How accurate is a due date from an ultrasound?

First-trimester ultrasounds are the most accurate, typically within 5–7 days. Later ultrasounds become less reliable for dating because babies grow at different rates.

What is CRL and when is it used?

CRL is crown-rump length, measured 6–14 weeks. The Robinson–Fleming formula converts CRL mm into gestational age, which the calculator then turns into a due date.

Can I use weeks and days instead of CRL?

Yes. Enter the ultrasound date and the gestational age the sonographer reported (weeks + days).

Ultrasound Due Date Calculator

Calculate your due date from a scan date plus CRL or gestational age.

Ultrasound date

CRL (mm) — optional

Auto-fills weeks + days for 6–14 week scans.

Weeks at scan

Days at scan

Estimated due date

Wednesday, March 17, 2027

Based on scan at 12 weeks 0 days.

This is an estimate for information only — only about 1 in 20 babies arrive on their due date. Always confirm dates with your midwife or doctor.

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Ultrasound Due Date Calculator

Frequently asked questions

How accurate is a due date from an ultrasound?

What is CRL and when is it used?

Can I use weeks and days instead of CRL?