Conception Calculator
Estimate when you likely conceived — from a due date or your last period — along with your fertile window, ovulation day and other key dates.
Your details
How to use this calculator
1. Choose your starting point
Pick "Last period" if you know the first day of your last menstrual period, or "Due date" if a provider has given you an estimated due date.
2. Enter the date
Fill in the date for your chosen mode. For the last-period mode you can also set your average cycle length, which shifts the estimated ovulation day.
3. Read the estimates
The big date is the most likely conception day. The cards show your fertile window, ovulation day, and the matching last-period and due dates.
Why these are estimates
Ovulation timing varies between cycles, and conception can occur a few days after intercourse. The fertile window covers that uncertainty rather than pinning a single day.
About conception dating
How is the conception date estimated?
Conception usually happens around ovulation. From a due date it is estimated as 266 days earlier. From your last period it is the period date plus your cycle length minus 14 days, which is the typical day of ovulation.
What is the fertile window?
It is the span of days when intercourse is most likely to lead to pregnancy: roughly the five days before ovulation through the day after, because sperm can survive several days while the egg lives about a day.
Not medical advice
These dates are estimates for general information only and cannot establish exactly when conception occurred. For confirmation or any concerns, consult a qualified healthcare professional.