Crown-Rump Length Calculator
Turn the crown-rump length (CRL) from your first-trimester ultrasound into an accurate gestational age and estimated due date — plus see exactly where your baby’s measurement sits on the normal growth curve.
Find this on your ultrasound report, usually labelled “CRL”. Valid for about 5–84 mm (roughly 6–14 weeks).
Enter your baby’s crown-rump length above to see the gestational age, due date, and growth curve.
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Where your baby sits on the growth curve
The shaded band is the normal CRL range for each week. Your measurement is the marked point.
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CRL reference by week
This is an educational estimate, not a diagnosis. CRL dating is most reliable between about 6 and 14 weeks and is accurate to within roughly 3–5 days. A single measurement can’t assess your baby’s health — only your sonographer and provider can, using the full scan. Always follow the gestational age and due date on your official scan report.
How the crown-rump length calculator works
In the first weeks of pregnancy, every baby grows at almost exactly the same rate. That’s the quiet magic behind crown-rump length dating: because there’s so little natural variation early on, measuring your baby from the top of the head (the crown) to the bottom of the buttocks (the rump) gives a remarkably precise read on how many weeks along you are. This calculator takes that CRL measurement from your scan report and converts it into a gestational age using the Robinson–Fleming formula — the same equation used by ultrasound services worldwide.
Enter your CRL in millimetres, centimetres, or inches. Add your scan date and the tool also works out your estimated due date; add your last period and it compares the two dating methods for you. You’ll also see your measurement plotted on the normal growth curve, so you can picture exactly where your baby falls. If you’re past 14 weeks, dating moves to other measurements — our Ultrasound Gestational Age Calculator handles those.
Quick answer
To estimate gestational age from crown-rump length, the Robinson–Fleming formula is GA (days) = 8.052 × √(CRL × 1.037) + 23.73, with CRL in millimetres. For example, a CRL of 45 mm works out to about 11 weeks 2 days, and 55 mm to about 12 weeks 1 day. First-trimester CRL dating is accurate to within roughly 3–5 days — more reliable than dating from your last period.
What is crown-rump length?
Crown-rump length is simply the straight-line distance from the top of your baby’s head to the bottom of the buttocks, measured on an early ultrasound. It’s taken with the baby lying in a neutral position, in profile, and it’s the single most accurate way to date a pregnancy between about 6 and 14 weeks. The legs aren’t included — partly because they’re curled up at this stage, and partly because the head-to-rump length follows such a predictable growth pattern. You’ll usually see it on your dating-scan or 12-week-scan report, labelled “CRL” and given in millimetres.
Why CRL beats your last period for dating
Dating from your last menstrual period assumes you ovulated on day 14 of a textbook 28-day cycle — but real cycles vary, and many people can’t pinpoint their last period exactly. CRL sidesteps all of that by measuring the baby directly. When the date from your last period and the date from an early scan disagree by more than about 5 to 7 days, obstetric guidelines (ACOG, NICE) recommend going with the CRL date. If you’d like to run the dating the other way, our Reverse Due Date Calculator works backward from a due date to your last period and conception.
CRL measurement chart by week
These are typical crown-rump lengths for each week of early pregnancy, based on the Robinson–Fleming curve. Individual babies vary, so treat them as a guide.
| Gestational age | Typical CRL (mm) | Typical CRL (inches) | Comparable size |
|---|---|---|---|
| 6 weeks | ~5 mm | ~0.2 in | A grain of rice |
| 7 weeks | ~9 mm | ~0.35 in | A blueberry |
| 8 weeks | ~15 mm | ~0.6 in | A kidney bean |
| 9 weeks | ~23 mm | ~0.9 in | A grape |
| 10 weeks | ~32 mm | ~1.3 in | A strawberry |
| 11 weeks | ~42 mm | ~1.6 in | A fig |
| 12 weeks | ~54 mm | ~2.1 in | A lime |
| 13 weeks | ~67 mm | ~2.6 in | A pea pod |
| 14 weeks | ~82 mm | ~3.2 in | A lemon |
When CRL dating stops being used
Once your baby’s CRL passes about 84 mm (roughly 14 weeks), growth rates between healthy babies start to diverge, so a single length becomes less reliable for dating. From that point, sonographers switch to a combination of measurements — biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). If your dating scan already set your due date, that date generally stays fixed for the rest of the pregnancy even as these later measurements track your baby’s growth. To see what’s happening week by week from here, our Pregnancy Week by Week Calculator is a lovely companion.
