Crown Rump Length Calculator
Use this crown rump length calculator to turn an early ultrasound CRL measurement into gestational age in weeks and days and an estimated due date.
Updated October 21, 2026
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Gestational Age Is Calculated From Crown Rump Length
Crown rump length (CRL) is the length of the embryo or fetus from the top of the head (the crown) to the bottom (the rump), measured on an ultrasound scan in the first trimester. This one measurement is the most accurate way to work out how far along a pregnancy is³. The calculator turns it into an age on the scan date, then counts forward to a due date.
What Crown Rump Length Measures
CRL measures size, and the formula converts that size into age. It is used from about 6 weeks up to a CRL of 84 mm, which is about 14 weeks³. Above 84 mm, CRL becomes less accurate for dating, so scans use head and body measurements instead³.
The Formula
The main result uses the Robinson and Fleming method, and a second line shows the INTERGROWTH-21st method for comparison:
Robinson: gestational age (days) = 8.052 x √CRL + 23.73
INTERGROWTH-21st: gestational age (days) = 40.9041 + 3.21585 x √CRL + 0.348956 x CRL
Due date = scan date + (280 - gestational age in days)
CRL is in millimeters. Robinson and Fleming built their formula in 1975 from 334 scans taken between 6 and 14 weeks¹. INTERGROWTH-21st is an international standard published in 2014, and it covers CRL from 15 to 95 mm².
How This Calculator Computes Your Result
Enter the CRL from your scan report in mm or cm (cm is multiplied by 10) and the scan date. The tool accepts 5 to 84 mm. It rounds the age to the nearest whole day, shows it as weeks and days (11+1 means 11 weeks 1 day), and adds the days left to reach 40 weeks to give your due date. From 15 mm, it also shows the INTERGROWTH-21st age and due date. If you add the first day of your last period, it shows the gap between your period age and your scan age.
What Your Result Means
Your result is the estimated age of the pregnancy on the scan date. Gestational age is counted from the first day of the last period, not from conception, so at 6 weeks the embryo is about 4 weeks past conception. This table shows typical results from both methods:
CRL (mm) | Robinson age | INTERGROWTH-21st age |
5 | 6 weeks 0 days | not covered |
10 | 7 weeks 0 days | not covered |
20 | 8 weeks 4 days | 8 weeks 6 days |
30 | 9 weeks 5 days | 9 weeks 6 days |
45 | 11 weeks 1 day | 11 weeks 1 day |
60 | 12 weeks 2 days | 12 weeks 3 days |
70 | 13 weeks 0 days | 13 weeks 1 day |
84 | 14 weeks 0 days | 14 weeks 2 days |
If the scan age and your period age are close, the scan has confirmed your dates. If they are further apart than ACOG's limit for your stage, ACOG guidance supports using the scan date, and your care team decides (see below).
When the Scan Changes Your Due Date
The American College of Obstetricians and Gynecologists (ACOG) says first trimester ultrasound is the most accurate way to set or confirm a due date³. Its Committee Opinion 700 (2017, reaffirmed 2022) gives these limits for using a CRL scan instead of the period date³:
Age at the scan | Gap that supports using the scan date |
Up to 8 weeks 6 days | more than 5 days |
9 weeks 0 days to 13 weeks 6 days | more than 7 days |
So a CRL that puts you 4 days behind your period date at 12 weeks does not meet the limit. A gap of 9 days at the same stage is beyond it. The calculator shows which limit applies to your result, but your care team decides your official due date. ACOG also says that once a due date is set from an early scan, later changes should be rare³.
How CRL Is Measured on the Scan
You do not measure CRL yourself. The sonographer measures it as a straight line from crown to rump. The yolk sac should not be included, because that makes the baby look older in very early pregnancy⁴. ACOG advises using the average of three separate CRL measurements when possible³. If the baby stays curled up at 13 weeks or later, the UK guidance says head circumference may be the better measurement for dating⁴. When you enter your number, use the final CRL on the report, not the size of the gestational sac.
Why Your Scan Report May Differ by a Day
Clinics and ultrasound machines use different dating charts. The UK's professional body (BMUS) adopted a version of the Robinson formula that multiplies CRL by 1.037, which makes results 0 to 2 days older than the plain formula used here⁴. Using the published INTERGROWTH-21st formula, results are within about 1 day of Robinson between about 30 and 70 mm, about 2 days older near 84 mm, and up to 4 days older near 15 mm². A 1 or 2 day difference between this tool and your report is a chart difference, not a sign that something is wrong.
How Much Older INTERGROWTH-21st Dates a Pregnancy Than Robinson
For the same CRL, INTERGROWTH-21st gives an older age than Robinson. The gap is largest at the small end and smallest from about 30 to 70 mm.
- Days older on INTERGROWTH-21st
Show data table
| Crown rump length (mm) | Days older on INTERGROWTH-21st (days) |
|---|---|
| 15 mm | 3.7 |
| 20 mm | 2.5 |
| 25 mm | 1.7 |
| 30 mm | 1.2 |
| 40 mm | 0.5 |
| 50 mm | 0.4 |
| 60 mm | 0.7 |
| 70 mm | 1.1 |
| 80 mm | 1.8 |
| 84 mm | 2.2 |
A Note on Accuracy
First trimester CRL dating is accurate to about 5 to 7 days either way, and it is more accurate earlier in pregnancy³. Robinson and Fleming found one measurement dated a pregnancy within 4.7 days (95% of the time), and three measurements within 2.7 days¹. The formula assumes typical early growth, so it cannot tell a dating difference from a growth problem. If you conceived through IVF, your due date is set from the embryo transfer date instead³. This is an educational estimate, and your care team sets your official due date.
Frequently Asked Questions
What is a normal crown rump length at 12 weeks?
At exactly 12 weeks the average CRL is about 56 mm, and 94% of pregnancies in the INTERGROWTH-21st standard fall between about 44 and 67 mm².
How accurate is crown rump length for dating a pregnancy?
CRL is the most accurate dating method, good to about 5 to 7 days either way in the first trimester³. It is most precise when measured early and averaged over three readings¹ ³.
Is the ultrasound due date more accurate than the last period due date?
Yes. ACOG calls first trimester ultrasound the most accurate way to set or confirm a due date³. If the gap between the two is larger than ACOG's limit for your stage, the scan date is the one generally used³.
Can my due date change after a later ultrasound?
Once a due date has been set from an early CRL scan, ACOG advises that later changes should be rare³. Your care team makes that call.
Sources
A Critical Evaluation of Sonar Crown-Rump Length Measurements, Robinson and Fleming, 1975
International Standards for Early Fetal Size and Pregnancy Dating Based on Crown-Rump Length, Papageorghiou et al., 2014
Methods for Estimating the Due Date (Committee Opinion 700), ACOG, 2017
Fetal Size and Dating: Charts Recommended for Clinical Obstetric Practice, Loughna et al., 2009