Pregnancy Due Date Calculator
Estimate your baby's due date from your last period using Naegele's rule.
This runs entirely in your browser. None of your dates are uploaded or stored anywhere.
How to use this tool
Find the first day of your last menstrual period and enter it in the date field. If your cycle is not the typical 28 days, set your average cycle length so the estimate can adjust. Press Calculate due date and the tool returns your estimated delivery date, how many weeks and days along you are today, your current trimester, an estimated conception window, and the number of days left until your due date. You can copy or download the summary to share with your doctor or keep for your records.
The formula: Naegele's rule
Naegele's rule is the standard method used in most pregnancy due date calculators. It adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). A common shortcut is to take the LMP, subtract three months, and add seven days, which lands on the same date. The rule assumes a regular 28-day cycle with ovulation around day 14, so conception is estimated at roughly LMP + 14 days. If your cycle is longer or shorter than 28 days, this tool shifts the due date by the difference (cycle length minus 28) so the delivery date estimate fits your body better.
A real example
Say the first day of your last period was January 1, 2026, and your cycle is the standard 28 days. Adding 280 days lands on October 8, 2026, which is your estimated baby due date. Ovulation, and therefore the conception window, is estimated around January 15, 2026 (LMP plus 14 days). If today were March 26, 2026, you would be about 12 weeks along, placing you near the end of the first trimester with roughly 196 days remaining until the due date.
Common questions
How accurate is a due date from Naegele's rule?
Only about one in twenty babies arrives exactly on the estimated due date. The date marks the middle of a normal delivery window, and a full-term birth can happen anytime from about 37 to 42 weeks. Treat it as a target, not a deadline.
What if I do not remember my last period date?
This calculator needs the first day of your last menstrual period to work. If you are unsure, an early ultrasound dating scan is the most reliable way to estimate your due date, and your provider can confirm it.
Why does my cycle length change the result?
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer, you ovulate later, so conception and the due date shift later by the same number of days. This tool adjusts automatically when you change the cycle length field.
How is the conception window estimated?
The estimated conception date is your last period date plus 14 days (adjusted for your cycle length), since ovulation usually occurs about two weeks into a cycle. We show a short window around that date because the fertile period spans several days.
Is this a substitute for medical advice?
No. This calculator gives an educational estimate only. Your due date and pregnancy care should always be confirmed and monitored by a qualified healthcare provider.