Reverse Due Date Calculator

A reverse due date calculator works in the opposite direction of a standard due date calculator. Instead of calculating your due date from conception, it takes your known due date and works backwards to estimate when conception likely occurred — typically 280 days (40 weeks) before the due date.

The standard formula is simple: subtract 280 days (or 40 weeks) from your estimated due date. Since a full-term pregnancy lasts approximately 280 days from conception, reversing this gives you the estimated date of conception. This calculator does that math for you automatically.

The date of conception is the day a sperm cell fertilizes an egg. This typically occurs within 24–48 hours of ovulation. Since sperm can survive in the body for up to 7 days, the conception date may not align exactly with the day intercourse took place, which is why a window of dates is given.

No — it's not possible to determine the exact day of conception with certainty. The calculator provides an estimate based on the standard 280-day pregnancy formula. Factors like irregular cycles, late ovulation, and the lifespan of sperm all introduce variability. Your doctor or an early ultrasound can help narrow down the timing.

Knowing your conception date can help confirm paternity timelines, provide peace of mind, and satisfy natural curiosity. It can also help identify the ovulation window, which is useful for future pregnancy planning. However, for medical purposes, your healthcare provider will rely on ultrasound measurements rather than a calculated conception date.

If your due date is August 1st, subtracting 280 days places your estimated conception date around October 25th of the previous year. The likely intercourse window would be roughly October 18th–25th, accounting for the days sperm can remain viable before fertilizing an egg.

Yes, due dates are estimates and can vary. Early ultrasounds (before 14 weeks) are the most accurate method for dating a pregnancy. Due dates calculated from the last menstrual period assume a regular 28-day cycle, which not everyone has. A difference of 1–2 weeks is quite common and does not indicate a problem.

To find out how many weeks pregnant you are today, this calculator counts the days from your estimated conception date to today and divides by 7. Because pregnancy is dated from the last menstrual period (about 2 weeks before conception), the calculator adds those 2 weeks to align with standard obstetric dating.