Between about 7 and 13+6 weeks, the length of the embryo from head to rump is the most accurate way to date a pregnancy – accurate to within about 5 days. Major guidelines recommend changing the due date if the CRL date differs from the period date by more than 5–7 days.
How to use the Crown-Rump Length Calculator
- Crown-rump length
- Date of the scan
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the CRL in millimetres and the date of the scan.
Inputs explained
| Field | What to enter |
|---|---|
| Crown-rump length | In mm |
| Date of the scan | A date |
What each result means
- Gestational age = 8.052×sqrt([Crown-rump length]) + 23.73
- Gestational age – weeks = floor([Gestational age]÷7)
- …plus days = mod(round([Gestational age]), 7)
- Due date = [Date of the scan] – round([Gestational age]) + 280
Example values
How the main result changes when Crown-rump length changes and everything else stays at the example values above:
| Crown-rump length (mm) | Gestational age – weeks (weeks) |
|---|---|
| 22.5 | 8 |
| 33.75 | 10 |
| 45 | 11 |
| 56.25 | 12 |
| 67.5 | 12 |
| 90 | 14 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Gestational age (days) = 8.052 × √CRL (mm) + 23.73
Due date = scan date − gestational age + 280 days
Worked example
Inputs — Crown-rump length: 45 mm; Date of the scan: 2026-10-25.
Results — Gestational age = 78 days; Gestational age – weeks = 11 weeks; …plus days = 1 days; Due date = Sat, May 15, 2027.
Key terms
- CRL
- Crown-rump length.
Frequently asked questions
What is a normal CRL at 12 weeks?
About 45–60 mm.
How accurate is CRL dating?
Within about ±5 days in the first trimester.
Why is CRL not used after 14 weeks?
The fetus curls, so head circumference is more accurate later.