Plot No. 52 · Health & Fitness
Pregnancy Due Date Calculator
Estimate your due date from the first day of your last menstrual period using Naegele's rule. The calculator adds 280 days, which is 40 weeks, to that date. It also shows how many weeks pregnant you are today and which trimester you are in.
An estimate using Naegele's rule, not medical advice. Only about 4 percent of babies arrive on the exact date, and a dating scan is more reliable.
The link saves your inputs so you can bookmark or share this exact result.
How the due date is worked out
The calculator uses Naegele's rule, the standard method midwives apply. It adds 280 days, which is exactly 40 weeks, to the first day of your last menstrual period. That date becomes your estimated due date. Pregnancy is counted from the last period rather than conception, which is why the 40-week count starts about two weeks before you actually conceived.
Using today's date, it also shows how far along you are now in weeks and days, and which trimester that falls in: the first trimester runs up to 13 weeks, the second from 13 to 27 weeks, and the third from 28 weeks onward. Because the page is built before it loads in your browser, today's figures appear once the page has finished loading.
How to use the due date calculator
- Find the first day of your last menstrual period.
- Enter that date in the date field.
- Read your estimated due date, your current week and day count, and your trimester.
Worked examples
Last period on 1 January 2026
Inputs: First day of last period set to 1 January 2026
Result: Estimated due date of Thursday 8 October 2026, which is 280 days later.
Checking the trimester
Inputs: A last period 15 weeks before today
Result: Gestational age of 15w 0d, placing the pregnancy in the second trimester, which begins at 13 weeks.
Limitations and common mistakes
Edge cases and limitations
- Naegele's rule assumes a regular 28-day cycle, so longer or shorter cycles shift the real date by several days.
- Only about 4 percent of babies arrive on the exact estimated date; a full-term birth is anywhere from 37 to 42 weeks.
- This is an estimate, not medical advice; an early dating scan is the more accurate guide.
Common mistakes
- Entering the date you found out you were pregnant rather than the first day of your last period.
- Expecting the baby precisely on the due date instead of treating it as the middle of a wide window.
Frequently asked questions
How is a due date calculated?
The standard method, Naegele's rule, adds 280 days, or 40 weeks, to the first day of your last menstrual period. So a last period starting on 1 January gives an estimated due date around 8 October the same year.
How accurate is the due date?
Only about 4 in 100 babies arrive on their exact estimated date. Most births happen within two weeks either side, so a full-term arrival anywhere from 37 to 42 weeks is normal. A dating scan usually refines the estimate.
What if my cycle is not 28 days?
Naegele's rule assumes a 28-day cycle with ovulation around day 14. If your cycles are longer or shorter, ovulation shifts and the estimate can be off by several days, which is why an early ultrasound is the more reliable guide.
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