Ultrasound Due Date Calculator
Enter your scan date and the baby’s gestational age to back-calculate your due date.
Ultrasound due date from scan age — runs offlineEstimates only — not medical advice. Confirm all dates and milestones with your doctor or midwife. Read our full disclaimer.
How this due date is calculated
Enter the ultrasound (scan) date and your baby’s gestational age in days from the scan. The due date is back-calculated as scan date + (280 − gestational age in days).
First-trimester ultrasound is the gold standard for dating because embryo and early fetal size follow a tight, predictable curve before individual genetics and maternal factors diverge. Measuring crown-rump length (CRL) around 7–13 weeks pins gestational age to within a few days, which is why providers trust an early scan over a remembered last period whenever the two disagree. This page simply reverses that measured age back to a due date.
What to expect
An ultrasound due date works backwards from a measurement taken on a specific scan day. You enter the scan date and the baby's gestational age in days (for example, 8 weeks 0 days = 56 days), and the calculator adds the remaining days to reach 280.
The formula is: due date = scan date + (280 − gestational age in days). Because the scan already represents some days of pregnancy, subtracting them from 280 leaves the days still to go, which are then added to the scan date.
Example: a scan on January 1 showing gestational age 56 days (8 weeks) gives a due date of about October 8 — scan date plus 224 days. Early scans (before about 13 weeks) are the most accurate for dating because the baby's size is very predictable at that stage.
Later scans are better for checking growth and position than for setting the due date. If an early ultrasound and your LMP disagree by more than a few days, most providers use the early ultrasound as the official due date.
The gestational age is usually given by your sonographer as weeks and days. Convert it to total days (weeks × 7 + days) before entering it above, or enter the days your clinic provided.
Important note
These calculators use standard formulas — Naegele's rule (LMP + 280 days) and common IVF conventions (transfer date + 260–263 days). They give estimates for planning only. They are not a substitute for prenatal care, and only your provider can confirm your due date and track your baby's development. See the disclaimer for details.
FAQ
Is an ultrasound due date more accurate than LMP?
An early ultrasound (first trimester) is generally the most accurate way to date a pregnancy, because the baby’s size follows a predictable curve. Your provider will usually adopt it if it differs from the LMP estimate.