Practical guides · Tool
Ovulation calculator: your most fertile days
Enter the first day of your last period and the usual length of your cycle. The calculator estimates your fertile days for the next 3 cycles. It is an estimate: further down, you will see how to be more precise.
A guide only, not medical advice. The date you enter stays on your phone.
How it is worked out
Ovulation usually comes about 14 days before your next period. So the calculator starts from the date your next period is due and counts 14 days back. In a 28-day cycle, ovulation falls around day 14 or 15.
Your fertile days run from 5 days before ovulation to the day after it. Sperm live about 3 days inside the body, sometimes 5. The egg lives about one day. The best chances are in the two days before ovulation.
The limits of the calculation
Even when your cycle is regular, the day you ovulate moves from month to month. One study found that only about 30% of women have all their fertile days between day 10 and day 17. The time from ovulation to the next period also varies: 12 days on average, but from 7 to 17 days from one woman to another.
How to be more precise
- Watch your cervical mucus. Before ovulation, the discharge becomes wetter, clear, slippery and stretchy, like raw egg white. This costs nothing, and it predicts your fertile days as well as tests from a pharmacy.
- Ovulation tests. They measure a hormone in urine (LH). When the test is positive, ovulation usually comes within two days.
- Your morning temperature. It rises by 0.3 to 0.6 °C after ovulation. It confirms that ovulation has happened, but it cannot predict it.
If your cycle is irregular
For an adult woman, a cycle of 24 to 38 days is normal. If your cycles are often shorter or longer, or change by more than 7 days, the calculation is not reliable. Watch your mucus instead, or talk to a midwife or a gynaecologist.
For insemination
Insemination should happen as close to ovulation as possible. At a clinic, the team follows your ovulation with blood tests, urine tests or ultrasound, which is far more precise than a calendar. See fertility treatment in South and Southeast Asia.
Sources
- American College of Obstetricians and Gynecologists (ACOG), fertility awareness-based methods, reviewed September 2026.
- American Society for Reproductive Medicine (ASRM), Optimizing natural fertility, 2022.
- Wilcox et al., the fertile window in the menstrual cycle, BMJ, 2000; Bull et al., over 600,000 cycles, npj Digital Medicine, 2019.
- International Federation of Gynecology and Obstetrics (FIGO), definition of a normal cycle, 2018.
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