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Due Date from Ultrasound Calculator

Find days and weeks left to your due date from a scan's gestational age.

Result

179.00 days
25.57 weeks

How it is calculated

The tool counts forward from the gestational age on your scan report to a 40-week pregnancy (280 days):

age today (days) = weeks × 7 + days + days since scan

days to due = 280 − age today, and weeks remaining = days to due ÷ 7

Enter the scan's gestational age as weeks (0 to 42) and extra days (0 to 6), plus how many days have passed since the scan. Both results are shown with two decimals, so 25.57 weeks means 25 weeks and about 4 days. The tool gives only the time left, not a calendar date. It returns no result if any field is negative or out of range, or if the age today already passes 280 days; exactly 280 days gives 0.00.

Worked example

The scan report says 12 weeks + 3 days, and the scan was 14 days ago.

Gestational age at the scan = 12 × 7 + 3 = 87 days. Age today = 87 + 14 = 101 days (14 weeks + 3 days).

Days to due = 280 − 101 = 179.00 days.

Weeks remaining = 179 ÷ 7 = 25.57 weeks, which is about 25 weeks and 4 days.

Try this example →

Common mistakes

  • Typing 12.3 in the weeks field for "12 weeks 3 days". The field reads it as 12.3 weeks (86.1 days), not 12 weeks and 3 days. Weeks and days go in separate fields, and days must be 0 to 6.
  • Putting the number of days since the scan into the days field, which only takes 0 to 6, instead of the "days since scan" field.
  • Using the conception date or the ultrasound's own due date as the starting point. The tool starts from the gestational age as printed on the report, which is counted from the last period.
  • Reading 25.57 weeks as 25 weeks and 57 days. The decimals are a fraction of a week; multiply by 7 (0.57 × 7 ≈ 4 days).
  • Treating the 280-day figure as the day of birth. It is the estimated full-term date, and most babies arrive before or after it.

Sources

About this calculator

An early ultrasound measures gestational age directly from fetal size — crown-rump length in the first trimester, or biparietal diameter and femur length later — which is more accurate than dating from last menstrual period alone, especially for people with irregular cycles or uncertain conception dates. From that measured gestational age, the estimated due date is simply 40 weeks from the calculated start of pregnancy, adjusted for how many weeks along the scan showed you to be.

Obstetricians use the ultrasound-based due date as the official pregnancy dating reference once it’s established, and it typically takes priority over an LMP-based estimate when the two disagree by more than about a week, since ultrasound dating in the first trimester carries a much narrower margin of error. Expectant parents use this figure to track weeks remaining and time key milestones like the anatomy scan and third-trimester visits.

This calculator takes the gestational age measured at your ultrasound (weeks and days) and the number of days that have passed since that scan, and returns the days and weeks remaining until the 40-week due point. It does not show a calendar date; to get your due date, add the days remaining to today’s date.

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