Crown-rump Length Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Steven Wooding and Aleksandra Zając, MD
Based on research by
H. P. Robinson, J. E. E. Fleming A Critical Evaluation of Sonar Crown-rump Length Measurements An International Journal of Obstetrics and Gynaecology (Sep 1975)See 1 more source
R Napolitano, J Dhami,EO Ohuma, C Ioannou, A Conde-Agudelo, SH Kennedy, J Villar, AT Papageorghiou Pregnancy dating by fetal crown–rump length: a systematic review of charts An International Journal of Obstetrics and Gynaecology (Jan 2014)
Last updated: Jul 20, 2022

Our crown-rump length calculator is the most accurate way to assess embryonic and fetal age and the duration of the early pregnancy.

Our tool will help you calculate the gestational age from CRL; we'll also talk about how accurate is an ultrasound in determining the age of the fetus at different stages of the pregnancy. 👶

Read on to find out even more:

  • Discover the CRL meaning.
  • Figure out details of the CRL ultrasound. 🏥
  • Learn how to calculate gestational age with crown rump length.
  • Take a look at the CRL measurement chart.

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment. If any health condition bothers you, consult a physician.

What is baby CRL and how to measure it?

CRL stands for the crown-rump length – the distance measured from the top of the head to the buttocks of an embryo (up to the 8th week after the conception) or a fetus (8-14th week of the pregnancy).

Why do we use it? 🤰🏾

To put it short: The CRL is used to determine the gestational age in the early stages of pregnancy. It's the most efficient way to find out your baby's age – the doctor should always compare it to the date of your LMP (last menstrual period).

When do we use it?

Perfect conditions for estimating your baby's age and use of the CRL charts are:

  • Natural conception;
  • CRL measurement between 9+0 and 13+6 weeks of pregnancy;
  • Known last menstrual period;
  • Regular menstrual cycles; and
  • Without the use of the pill or breastfeeding during the previous 2 months.

Discovering the baby's age using CRL and ultrasound

So how exactly does it work? 🤷

Once your pregnancy test came out positive, your head was probably pounding with conflicting emotions and excitement; your next step should be a visit to your OB-GYN (obstetrics (pregnancy) and gynecology (female reproductive health) doctor) to confirm the age of the pregnancy (gestational age).

  1. If your pregnancy is early on, your physician will: 🔅
    • Ask you about the date of the first day of your last period;
    • Perform an ultrasound examination, find the CRL and compare it to the crown-rump length chart; and
    • Perform a full physical examination & take the detailed history.

💡 Visit your gynecologist early on! Late pregnancy measurement are
not as accurate as they should be – on top of that, frequent visits to your doctor will make you sure that your baby's safe and sound!

  1. If your pregnancy is more advanced, your physician will: 🔆
    • Ask about your last period;
    • Perform the ultrasound and check the length of your baby's head, bones, and belly – all these values will be then computed using the fetal weight percentile tool; and
    • Perform a full physical examination, e.g., check the fundal height & take the detailed history.

All this information is then computed, using special equations (usually built-in into the ultrasound machine), and compared against your last menstrual period date.

Look here to find out the specific formulas we used in the CRL calculator.

As a rule:

  • I trimester (first trimester) measurements are used to determine the baby's age and the estimated date of delivery (or use the due date calculator).
  • II + III trimester measurements are used to assess the baby's growth, wellbeing, and rule out the possibility of any diseases or malformations.

How to use the crown-rump length calculator?

Our crown-rump length and gestational age calculator gives you a straightforward answer to your question – enter your CRL measurements into the proper field and await your result; in the format of hours, days, weeks, or mixed!

Our tool contains the functionality of a crown-rump length calculator in cm – you may choose any from 3 different units of the CRL.

Want to know more about the wonderful process of pregnancy? Let's take a tour from the beginning to the grand finale: 🍼

How to calculate gestational age with crown-rump length?

Here comes the time when we'd like to calculate gestational age from the CRL ourselves (or just replicate our CRL calculator's results 🙃); what's our first step?

  1. Choosing the best equation possible. There are plenty of different formulas that we can use for assessing fetal age.

    We decided to use the corrected Robinson and Fleming equation. The INTERGROWTH–21st Project, an international study of fetal growth, ranked it among the 4 most accurate fetal assessment equations.

    On top of that, this equation is one of the easiest formulas found. 😉

    Gestational age (in days) = 8.052 * √(CRL * 1.037) + 23.73,


    • CRL is given in millimeters (mm).
  2. A second option is to use a special CRL measurement chart; it's definitely easier than calculations but still more troublesome than using our crown-rump length calculator. 🙊

    The crown-rump length chart:

    CRL (in mm)Gestational age (weeks+days)
    56 + 0
    107 + 1
    157 + 6
    208 + 4
    259 + 2
    309 + 6
    3510 + 2
    4010 + 6
    4511 + 2
    5011 + 5
    5512 + 1
    6012 + 3
    6512 + 6
    7013 + 1
    7513 + 4
    8013 + 6
    8514 + 1

    Hey, we'd like you to know that this particular CRL chart is also based on the already mentioned Robinson and Fleming equation :)

Łucja Zaborowska, MD, PhD candidate
Crown rump length
Gestational age
Pregnancies over 14 weeks require different kind of measurements. Try the fetal weight tool. 🤰
Check out 32 similar fertility & pregnancy calculators 🤰
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