How to Calculate Your Pregnancy Due Date
The standard due-date estimate is simple arithmetic: 280 days from the first day of your last menstrual period. But cycle length shifts it, and knowing your gestational age week-by-week is usually what you actually want. Here's how it all works.
Last updated: July 2026
Add 280 days (40 weeks) to the first day of your last menstrual period — Naegele's rule — and adjust by your cycle length minus 28 days. If you know the conception date, add 266 days instead. A due date calculator does this instantly and also shows your current week of pregnancy and trimester dates.
Calculate your estimated due date from your last period or conception date, plus gestational age and trimester dates. Free, instant, in-browser.
Step by step
- Open the free Pregnancy Due Date Calculator.
- Enter the first day of your last period (or switch to conception date).
- Set your average cycle length if it isn't 28 days.
- Read your estimated due date, current week and trimester ranges.
The 280-day rule (Naegele's rule)
Pregnancy is dated from the last menstrual period, not conception, because that date is usually known. Naegele's rule adds one year, subtracts three months and adds seven days — equivalent to 280 days. Ovulation typically happens around day 14 of a 28-day cycle, which is why a conception-based estimate uses 266 days instead.
Why cycle length matters
With a 35-day cycle, ovulation happens around a week later than the textbook day 14, pushing the realistic due date about a week later too. Adding (your cycle length − 28) days to the standard estimate corrects for this.
Estimates, not promises
Only around 4–5% of babies arrive on their due date; most are born within two weeks either side. First-trimester ultrasound dating is more accurate than any calendar method, and your provider may adjust your date after a scan.
FAQ
How many weeks pregnant am I?
Count the weeks since the first day of your last period — the calculator shows this as gestational age in weeks and days.
When do trimesters start and end?
Trimester 1 runs to the end of week 13, trimester 2 to the end of week 27, and trimester 3 to birth.
Is my information stored?
No — the dates are calculated in your browser and never uploaded.
Related tools
Ovulation & Due Date Calculator
Calculate your fertile window, ovulation date and estimated due date. Free, private, in-browser.
Date Difference Calculator
Calculate the exact difference between two dates in years, months, days and total days, weeks and hours. Free and instant.
Days Until Calculator
Calculate how many days, weeks, months and hours until (or since) a specific date. Free, instant countdown calculator.