About the CRL Gestational Age Calculator
This CRL gestational age calculator converts a first-trimester crown-rump length (CRL) — the straight-line length of the embryo or fetus from the top of the head to the bottom of the rump — into gestational age in weeks and days, the equivalent first day of your last period, and an ultrasound due date. You can choose the Robinson & Fleming formula (the classic reference used by many scanners), Hadlock’s 1992 real-time ultrasound curve, or the INTERGROWTH-21st international standard.
It is for expectant parents reading a dating-scan report and for students checking how dating works. Add the scan date and, optionally, your last menstrual period (LMP) to see the difference in days. The tool applies the American College of Obstetricians and Gynecologists (ACOG) guidance: before 9 0/7 weeks a difference of more than 5 days is a reason to change the due date, and from 9 0/7 to 13 6/7 weeks a difference of more than 7 days.
ACOG notes that first-trimester CRL dating is the most accurate way to establish gestational age (about ±5–7 days), that the measurement should ideally be the mean of three CRL measurements, and that beyond 84 mm (about 14 weeks) other measurements such as head circumference should be used instead. Your sonographer’s or doctor’s dating always takes precedence over this estimate.
How to use the crl gestational age calculator
- 1Find the CRL on your dating-scan report (in millimetres).
- 2Choose a formula — leave Robinson & Fleming if you are unsure.
- 3Enter the date of the scan.
- 4Optionally add the first day of your last period to compare the two dates.
- 5Read the gestational age, ultrasound due date and the ACOG redating check.
Formula and method
Each formula is a regression curve fitted to pregnancies with known dates. Robinson & Fleming (1975) is the long-standing reference built into many ultrasound machines; Hadlock (1992) was derived with high-resolution real-time ultrasound for CRLs from 2 mm to 12 cm; INTERGROWTH-21st (2014) is a prescriptive international standard built from healthy pregnancies in eight countries for CRLs of roughly 15–95 mm. The three agree within a day or two through most of the first trimester. Some software uses a “modified Robinson” version that multiplies CRL by 1.037 before taking the square root, which adds up to about a day.
Gestational age is rounded to the nearest whole day. The due date assumes the conventional 280-day (40-week) pregnancy counted from the equivalent LMP. When you add your LMP, the difference is ultrasound age minus LMP age; ACOG Committee Opinion 700 (reaffirmed 2025) supports changing the due date when that difference exceeds 5 days before 9 0/7 weeks, or 7 days from 9 0/7 to 13 6/7 weeks. This tool uses the ultrasound age to pick the threshold.
- CRL
- Crown-rump length in millimetres
- L
- Crown-rump length in centimetres (Hadlock)
- GA
- Gestational (menstrual) age on the day of the scan
Worked examples
45 mm CRL that matches the LMP
Robinson & Fleming gives 8.052 × √45 + 23.73 ≈ 77.7, so about 78 days or 11 weeks 1 day on 20 March 2026. Counting back 78 days lands on 1 January 2026 — exactly the reported LMP — so both methods give a due date of 8 October 2026.
Early scan smaller than dates
A 12 mm CRL gives 8.052 × √12 + 23.73 ≈ 51.6, or 52 days (7 weeks 3 days), while the LMP puts the pregnancy at 59 days. The ultrasound is 7 days behind; because the scan is before 9 weeks, any difference over 5 days supports moving the due date to 15 October 2026.
INTERGROWTH-21st at 62 mm
INTERGROWTH-21st gives 40.9041 + 3.21585 × √62 + 0.348956 × 62 ≈ 87.9, or 88 days (12 weeks 4 days). The LMP says 91 days, a 3-day difference — inside the 7-day window for scans between 9 and 14 weeks — so the LMP due date of 8 October would normally be kept.
Frequently asked questions
What is crown-rump length (CRL)?+
CRL is the longest straight-line measurement of the embryo or fetus from the top of the head (crown) to the bottom of the buttocks (rump), excluding the legs. It is measured on a first-trimester ultrasound and is the most accurate way to date a pregnancy.
How accurate is dating by CRL?+
ACOG states that CRL measurement up to and including 13 6/7 weeks has an accuracy of about ±5–7 days, and it is more accurate the earlier it is done. Averaging three separate CRL measurements improves precision.
When should my due date be changed after a dating scan?+
ACOG guidance is to change the due date to the ultrasound date if they differ by more than 5 days before 9 0/7 weeks, or by more than 7 days from 9 0/7 to 13 6/7 weeks. Smaller differences usually keep the LMP date. After IVF, the transfer date is used instead.
Why is CRL not used after 14 weeks?+
Beyond about 84 mm the fetus curls and stretches, so CRL becomes less reliable. ACOG recommends switching to second-trimester biometry — biparietal diameter, head circumference, abdominal circumference and femur length — for dating.
Why do different CRL charts give slightly different ages?+
Each chart was built from a different population and ultrasound technology: Robinson in 1975, Hadlock in 1992 and INTERGROWTH-21st in 2014. Differences are usually only a day or two, which is well within the accuracy of the measurement itself.
What CRL is normal at 12 weeks?+
At 12 weeks 0 days the three formulas predict a CRL of roughly 53–56 mm, and about 43–45 mm at 11 weeks 1 day. Individual babies vary, so a single measurement is used to date the pregnancy rather than to judge growth.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.
Sources
- ACOG Committee Opinion 700 — Methods for Estimating the Due Date
- Robinson HP, Fleming JE. A critical evaluation of sonar “crown-rump length” measurements (BJOG 1975)
- Hadlock FP et al. Fetal crown-rump length: reevaluation of relation to menstrual age (Radiology 1992)
- Papageorghiou AT et al. INTERGROWTH-21st international standards for early fetal size and dating by CRL (UOG 2014)