Crown-Rump Length Calculator (CRL to gestational age and due date)

Understanding the crown-rump length calculator — concept and principle

This calculator estimates gestational age and the due date from the crown-rump length (CRL) measured on a first-trimester ultrasound, using the INTERGROWTH-21st international standard, and shows the classic Robinson–Fleming estimate for comparison. Expectant parents reading a scan report and sonography students use it.

Early-pregnancy length = clock — with the default inputs: 11 weeks + 1 day.

How it is calculated

INTERGROWTH-21st: gestational age (days) = 40.9041 + 3.21585 × √CRL + 0.348956 × CRL, CRL in mm, valid for 15–95 mm
Robinson–Fleming: days = 8.052 × √CRL + 23.73
Gestational age is shown as completed weeks + days
Due date = scan date − gestational age + 280 days
Equivalent first day of last period = scan date − gestational age

The principle

In the first trimester, embryos and fetuses grow at very similar rates, so their length is a precise clock. Crown-rump length is the straight-line length from the top of the head to the bottom of the rump. The INTERGROWTH-21st project measured healthy pregnancies in several countries to produce an international standard; the older Robinson–Fleming formula from 1975 gives very similar results. Between about 9 and 14 weeks, CRL dating is the most accurate ultrasound method.

Worked example

Defaults: CRL 45 mm, scan today (shown for 10 November 2026).

√45 = 6.708; GA = 40.9041 + 3.21585 × 6.708 + 0.348956 × 45 = 78.2 days, rounded to 78 days = 11+1.
Robinson–Fleming = 8.052 × 6.708 + 23.73 = 77.7 days; difference = 0.4 days.
Equivalent first day of last period = scan − 78 days = Mon, Aug 24, 2026.
Due date = scan − 78 + 280 days = Mon, May 31, 2027.

How to read the result

When a first-trimester scan and the last-period date disagree, the clinic decides which to use according to its dating rules; often the scan is preferred when the difference is large. CRL outside 15–95 mm needs other measurements, such as head circumference.

Common mistakes

Using the formula outside 15–95 mm.
Measuring a curved or flexed fetus, which shortens CRL.
Writing 11+1 as 11.1 weeks.
Changing the due date with later scans without clinical advice.

Keep in mind: Not a diagnosis. Pregnancy dating is set by your care team from the scan and your history.

Frequently asked questions

How accurate is dating by CRL?

It is the most accurate ultrasound dating method in the first trimester.

What does 11+1 mean?

Eleven completed weeks and one day.

Why compare two formulas?

They usually agree within about a day, which shows the estimate is stable.

Sources: Papageorghiou AT, Kennedy SH, Salomon LJ, et al. (INTERGROWTH-21st), Ultrasound Obstet Gynecol 2014;44:641–648 · Robinson HP, Fleming JEE, Br J Obstet Gynaecol 1975;82:702–710

Checked 9 Nov 2026. Papageorghiou AT, Kennedy SH, Salomon LJ, et al. (INTERGROWTH-21st), “International standards for early fetal size and pregnancy dating based on ultrasound measurement of crown–rump length in the first trimester of pregnancy”, Ultrasound Obstet Gynecol 2014;44:641–648 (GA equation, CRL 15–95 mm); Robinson HP, Fleming JEE, Br J Obstet Gynaecol 1975;82:702–710 (standard value).

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