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Crown-Rump Length Calculator

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Convert a first-trimester ultrasound crown-rump length (CRL) into gestational age using the Robinson and Fleming formula, plus an estimated due date instantly.

Crown-Rump Length
mm
10 mm84 mm

Valid for first-trimester CRL measurements of roughly 10–84mm (≈6–13 weeks gestation).

7th July 2026

Add the scan date to also see an estimated due date.

Gestational Age

0w 0d

Measurement Validity

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Informational and educational use only — not a substitute for your radiologist's or OB/GYN's interpretation of your ultrasound report.

What is a Crown-Rump Length?

A Crown-Rump Length Calculator converts a first-trimester ultrasound CRL measurement — the distance from the top of the fetal head to the bottom of the torso — into an estimated gestational age, using the Robinson and Fleming formula published in 1975. This remains one of the most accurate and widely used methods for dating early pregnancy in clinical obstetric practice.

CRL-based dating is considered more reliable than dating purely from your last menstrual period, because it measures fetal size directly at a developmental stage when growth is remarkably consistent across nearly all pregnancies — before individual variation in size becomes more pronounced later on. This is why many providers will adjust or confirm your estimated due date based on a first-trimester dating ultrasound.

This calculator is informational and educational only, not a substitute for your radiologist's or OB/GYN's official interpretation of your ultrasound report. The Robinson and Fleming formula is validated for CRL measurements of roughly 10 to 84 millimeters (approximately 6 to 13 weeks gestation); outside that range, your provider likely used a different dating method appropriate to that stage of pregnancy.

How to use this Crown-Rump Length calculator

  1. Locate your CRL measurement in millimeters from your ultrasound report.

  2. Enter the CRL value using the slider or exact-entry field.

  3. Optionally enter your scan date to also calculate an estimated due date anchored to this measurement.

  4. Read your Gestational Age — shown in weeks and days — in the result card.

  5. Check the Measurement Validity note to confirm your CRL falls within the formula's validated range.

  6. Compare against your LMP-based estimate using the EDD Calculator to see whether the two methods roughly agree or suggest a redating.

  7. Confirm any due date adjustment with your OB/GYN, since your official ultrasound report and provider's clinical judgment take precedence over this general calculation.

Formula & Methodology

Robinson & Fleming (1975) Formula
Gestational Age (days) = 8.052 × √(CRL in mm) + 23.73

Weeks + Days Conversion
Weeks = floor(Gestational Age in days ÷ 7) Extra Days = Gestational Age in days mod 7

Estimated Due Date (if scan date provided)
Due Date = Scan Date + (280 − Gestational Age in days)

Validity Range: CRL 10-84mm (approximately 6-13 weeks gestation)

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Worked Example:

An ultrasound at a first-trimester dating scan measures a CRL of 45mm, taken on a given scan date.

| Variable | Value |
|---|---|
| CRL | 45 mm |

Gestational Age = 8.052 × √45 + 23.73 = 8.052 × 6.708 + 23.73 ≈ 54.02 + 23.73 = 77.75 → 78 days

78 days = 11 weeks, 1 day

CRL of 45mm falls within 10-84mm → Within the validated range

If the scan date was, say, June 1st: Due Date = June 1st + (280 − 78) = June 1st + 202 days ≈ December 20th

This estimated due date, anchored to the ultrasound measurement, is generally considered more reliable than pure LMP-based dating at this early stage. Compare it against the EDD Calculator result from the same LMP to see whether your provider might discuss redating your pregnancy.

Frequently Asked Questions

Crown-rump length is the measurement, in millimeters, from the top of the fetal head (crown) to the bottom of the torso (rump), taken during a first-trimester ultrasound. It is the most accurate single measurement for dating a pregnancy in the first trimester, more reliable than dating from last menstrual period alone.
This calculator uses the Robinson and Fleming formula, published in the British Journal of Obstetrics and Gynaecology in 1975: gestational age in days equals 8.052 times the square root of the CRL in millimeters, plus 23.73. This formula remains one of the most widely used and validated methods for first-trimester ultrasound dating in clinical practice today.
The Robinson and Fleming formula is validated for CRL measurements of roughly 10 to 84 millimeters, corresponding to approximately 6 to 13 weeks of gestation. Measurements outside this range fall outside the formula's validated accuracy and the calculator will flag this so you know to interpret the result with extra caution.
Menstrual-cycle-based dating assumes a textbook-regular cycle and precise recall of your last period's first day, both of which vary in practice. CRL measures the fetus directly at a stage when growth rates are highly consistent across nearly all pregnancies, making it the most accurate dating method available, which is why providers often adjust a pregnancy's estimated due date based on a first-trimester ultrasound.
Often, yes, especially if the CRL-based gestational age differs from LMP-based dating by more than about 5-7 days. Many practices follow ACOG guidance to redate a pregnancy based on first-trimester ultrasound when there's a meaningful discrepancy, since the ultrasound-based estimate is generally considered more reliable at this stage.
No. This calculator applies the same published formula your radiologist or sonographer's software likely uses, but your official ultrasound report — reviewed and interpreted by your OB/GYN or radiologist — is the authoritative source for your pregnancy dating and any due date adjustment. Use this tool for general understanding, not as a replacement for that report.
If your CRL measurement falls below 10mm or above 84mm, the Robinson and Fleming formula's accuracy has not been specifically validated for that value, and your provider likely used a different dating method (such as gestational sac diameter for very early pregnancies, or biparietal diameter for later ones) for your actual clinical dating.
If you provide your scan date along with the CRL measurement, the calculator computes your due date as the scan date plus (280 minus the calculated gestational age in days) — effectively projecting forward to a standard 280-day (40-week) gestation from your CRL-derived starting point.
No, and this is intentional. CRL-based dating is specifically a first-trimester technique because growth rates are remarkably consistent across almost all pregnancies at this early stage, before individual growth variation (influenced by genetics, sex, and other factors) becomes more pronounced later in pregnancy.
Enter your CRL measurement in millimeters from your ultrasound report, using the slider or exact-entry field. Optionally add your scan date to also see an estimated due date. The calculator instantly shows your gestational age in weeks and days, a validity note, and your due date if a scan date was provided.
No. All calculations happen entirely within your browser. None of the values you enter are transmitted to or stored on any server, and everything is cleared when you close the page.
Also known as
CRL calculatorCRL to gestational ageRobinson Fleming formula calculatorfirst trimester ultrasound datingcrown rump length to due date