Pregnancy Due Date Calculator
Estimate your due date, current gestational age, and trimester using the standard Naegele's Rule formula.
This is an estimate based on Naegele's Rule, a standard obstetric formula. It is not a substitute for advice, diagnosis, or care from a doctor or midwife.
How to Estimate Your Due Date
- Enter the first day of your last menstrual period (LMP).
- If you know your average cycle length and it isn't 28 days, adjust the "Average Cycle Length" field.
- Your estimated due date (280 days from LMP, adjusted for cycle length) appears automatically.
- Your current gestational age and trimester are calculated from today's date.
Frequently Asked Questions
How accurate is this estimate?
It's a standard estimate based on Naegele's Rule (LMP plus 280 days), which is the same method most healthcare providers use for an initial due date. Only about 1 in 20 babies are actually born on their exact due date, so treat this as a helpful estimate, not a guarantee.
What if my cycle isn't 28 days?
Naegele's Rule assumes a 28-day cycle. If your average cycle is longer or shorter, enter your actual cycle length and the calculator adjusts the due date by adding the difference between your cycle length and 28 days.
What is Naegele's Rule?
It's a standard obstetric formula that estimates a due date as 280 days (40 weeks) from the first day of your last menstrual period, assuming a typical 28-day cycle and ovulation around day 14.
Why does the calculator show gestational age in weeks and days?
Pregnancy is conventionally tracked in weeks and days starting from the first day of your last period, not from conception, so gestational age is calculated as the number of days between your LMP and today, shown as weeks and remaining days.
Is this a substitute for medical advice?
No — this tool provides a mathematical estimate based on a standard formula. It is not a substitute for advice, diagnosis, or care from a doctor or midwife, who can confirm your due date with an ultrasound and adjust it based on your individual pregnancy.