Pregnancy Due Date Calculator is a free tool that works out your baby’s due date from the first day of your last period, adjusted for your cycle length. It also estimates the conception date and tells you the current week and trimester. Everything is calculated in your browser and nothing is uploaded.
How to work out a due date
- Enter the First day of last period — the default guesses 12 weeks ago; correct it.
- Set your usual Cycle length in days (28 if unsure).
- Read the Due date, the Likely conception day and where you are Right now.
- If an early scan disagrees by more than a week, use the scan’s date instead — ask your midwife.
A worked example: New Year’s Day to early October
With a last period of 1 January 2026 and a 28-day cycle, the rule adds 280 days to reach 8 October 2026. Conception lands around 15 January — 266 days before the due date — and on 1 April the pregnancy is exactly 12 weeks and 6 days, the cusp of the second trimester. A 35-day cycle would push the due date a full week later, to 15 October, which is why the cycle box matters.
Due date or conception date? It does both directions
Most people arrive knowing their last period and wanting the due date, but just as many arrive holding a scan slip with a due date and wondering when it all started. The same 280-day ruler reads both ways, so this page answers both. If you are still trying, the other direction of planning starts earlier: find the fertile days with the Ovulation Calculator instead.
Frequently asked questions
How accurate is a due date from the last period?
Within about two weeks for most pregnancies, and only around 1 in 20 babies arrives on the exact date. An early ultrasound that disagrees by more than a week usually replaces the period-based date — ask your midwife or doctor which to use.
Why does it ask for cycle length?
Ovulation — and therefore conception — shifts with longer or shorter cycles. The calculator moves the due date by the difference from a 28-day cycle, which a fixed 280-day count would miss.
How is the conception date estimated?
Roughly 266 days before the due date, which matches ovulation in a 28-day cycle. It is an estimate of the fertile window, not proof of a date — sperm survive several days, so conception rarely equals intercourse day.
Is this medical advice?
No. It runs Naegele’s rule on the dates you enter. Dating, screening schedules and decisions about induction belong with your midwife or doctor, especially with irregular cycles, IVF or complications.
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