Pregnancy Due Date Calculator
Estimate a due date from the first day of your last period
This pregnancy due date calculator adds 280 days to the first day of your last menstrual period, then moves the date by one day for each day your cycle length differs from 28.
If you know the ovulation date, use the due date by ovulation calculator. That page adds 266 days to the ovulation date.
Calculator Inputs
Usually 28 days
Results
Enter values above to see your results
These dates are estimates for planning, not a medical diagnosis. An ultrasound is more accurate, and only a small share of babies are born on the exact due date.
How to use this calculator
- Enter the first day of your last menstrual period, not the day you think you conceived.
- Change the cycle length only if your cycles are usually shorter or longer than 28 days. A 30-day cycle moves the due date two days later.
- If you know an ovulation or conception date, use the ovulation due date calculator instead. The week calculator is the better page when the question is “how far along,” not “what is the date.”
How the calculation works
Naegele’s rule dates a pregnancy as 280 days (40 weeks) from the last menstrual period. The period is used because most people know that day, while ovulation is often unknown. Conception is typically about 14 days after a 28-day period starts, so 280 days from the period is 266 days from conception.
Cycle adjustment = cycle length − 28. A 32-day cycle adds 4 days. A 26-day cycle subtracts 2 days. The adjustment assumes ovulation still happens 14 days before the next period.
Weeks pregnant are the days since the period date, divided by 7. That count does not include the cycle adjustment. The second trimester starts at week 13 and the third at week 28 on this page.
What the date does and does not mean
The result is an estimated due date for planning. Only a small share of births happen on that exact day. A first-trimester ultrasound is the dating method clinics prefer when the period is uncertain.
“Baby calculator” and “due date calculator” on this site are this page. A separate conception or ovulation date belongs on the ovulation due date tool so the two methods are not mixed into one unlabeled number.
Worked example
Period on 1 January 2026, 28-day cycle
1 Jan 2026 + 280 days= 8 Oct 20261 Jan 2026 + 14 days= 15 Jan 202615 Jan 2026 + 266 days= 8 Oct 2026Tips & Best Practices
- •Only about 4% of babies are born on the exact due date. Most arrive within two weeks on either side.
- •An early ultrasound replaces both the period estimate and the ovulation estimate when the dates disagree.
Frequently Asked Questions
How is a pregnancy due date calculated?
Add 280 days to the first day of the last menstrual period when the cycle is 28 days. Add or subtract a day for each day the cycle differs from 28.
What if my cycle is not 28 days?
A longer cycle moves the estimate later and a shorter cycle moves it earlier, by the difference from 28 days. If you know the ovulation date, that date plus 266 days is the more direct estimate.
How many weeks pregnant am I?
Count the days since the first day of your last period and divide by 7. The pregnancy week calculator leads with that number. Gestational age counted this way is about two weeks ahead of the time since conception.
Guides & Articles
Due Date From Ovulation or Conception Date
Add 266 days to an ovulation date or a conception date. A worked example, why both dates use the same rule, and when the period method is the better start.
How Accurate Are Pregnancy Due Dates?
A due date is the 40-week mark from a period, or 38 weeks from ovulation. Most births are not on that exact day. What moves the estimate, and what replaces it.
How Many Days Is a Pregnancy?
280 days (40 weeks) from the last menstrual period, or 266 days (38 weeks) from conception. Same model, different starting point, plus the term labels clinicians use.
Related Calculators
Due Date by Ovulation
Estimate a due date from an ovulation date or a conception date.
Pregnancy Week Calculator
See how many weeks and days pregnant you are from the first day of your last period.
Ovulation Calculator
Track your fertility window and find your most fertile days to increase chances of conception
Pregnancy Weeks to Months
Convert pregnancy weeks and days into a pregnancy month and a trimester.