When are your fertile days?
Fertile days are the days in your cycle when conception is possible. An egg lives roughly 12 to 24 hours after ovulation, while sperm can survive in the body for up to five days. That's why the fertile window covers roughly five days before ovulation, ovulation day, and the day after – about six days per cycle in total.
The highest chance of conceiving falls in the two to three days before ovulation and on ovulation day itself.
How does the calculator work out ovulation?
The second half of the cycle (the luteal phase, from ovulation to the next period) is fairly consistent for most women and lasts around 14 days. The calculator predicts ovulation by subtracting 14 days from the cycle length. For a 28-day cycle that's roughly day 14; for a 32-day cycle, day 18.
The first half of the cycle (the follicular phase) is the part that varies most from cycle to cycle. That's why the estimate is most reliable when your cycle is regular; with irregular cycles, read the result as a rough estimate.
Signs of ovulation and more precise methods
Around ovulation, some women notice a clearer, stretchier discharge, mild one-sided lower abdominal discomfort, or a shift in basal body temperature, which rises by roughly 0.2–0.5 °C after ovulation.
Ovulation predictor kits (LH tests), which measure the rise in luteinizing hormone in urine, along with temperature and discharge tracking, pin down ovulation more precisely than a calendar estimate. If you've been trying to conceive for a while or your cycle is very irregular, talk to a gynecologist.
Frequently asked questions
How many days after my period is ovulation?
That depends on cycle length, not on the period itself. Ovulation falls roughly 14 days before your next period – around day 14 of a 28-day cycle, counting from the first day of your period.
How long do fertile days last?
The fertile window lasts about six days: five days before ovulation, ovulation day, and the day after.
When am I most fertile?
In the two to three days before ovulation and on ovulation day itself.
Can I use a fertile days calculator as contraception?
No. A calendar-based estimate is built on an average and doesn't account for variation in any given cycle, so it isn't a reliable method of contraception.
What if my cycle is irregular?
Enter the average length of your last few cycles and read the result as a rough estimate. Ovulation predictor kits and tracking body signs are more precise.
Sources
- Wilcox AJ et al. – Timing of sexual intercourse in relation to ovulation (N Engl J Med, 1995)
- NHS – Periods and fertility in the menstrual cycle
- ACOG – Fertility Awareness-Based Methods of Family Planning
Updated: October 2026
For information only, not a substitute for medical advice.