How to use the ovulation calculator
- Enter the first day of your last period – the first day of full bleeding, not spotting.
- Choose your average cycle length: the days from the first day of one period to the day before the next.
- Leave the luteal phase at 14 days unless you have tracked yours (from ovulation tests or temperature charts).
- Press Calculate for your next fertile window, ovulation date and period, a picture of the cycle, and the next six cycles.
How ovulation is estimated
The second half of the cycle – the luteal phase, from ovulation to the next period – is fairly constant at about 14 days. The first half varies from person to person. So ovulation is counted back from the next period:
\text{ovulation day} = \text{cycle length} - \text{luteal phase}An egg lives about 12–24 hours, but sperm can survive up to five days, so the fertile window is the five days before ovulation, the day itself and the day after. The two days before ovulation give the highest chance.
- Ovulation: day 30 − 14 = day 16, which is Friday, October 16, 2026.
- Fertile window: October 11, 2026 to October 17, 2026.
- Next period: October 31, 2026.
Ovulation day by cycle length
| Cycle length | Ovulation day | Fertile days |
|---|---|---|
| 21 days | day 7 | days 2–8 |
| 24 days | day 10 | days 5–11 |
| 26 days | day 12 | days 7–13 |
| 28 days | day 14 | days 9–15 |
| 30 days | day 16 | days 11–17 |
| 32 days | day 18 | days 13–19 |
| 35 days | day 21 | days 16–22 |
Signs of ovulation
- Cervical mucus becomes clear, slippery and stretchy, like raw egg white, in the most fertile days.
- Ovulation test kits detect the rise in luteinising hormone (LH) 24–36 hours before ovulation.
- Basal body temperature rises by about 0.2–0.5 °C after ovulation and stays up until the next period – it confirms ovulation afterwards.
- Some people feel a one-sided twinge (mittelschmerz) or breast tenderness.
Combining the calendar with one of these signs pinpoints ovulation much better than the calendar alone.
Limits
Calendar predictions assume your cycles are regular. Stress, illness, travel, breastfeeding and conditions such as polycystic ovary syndrome (PCOS) can shift ovulation, and cycles of different lengths make predictions less reliable. This calculator is not a method of contraception. If you have been trying to conceive for a year (six months if you are 35 or over), see a doctor. Once you are pregnant, the due date calculator works out when your baby is due.
Frequently asked questions
When do I ovulate?
About 14 days before your next period. In a 28-day cycle that is day 14; in a 32-day cycle, day 18.
What are my most fertile days?
The five days before ovulation and the day of ovulation, with the best chances on the two days before. The calculator shows these dates for your next six cycles.
Can I get pregnant right after my period?
It is unlikely but possible with short cycles, because sperm can live up to five days. In a 21-day cycle, ovulation is around day 7.
How accurate is an ovulation calculator?
It is a good guide for regular cycles, but ovulation can move by several days. Ovulation test kits or tracking cervical mucus give a more precise answer.
When should I take a pregnancy test?
From the day your period is due. Testing earlier can give a false negative, because the pregnancy hormone may not have built up yet.