Crown-Rump Length (CRL) Calculator — Gestational Age from Ultrasound
Enter the crown-rump length from an early-pregnancy ultrasound to get the estimated gestational age in weeks and days, pregnancy progress and days remaining to the due date.
mm
11 weeks 1 days — Robinson & Fleming (1975)
- 1
√CRL (square root of crown-rump length)
√45 = 6.7082Robinson & Fleming formula: GA (days) = 8.052 × √CRL(mm) + 23.73 - 2
Scaled term: 8.052 × √CRL
8.052 × 6.7082 = 54.014 - 3
Gestational age in days
54.014 + 23.73 = 77.74 - 4
Gestational age in weeks
⌊77.74 ÷ 7⌋ = 11
How does this calculator work?
Crown-rump length converts to gestational age via GA (days) = 8.052 × √CRL(mm) + 23.73 (Robinson & Fleming). Enter the CRL in millimetres from the first-trimester ultrasound scan to get weeks and days of pregnancy plus the estimated days remaining to the 40-week due date.
Formula
How this is calculated
Crown-rump length (CRL) is the longest measurement of an embryo or fetus from the top of the head to the bottom of the rump, taken at a natural curve. It is the most accurate method of estimating gestational age in the first trimester (roughly 6–14 weeks, CRL 10–84 mm), because fetal size is uniform at this early stage and is minimally affected by genetics or nutrition — unlike later measurements.
The calculator applies the Robinson & Fleming (1975) formula: GA (days) = 8.052 × √CRL(mm) + 23.73. This was derived from a prospective ultrasound study and remains the most widely cited equation for first-trimester dating. The result is split into completed weeks plus extra days (e.g., 77.7 days → 11 weeks 0 days), and days remaining is 280 minus the estimated gestational age, using Naegele's 40-week rule.
The formula carries an inherent uncertainty of roughly ±3–5 days; always verify results with your sonographer or clinician. Beyond 14 weeks (CRL > ~84 mm) the fetus begins to flex, making CRL less reproducible; biparietal diameter (BPD) or head circumference (HC) are used instead. This calculator accepts CRL up to 130 mm but accuracy declines above ~84 mm.
Frequently asked questions
CRL is the standard first-trimester measurement for dating a pregnancy. It is more reliable than last-menstrual-period (LMP) dating when cycles are irregular, and is routinely taken at the 11–14 week dating or nuchal-translucency (NT) scan.
Robinson & Fleming (1975) derived GA (days) = 8.052 × √CRL(mm) + 23.73 from a prospective ultrasound study. It is the most widely cited equation for CRL-based gestational-age estimation and is accurate to within about ±3–5 days in the first trimester.
Above 84 mm (about 13–14 weeks) the fetus starts to flex, making CRL less reproducible. Sonographers switch to head circumference (HC) or biparietal diameter (BPD) for dating beyond 14 weeks. The calculator accepts up to 130 mm but accuracy decreases above this threshold.
TG we-Calculate Editorial Team. (2026). Crown-Rump Length (CRL) Calculator — Gestational Age from Ultrasound [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/crown-rump-length-calculator
TG we-Calculate Editorial Team. "Crown-Rump Length (CRL) Calculator — Gestational Age from Ultrasound." TG we-Calculate. 2026. https://we-calculate.com/calculator/crown-rump-length-calculator.
TG we-Calculate Editorial Team, "Crown-Rump Length (CRL) Calculator — Gestational Age from Ultrasound," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/crown-rump-length-calculator
@misc{wecalculate_crown_rump_length_calculator, title = {Crown-Rump Length (CRL) Calculator — Gestational Age from Ultrasound}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/crown-rump-length-calculator}}, year = {2026}, note = {TG we-Calculate} }
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