Comprehensive Calculator Guide
This calculator turns the crown-rump length (CRL) on your scan report into gestational age in weeks and days and an estimated due date. A 45 mm CRL means 78 days — 11 weeks and 1 day — so a scan on 27 October 2026 puts the due date around 17 May 2027. It’s the most accurate way to date a pregnancy in the first trimester.
The Robinson formula and early dating
Robinson and Fleming (1975): gestational age in days = 8.052 × √CRL (mm) + 23.73, most accurate for about 10–84 mm (weeks 7–14). Some machines use newer curves that differ by a day or two.
Early CRL is accurate to about 5–7 days. ACOG advises going by the scan if it differs from period-based dating by more than 5 days before week 9, or 7 days in weeks 9–14.
How to Use
- Enter the CRL in millimetres from your scan report.
- Enter the scan date, or leave it empty for today.
- Press “Work out the dates” to see gestational age and the due date.
Formula Used
Gestational age (days) = 8.052 × √CRL (mm) + 23.73 — Robinson 1975
Practical Examples
- Example 1
- CRL 45 mm: 78 days (11+1); due around 17 May 2027 for a 27 Oct 2026 scan.
- Example 2
- CRL 12 mm: 52 days (7+3); due around 12 June 2027.
Important Tips
- If the report gives centimetres, multiply by 10.
Common Mistakes to Avoid
- Using the formula after week 14.
Frequently Asked Questions
- How do I get gestational age from CRL?
- 8.052 × √CRL in mm + 23.73 gives the age in days.
- What does CRL mean on a scan?
- The baby’s length from crown to rump, measured in the first trimester to date the pregnancy.
- Is the scan or my period more accurate?
- An early scan, especially with irregular cycles.