Related maternal health calculators
This tool is part of the Maternal Health Calculators hub. These companion tools cover ultrasound dating, cycle tracking, and pregnancy planning — each answers a different question, so choose whichever fits where you are.
Frequently asked questions
The Robinson–Fleming formula converts CRL in millimetres to gestational age in days: GA = 8.052 × √(CRL × 1.037) + 23.73, then divided into weeks and days. This calculator does the maths and also gives your estimated due date.
Between about 6 and 14 weeks, CRL dates a pregnancy to within roughly 3–5 days — the most accurate method available. It’s more reliable than last-period dating, which can be off by two to three weeks if cycles are irregular.
It depends on how far along you are. Roughly, CRL is about 5 mm at 6 weeks, 15 mm at 8 weeks, 32 mm at 10 weeks, 54 mm at 12 weeks, and 82 mm at 14 weeks. The chart and growth curve above show where your measurement falls.
In early pregnancy the baby’s legs are curled up and hard to measure consistently. The head-to-rump length is easier to capture reliably and follows a very predictable growth pattern, which makes it ideal for dating.
When an early scan and your last period disagree by more than about 5 to 7 days, guidelines recommend using the CRL-based date, because the scan measures the baby directly. Your provider will usually adjust your due date to the scan.
Not necessarily. A CRL a little smaller or larger than expected often just means your dates were slightly off, and the scan corrects them. Your sonographer looks at the whole picture, including the heartbeat, so any real concern is assessed in context — not from one number.
No. Once CRL passes about 84 mm (around 14 weeks), babies grow at different rates and a single length becomes unreliable for dating. Sonographers then use head, abdomen, and femur measurements instead.
Yes — each twin is measured separately and gets its own CRL. Identical twins usually have nearly matching lengths; small differences can be normal. A larger gap between twins is something the sonographer will look into further.
It uses the corrected Robinson–Fleming (1975) equation, the international standard adopted by NICE, RCOG, and ISUOG. Other common formulas such as Hadlock and INTERGROWTH-21st agree with it to within one or two days across the CRL range.
The maths matches the clinical standard, so your result should agree with your scan report to within a day or two. Still, it’s an educational estimate. Your official gestational age comes from your sonographer, who accounts for measurement quality and your full history.
Methodology
This calculator estimates gestational age from crown-rump length using the corrected Robinson–Fleming equation: gestational age in days = 8.052 × √(CRL in mm × 1.037) + 23.73, then expressed as completed weeks plus days. Centimetre and inch entries are converted to millimetres first. When a scan date is provided, the estimated due date is calculated as the scan date plus (280 − gestational age in days), consistent with a standard 280-day pregnancy. When a last menstrual period is also entered, the tool compares the LMP-based gestational age with the CRL-based age and flags a difference of more than five days, in line with obstetric dating guidance. The expected CRL range shown on the growth curve is derived from the inverse of the same equation with an approximate ±10% reference band. CRL dating is applied over roughly 5–84 mm (about 6–14 weeks); beyond this, other biometric measurements are more appropriate. All results are educational estimates and are not a diagnosis or a substitute for your sonographer’s report or care from a qualified provider.
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References & trusted sources
- Robinson HP, Fleming JEE (1975) — A critical evaluation of sonar crown-rump length measurements
- ACOG — Methods for Estimating the Due Date (Committee Opinion 700)
- NHS — The 12-week dating scan
- Tan et al. (2023) — A new formula for estimating gestational age by crown-rump length
Last reviewed & updated: July 2026.