Ovulation occurs approximately 14 days before the next expected period, regardless of total cycle length. In 28-day cycles, that is around day 14. In 35-day cycles, around day 21. This calculation is an estimate for regular cycles; irregular cycles require complementary methods.
Fertile Window Calculator
Estimated ovulation, fertile window and cycle calendar — with guidance on irregular cycles and clear disclaimers.
How ovulation is estimated — and why irregular cycles make the calculation less reliable.
Ovulation occurs approximately 14 days before the next menstrual period, regardless of total cycle length. In a 28-day cycle, that is day 14. In a 35-day cycle, it is day 21. The fertile window includes up to 5 days before ovulation — the time sperm can survive in the reproductive tract — and 1 day after, which is the lifespan of the released egg.
Enter your LMP and usual cycle length to see the estimated fertile window.
- Enter the date of the first day of your last menstrual period (LMP).
- Adjust the usual cycle length (default: 28 days). The cycle can range from 21 to 45 days.
- See the estimated ovulation date, 7-day fertile window and a visual calendar with highlighted days.
Cycle phases and when ovulation occurs
The menstrual cycle begins on the first day of menstruation and divides into four phases: menstrual (days 1–5), follicular (days 6–13), ovulatory (around day 14 in 28-day cycles) and luteal (days 15–28).
Ovulation is triggered by an LH (luteinizing hormone) surge that occurs 24–36 hours before the egg is released. The egg has a 12–24 hour lifespan; sperm survive for up to 5 days in the reproductive tract.
In longer or irregular cycles, the follicular phase is the one that extends — the luteal phase has a relatively constant duration of 14 days in most individuals.
- Regular cycle: 21 to 35 days.
- Cycles shorter than 21 or longer than 35 days warrant gynecological evaluation.
- PCOS, stress and thyroid disorders affect the timing of ovulation.
How to monitor ovulation more accurately
The calendar method estimates ovulation but does not confirm it. For more accurate monitoring, the main methods are: basal body temperature (BBT), cervical mucus (Billings method), urinary LH tests (OPK) and follicular ultrasound.
BBT requires daily measurement upon waking, before any activity. A rise of 0.2–0.5°C indicates that ovulation has already occurred (retroactive confirmation, not predictive).
Urinary LH tests are predictive: they detect the surge that precedes ovulation by 24–36 hours, making them more useful for those trying to conceive.
Your data stays on your device
This tool works entirely in your browser. No period date, cycle length or fertility information is sent to servers, collected for analysis or stored.
Sources and references for this tool
These references help contextualize formulas, standards, APIs and limitations used on this page. They do not replace professional validation when a result has legal, financial, medical or operational impact.
- NHS — Understanding Your Menstrual CycleNHS — NHS reference on menstrual cycle, ovulation and fertile window.
- Fertility Awareness Methods — WHOWorld Health Organization — WHO context on natural family planning methods and their effectiveness rates.
- Menstrual Cycle — StatPearls (NCBI)NCBI / StatPearls — Detailed physiology of the menstrual cycle and ovulatory phases.