CRL Calculator | Crown-Rump Length to GA & EDD

Enter a crown-rump length (CRL) to see gestational age and an estimated due date. You can also pick a target GA to see the expected CRL and a typical range.

Crown–Rump Length (CRL) → GA & EDD
Target GA for expected CRL (optional)
weeks days
Educational only, not medical advice: this explains how the calculator estimates gestational age (GA), due date, and an expected CRL range from a single crown–rump length measurement.

How the estimate works

  • CRL→GA model: the tool uses a classic first-trimester dating curve derived from Robinson and widely reproduced in later charts: GA (days) ≈ 23.73 + 8.052 × √(CRL in mm). It is most applicable for CRL about 5–84 mm (roughly 6–14 weeks). The app rounds to weeks + days for display.
  • EDD from a measurement date: when a measurement date is entered, EDD is calculated as measurement date + (280 − GA days), aligning with a standard 280-day LMP-based pregnancy length used in dating by early ultrasound.
  • Expected CRL for a target GA: the inverse of the same curve is used: CRL (mm) ≈ ((GA days − 23.73)/8.052)². A simple typical band of ±10% around the estimate is shown for education; individual labs may publish narrower or wider reference limits.

Units and inputs

  • Units: input accepts mm or cm. The calculator converts cm→mm using 1 cm = 10 mm before applying the model.
  • Dates: measurement date is optional; without it, GA is shown at the time of scan only and no EDD is produced.
  • Rounding: GA is rounded to whole days; CRL shown to 0.1 mm (or 0.1 cm if that unit is selected).

Assumptions & limitations

  • Applies to first-trimester, mid-sagittal CRL measured with a neutral fetal position by ultrasound. Image quality and measurement technique affect accuracy.
  • Different charts (Robinson, Hadlock, INTERGROWTH-21st, etc.) are closely aligned but not identical; follow the reference used by your ultrasound service.
  • Embryonic growth can vary; clinical dating should follow your sonographer’s report and local protocols.
Key sources
  • Robinson HP, Fleming JEE. “A critical evaluation of sonar ‘crown-rump length’ measurements.” Br J Obstet Gynaecol 1975;82:702–710. (Robinson CRL dating curve).
  • Altman DG, Chitty LS. “New charts for ultrasound dating of pregnancy.” Ultrasound Obstet Gynecol 1997;10:174–191. (First-trimester reference standards).
  • INTERGROWTH-21st Project: first-trimester dating and growth standards (overview and CRL resources): intergrowth21.tghn.org.
  • ACOG/AIUM guidance on ultrasound dating in early pregnancy: Methods for Estimating the Due Date.

How to use this calculator

  • Enter the CRL value (mm or cm). The tool auto-converts units.
  • Optional: add the measurement date to calculate an EDD.
  • Optional: choose a target GA to see the expected CRL and a typical range.
  • Read the outputs: gestational age at measurement, estimated due date, and expected CRL for GA.

Educational only. Ultrasound measurements vary with fetal position and equipment. Your clinician’s report takes precedence.

Last updated: September 21, 2025

How to use this calculator

  • Enter the CRL value (mm or cm). The tool auto-converts units.
  • Optional: add the measurement date to calculate an EDD.
  • Optional: choose a target GA to see the expected CRL and a typical range.
  • Read the outputs: gestational age at measurement, estimated due date, and expected CRL for GA.

Educational only. Ultrasound measurements vary with fetal position and equipment. Your clinician’s report takes precedence.

Last updated: September 21, 2025

Frequently asked questions

What is CRL and when is it used?

Crown-rump length (CRL) is the head-to-bottom length of the embryo/fetus on early ultrasound. It is the preferred way to date pregnancy in the first trimester.

How accurate is CRL dating?

CRL-based gestational age is typically more precise than LMP in early pregnancy, but there is still a normal margin of error. Small differences are expected between machines and sonographers.

Why does my GA not match my LMP dates?

Ovulation and implantation vary from cycle to cycle. If CRL and LMP disagree by more than common thresholds, clinicians usually use ultrasound dating. You can also compare with the Ultrasound Due Date Calculator.

Can I enter cm instead of mm?

Yes. The input accepts mm or cm and converts automatically for the calculation.

What if CRL is not visible yet?

Very early scans may not show a measurable CRL. A re-scan is usually scheduled in about 1–2 weeks. For timing context, you can review the Implantation Calculator or Pregnancy Test Accuracy.

Does this work for twins?

CRL is measured for each fetus separately. Dating may be based on the larger or earlier fetus depending on local practice. For multiples planning, see the Twin & Triplet Due Date Calculator.

What about risk after a heartbeat?

CRL is a growth measure, not a risk estimate. For risk context after cardiac activity is seen, use the Miscarriage Risk After Heartbeat Calculator.