Due Date Calculator
Enter the first day of your last period and your usual cycle length. We estimate your due date and show where you are in the pregnancy today.
This calculator gives general estimates for information only. It is not medical advice and does not replace a consultation with a doctor or other qualified health professional.
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The formula
Due date = first day of last period + 280 days (adjusted by cycle length: cycle - 28 days). This is Naegele's rule.
How the due date is calculated
A full pregnancy is counted as 40 weeks, or 280 days, from the first day of your last period. For a 28-day cycle, that is the whole calculation. If your cycle is longer or shorter, we shift the date by the difference, because ovulation happens later or earlier.
The three trimesters
- First trimester: up to the end of week 13
- Second trimester: weeks 14 to 27
- Third trimester: week 28 until birth
We also show full term from 37 weeks onward. Babies born between 37 and 42 weeks are considered term.
What to do next
Your first scan will usually confirm or adjust the date, and dating from a scan is considered more reliable than dating from your last period if the two disagree by more than a week or so.
This calculator is an estimate for information. It is not medical advice, so speak to your midwife or doctor about your own pregnancy.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| First day of last period; First day of last period: 2026-01-10; Average cycle length (days): 28 | Estimated due date: Saturday, October 17, 2026 |
| First day of last period; First day of last period: 2026-03-22; Average cycle length (days): 28 | Estimated due date: Sunday, December 27, 2026 |
| First day of last period; First day of last period: 2026-05-05; Average cycle length (days): 32 | Estimated due date: Friday, February 5, 2027 |
Due Date Calculator: frequently asked questions
How accurate is an estimated due date?
Only about 4 in 100 babies arrive on the due date. Most are born within two weeks either side. Treat the date as the middle of a window, and follow your midwife or doctor, who may adjust it after a scan.
Why do doctors count from the last period if conception happens two weeks later?
Because most people remember the start of their last period but not the exact day of conception. Pregnancy weeks are counted from that date, so you are already "two weeks pregnant" at conception.
Can I rely on this result as medical advice?
No. It is a general estimate for information only. It cannot replace a check-up, and your doctor can interpret the numbers for your own situation.
Is what I enter in this calculator saved or sent anywhere?
No. The calculation runs in your browser, so what you enter is not sent to our servers. We only count, anonymously, whether a tool ran successfully.
Is this calculator free to use?
Yes. It is free, needs no sign-up and has no usage limit.
Last reviewed: October 3, 2026