Due Date Calculator
Estimates your baby's due date by adding 280 days (40 weeks) to your last menstrual period (LMP). Use this during early pregnancy to plan prenatal appointments and milestones.
About this calculator
The most widely used method for estimating a due date is Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a regular 28-day cycle with ovulation on day 14. The formula is: Due Date = LMP + 280 days. Days remaining until delivery = Due Date − Today's Date. Because 280 days spans roughly 9 calendar months and 7 days, the result aligns with the standard 40-week gestational calendar. Only about 5% of babies are born exactly on their EDD, so the result is an estimate — typically within a two-week window on either side.
How to use
Suppose your last menstrual period began on March 1, 2025. Step 1: Add 280 days to March 1, 2025 → December 5, 2025 (your estimated due date). Step 2: If today is May 15, 2025, the days remaining = December 5 minus May 15 = 204 days. Enter March 1, 2025 as your LMP date, and the calculator instantly shows December 5, 2025 as your EDD plus the countdown in days.
Frequently asked questions
How accurate is the due date calculator based on last menstrual period?
The LMP-based due date is accurate to within about ±2 weeks for most pregnancies with regular 28-day cycles. It can be less precise if your cycles are irregular or longer/shorter than average. A first-trimester ultrasound (ideally at 8–12 weeks) is the gold-standard confirmation and can revise the estimate by a few days. Most healthcare providers use both methods together for the best accuracy.
What does 280 days mean in pregnancy and why is it used?
280 days equals 40 weeks, which is the internationally accepted average length of a human pregnancy measured from the first day of the last menstrual period. Clinicians count from the LMP rather than conception because the exact day of ovulation is rarely known. This 40-week framework divides neatly into three trimesters and aligns with the developmental milestones tracked at prenatal visits. Even though conception typically occurs around day 14, adding 14 days to the LMP start brings the same total.
When should I use a due date calculator instead of asking my doctor?
A due date calculator is a great first step as soon as you suspect you are pregnant — it gives you an immediate estimate before your first prenatal appointment. It helps you plan when to book your first ultrasound, schedule time off work, and begin prenatal care. However, your doctor or midwife will confirm or adjust the date using ultrasound measurements and clinical assessment. Use this tool for planning and orientation, not as a medical diagnosis.