Conception Calculator
28 days
Best Conception Dates--
Ovulation Date--
This calculator estimates the days you are most likely to conceive by locating your fertile window around ovulation. From the first day of your last period and your average cycle length, it works out the probable ovulation date and the short window — a few days before through one day after — when intercourse is most likely to result in pregnancy. It is a planning aid based on cycle averages, not a guarantee.
Formula
Ovulation day = last period date + (cycle length − 14) days
- last period date
- First day of your most recent menstrual period
- cycle length
- Average number of days from one period to the next
- fertile window
- Ovulation day − 5 days through ovulation day + 1 day
How it works
- Enter the first day of your last menstrual period and your average cycle length in days (28 is typical).
- Ovulation is estimated to occur about 14 days before the next period, so the tool sets it at the start of your period plus (cycle length − 14) days.
- It then marks the fertile window from five days before ovulation through one day after, since sperm can survive several days and the egg is viable for about a day.
Worked example
A last period starting January 1 with a 28-day cycle.
- Ovulation = January 1 + (28 − 14) = January 1 + 14 days = January 15.
- Fertile window start = ovulation − 5 days = January 10.
- Fertile window end = ovulation + 1 day = January 16.
Best conception dates January 10–16, with ovulation around January 15.
Frequently asked questions
- How is the fertile window defined here?
- The tool counts the five days before estimated ovulation plus the day after. Sperm can live up to about five days in the reproductive tract and the egg survives roughly a day, so this span captures the highest-probability days.
- What if my cycle is irregular?
- The estimate assumes a fairly regular cycle, since ovulation is anchored at 14 days before the next period. If your cycles vary widely, the predicted window will be less reliable, and ovulation tracking methods may help.
- Does conceiving in the window guarantee pregnancy?
- No. Timing intercourse in the fertile window improves the odds in any given cycle, but conception still depends on many factors. Many healthy couples take several months of trying.
- How does this differ from a due date calculator?
- This tool looks forward from your last period to predict when to conceive. A due date calculator instead projects forward from a known last period or conception to estimate when the baby will arrive.