Free Pregnancy Tool
Turn the crown-rump length from an early scan into weeks pregnant and a due date. Private - nothing leaves this page.
Shown as "CRL" on the scan report. If it's in cm, multiply by 10.
Weeks pregnant, counted from the last period.
Please enter a valid measurement.
| Week | Average CRL | Typical range | What's happening |
|---|---|---|---|
| Week 6 | 5.1 mm | 4.2 mm – 6.1 mm | Heartbeat often first visible |
| Week 7 | 9.8 mm | 8.1 mm – 11.6 mm | Early pregnancy / viability scan |
| Week 8 | 16.1 mm | 13.2 mm – 19 mm | Early pregnancy / viability scan |
| Week 9 | 23.8 mm | 19.5 mm – 28.1 mm | Baby starts to move (too small to feel) |
| Week 10 | 33 mm | 27.1 mm – 39 mm | Embryo becomes a fetus |
| Week 11 | 43.8 mm | 35.9 mm – 51.6 mm | NHS dating scan window opens |
| Week 12 | 56 mm | 45.9 mm – 66.1 mm | Typical NHS dating scan |
| Week 13 | 69.8 mm | 57.2 mm – 82.4 mm | End of first trimester |
| Week 14 | 85.1 mm | 69.8 mm – 100.4 mm | Dating scan window closes |
Crown-rump length is the length of the baby from the top of the head (crown) to the bottom (rump), measured on an ultrasound scan in the first trimester. It's the number on an early scan report that sonographers use to date the pregnancy.
In the first trimester babies grow at a very consistent rate, so length maps closely to age. The calculator uses the Robinson & Fleming formula - the standard used for decades in scan departments - to turn the CRL into weeks and days on the day of the scan. It then counts forward to 40 weeks for the due date. From about 84 mm (around 14 weeks), scans switch to head circumference instead.
The scan report is often handed over quickly, full of abbreviations. Knowing what the CRL means - and why the due date might have moved from the one worked out from the last period - makes it easier to follow the conversation at the next appointment.
No scan yet? The Due Date Calculator gives an estimate from the last period, and the IVF Due Date Calculator dates IVF pregnancies from the transfer day.
FAQ
What crown-rump length tells you, and why it's used to date a pregnancy.
A first-trimester CRL is the most accurate way to date a pregnancy, usually to within about 5 days. That's why a dating scan normally replaces a due date worked out from the last period if they differ by more than a few days.
Period-based dates assume ovulation on day 14 of a 28-day cycle. If ovulation happened later or earlier, the baby will measure younger or older than the calendar suggests. It's very common, and it's why the scan date is used.
The most common reason is simply later ovulation, so the pregnancy is younger than period dates suggest. If the difference is large, the clinic will usually offer a repeat scan 1-2 weeks later to check growth. Always follow the advice of your sonographer or midwife.
2 to 84 mm, which covers roughly 6 to 14 weeks. Above 84 mm, the NHS screening programme dates pregnancies by head circumference instead.
Close. Early fruit-and-vegetable size comparisons use crown-rump length, then switch to head-to-heel length later in pregnancy once the legs are measured too.
Find your due date from the first day of the last period, or from how many weeks pregnant you are today.
Use tool →See what the baby is approximately the size of at every week from 4 to 40 — no due date required.
Use tool →A practical guide to the first 12 weeks of pregnancy for partners. What is happening, what your partner needs, and how to show up well.
Read article →Most partners assume pregnancy lasts nine calendar months. Here is why doctors use weeks instead, and why 'five months pregnant' is harder to interpret than it sounds.
Read article →Built for dads & partners
Stop asking. Stop guessing.
Open the app and you're caught up - week by week, until your baby is here.
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