Crown Rump Length Calculator

Q: What CRL values does this calculator accept?

This calculator accepts CRL values from 2 mm to 95 mm . You can also enter the value in centimeters (0.2 to 9.5 cm) by clicking the cm button. The calculator converts centimeters to millimeters automatically before doing the math.

Q: How accurate is a CRL-based gestational age?

Between 7 and 13 weeks, CRL is accurate to within about 3 to 5 days of the true gestational age. Before 7 weeks, the baby is very small, so the margin of error is wider. After 13 weeks, accuracy drops because growth rates start to vary between babies.

Q: What do the colored flags mean?

The flags give you quick context about your CRL value. A green flag means your CRL is in the optimal dating range (7–13 weeks). A blue flag means the measurement is early and may have a wider margin of error. A yellow flag means the CRL is above 84 mm, where dating becomes less reliable. A red flag means the value is outside the accepted range.

Crown Rump Length Calculator

Updated Jul 19, 2026 By Jehan Wadia

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. CRL is most accurate between 7 and 13 weeks of pregnancy.

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). 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, usually within 3 to 5 days of the true gestational age. After about 14 weeks, the baby starts to curl and bend, which makes CRL less reliable. At that point, other measurements like head size and thigh bone length are used instead. As pregnancy progresses, tools like the Baby Percentile Calculator can help you track how your baby's growth compares to standard ranges.

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.
Tan et al. (2023) — A newer formula based on updated research. It uses a polynomial equation that may better fit modern ultrasound data.

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. If your pregnancy was achieved through assisted reproduction, our IVF Due Date Calculator can provide a due date estimate tailored to your transfer or retrieval date.

What Is the Estimated Due Date?

A full-term pregnancy lasts about 280 days (40 weeks) from the first day of the last menstrual period. 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. Keep in mind that only about 5% of babies are born on their exact due date. It is a best estimate, not a guarantee. You can also use our Due Date Calculator if you know your last menstrual period date and want a quick due date based on that. To monitor healthy weight changes throughout your pregnancy, the Pregnancy Weight Gain Calculator is another helpful companion tool. Tracking your ovulation cycle alongside CRL dating can also give you a clearer picture of your pregnancy timeline.

Formulas used

Robinson & Fleming – Gestational Age from CRL

\text{EGA (days)} = 8.052 \times \sqrt{\text{CRL} \times 1.037} + 23.73

Tan et al. – Gestational Age from CRL

\text{EGA (days)} = 37.31 + 1.39 \times \text{CRL} - 0.014 \times \text{CRL}^2 + 0.00007 \times \text{CRL}^3

Inverse Robinson & Fleming – CRL from Gestational Age

\text{CRL} = \frac{\left(\dfrac{\text{EGA} - 23.73}{8.052}\right)^{2}}{1.037}

Estimated Due Date

\text{EDD} = \text{Scan Date} + (280 - \text{EGA})

Frequently asked questions

What CRL values does this calculator accept?

This calculator accepts CRL values from 2 mm to 95 mm. You can also enter the value in centimeters (0.2 to 9.5 cm) by clicking the cm button. The calculator converts centimeters to millimeters automatically before doing the math.

What is the difference between the Robinson & Fleming and Tan et al. formulas?

Robinson & Fleming (1975) is an older formula that uses a square root equation. Tan et al. (2023) is a newer formula that uses a polynomial equation based on more recent data. Both are trusted in medicine. This calculator shows results from both so you can compare them. In most cases, the two results are very close.

Which formula should I trust more?

Both formulas are clinically accepted. Robinson & Fleming has been used for decades and is well established. Tan et al. is based on newer research and may better match modern ultrasound equipment. Your doctor will decide which one fits your situation best. Most of the time, the results differ by less than a day or two.

Why do the two formulas give slightly different results?

Each formula was developed using different study groups and different mathematical methods. Small differences in the data sets and equations lead to slight variations in the output. A difference of one or two days between the two is normal and not a cause for concern.

Can I use this calculator after 14 weeks of pregnancy?

CRL is not reliable after about 14 weeks because the baby starts to curl and bend. After that point, doctors use other measurements like biparietal diameter (BPD), head circumference (HC), and femur length (FL) to estimate gestational age. This calculator supports CRL values up to 95 mm, which roughly corresponds to 14 weeks.

How accurate is a CRL-based gestational age?

Between 7 and 13 weeks, CRL is accurate to within about 3 to 5 days of the true gestational age. Before 7 weeks, the baby is very small, so the margin of error is wider. After 13 weeks, accuracy drops because growth rates start to vary between babies.

What does the scan date do in the calculator?

The scan date anchors your results to the calendar. The calculator uses it to figure out your estimated due date. It takes 280 days (a full-term pregnancy), subtracts the gestational age from the CRL, and adds the remaining days to the scan date. If you leave this field blank, it defaults to today.

What do the colored flags mean?

The flags give you quick context about your CRL value. A green flag means your CRL is in the optimal dating range (7–13 weeks). A blue flag means the measurement is early and may have a wider margin of error. A yellow flag means the CRL is above 84 mm, where dating becomes less reliable. A red flag means the value is outside the accepted range.

How do I switch between millimeters and centimeters?

Click the mm or cm button next to the CRL input field. The calculator will automatically convert your current value to the new unit. All internal math is done in millimeters regardless of which unit you choose.

What does Mode 2 of the calculator do?

Mode 2 lets you enter a gestational age in weeks and days, and it tells you the expected CRL for that stage of pregnancy. This is useful if you want to check whether a measured CRL matches the expected size for a known gestational age.

Can this calculator diagnose a problem with my pregnancy?

No. This calculator is for educational purposes only. It estimates gestational age and expected CRL using published formulas. It cannot diagnose any medical condition. If your CRL does not match your expected gestational age, talk to your doctor. They will evaluate your results in context.

What if my CRL is smaller or larger than expected?

A CRL that is a few days off from the expected value is usually normal. A significantly smaller CRL could mean the pregnancy is earlier than thought, or in some cases it may signal a concern. A larger CRL could mean the pregnancy is further along. Your doctor will interpret the results and may schedule a follow-up scan.

Does the chart update when I change my CRL value?

Yes. Every time you enter a new CRL value or change the gestational age, the chart updates automatically. It plots both the Robinson & Fleming and Tan et al. curves and marks your specific data point on each curve.

What gestational age range does Mode 2 support?

Mode 2 supports gestational ages from 6 weeks 0 days to 14 weeks 6 days. This covers the window where CRL is a meaningful measurement for pregnancy dating.

Is the estimated due date from this calculator the same as my doctor's due date?

It may be close but not always the same. Doctors may adjust the due date based on your last menstrual period, other ultrasound measurements, IVF transfer dates, or clinical judgment. A difference of a few days between this calculator and your doctor's estimate is common and normal.

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