What Is the Reverse Due Date Calculator?
The reverse due date calculator will help you no matter if you're already pregnant, planning to conceive, or just curious to find out the day you were created — to discover how old you truly are. ⭐
We'll help you with:
- Calculating your conception date based on your baby's birthday or due date;
- Planning the conception date, so you can obtain the due date of your desire.
We try our best to make our calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment. If any health condition bothers you, consult a physician.
How to Use the Reverse Due Date Calculator
Our reverse due date calculator only requires one value from you. It can be either:
- Your due date or birthday of the baby; or
- Your conception date.
The calculator works both ways — you can enter your data anywhere you want!
💡 By default, the length of pregnancy is set to 40 weeks (280 days), but please adjust this if you know the exact duration.
Our reverse pregnancy due date calculator will show you an estimated time interval where the intercourse most likely occurred or should occur.
Why Is It an Interval and Not a Single Date?
Human fertility is a complicated subject, and it depends on many unpredictable variables. That's why we always have to give ourselves some uncertainty range to avoid any possible errors.
So what may go wrong?
- Ovulation does not always happen on the very same day each month; and
- Conception does not have to occur on the calculated day of ovulation.
How to Calculate the Conception Date Based on Birthday?
Firstly, we need to make a few assumptions:
- Menstrual cycle length is equal to 28 days; and
- Human pregnancy lasts exactly 40 weeks, that is 280 days.
(These are the common assumptions we need to make in obstetrics — for example, in Naegele's rule for assessing the gestational age and the estimated delivery date.)
The equation we need looks as follows:
Conception = Birthday date − 280 days
or,
Conception = Due date − 280 days
Now we need to estimate the error range for the intercourse date; let's calculate the earliest possible day the intercourse occurred:
Intercourse (earliest) = Conception − 7 days
This accounts for the fact that sperm can survive inside the female reproductive tract for up to 5–7 days. So the intercourse window runs from 7 days before the estimated conception date to the conception date itself.
Planning Your Pregnancy Calendar
If you're planning a pregnancy and want to target a specific due date, simply select "Conception Date" mode and enter your desired conception date. The calculator will tell you:
- Your target due date (conception + 280 days);
- The intercourse window you should aim for (up to 7 days before conception).
This lets you plan backwards from the birthday you'd like your child to have — whether it's a summer baby, avoiding a holiday, or aligning with family plans.
American and International Date Formats
The calculator supports two date display formats:
- American format — dates are shown as Month Day, Year (e.g., January 15, 2025). Use this if you're in the United States.
- International (Metric) format — dates are shown as DD/MM/YYYY (e.g., 15/01/2025). Use this if you're in Europe, Latin America, or most other countries.
FAQs
How accurate is the reverse due date calculator?
The calculator provides a statistical estimate based on a standard 40-week (280-day) pregnancy and a 28-day menstrual cycle. Actual dates vary because ovulation timing, sperm survival, and individual biological factors differ from person to person. Use the result as a guide, not a certainty.
Can I use this calculator to find out when I was conceived?
Yes! Simply enter your own birthday in the "Baby's Birthday / Due Date" field, and the calculator will estimate the date your parents likely conceived you, as well as the intercourse window.
What if my pregnancy was longer or shorter than 40 weeks?
You can adjust the Pregnancy Length field to match your actual or estimated pregnancy duration (between 20 and 50 weeks). This will recalculate the conception date accordingly.
Is this calculator for medical use?
No. This tool is for informational and entertainment purposes only. For medical decisions about your pregnancy, always consult a qualified healthcare professional or obstetrician.