the scan date. The result is the estimated due date. Few babies are born on the due date itself; it is a best estimate, not a guarantee.