Estimate your ovulation day and fertile window from your last period.
This is an estimate for planning conception, not a contraceptive method — it cannot reliably prevent pregnancy.
The calendar method places ovulation a luteal-phase length (typically 14 days) before your next period: ovulation = last period + (cycle length − luteal length).
The fertile window spans the five days before ovulation plus the day of ovulation, because sperm can survive several days in the reproductive tract.
It is an estimate based on averages. Cycles vary month to month, so ovulation tests or temperature tracking are more precise.
No. The fertile window shifts with cycle variation, so calendar timing alone is not a reliable contraceptive.
The time from ovulation to your next period — usually 12–14 days and fairly consistent for each person.
This calculator estimates your ovulation day and fertile window from the date your last period began and your typical cycle length. Use it to identify when you are most likely to conceive, to track patterns across three projected cycles, or simply to understand where you are in your cycle. Results are an estimate based on an average luteal phase — individual cycles vary, and this tool is not a contraceptive method.
The headline dates show your estimated ovulation day and the fertile window (the roughly six-day span ending the day after ovulation). The cycle wheel or timeline marks each phase so you can see how many days remain in your current cycle. The three-cycle table beneath projects the same dates for the next two full cycles. All dates assume your cycle is consistent; if your cycle is irregular, treat every date as an approximation.
Last menstrual period starts on 1 April 2026. Cycle length: 28 days. Luteal phase: 14 days (default).
Estimated ovulation: 15 April 2026 (day 15 of the cycle). Fertile window: 10 April – 16 April 2026 (days 10–16). Next period expected: 29 April 2026. These are estimates — actual timing can shift by several days.
The calculator uses the calendar method: ovulation is placed one luteal phase before the next expected period. For women with regular 28-day cycles the estimate is often within a day or two, but actual ovulation can shift by several days due to stress, illness, or natural variation. Ovulation tests or basal body temperature tracking are more precise if accuracy matters.
The fertile window is the period during which pregnancy is possible from intercourse. It runs from five days before ovulation through the day after, because sperm can survive in the reproductive tract for up to five days while the egg is viable for roughly 12–24 hours. The calculator marks this six-day span on the cycle wheel.
No. Calendar-based estimates are not a reliable method of birth control. The fertile window shifts with normal cycle variation, so timing intercourse by calendar alone carries a significant failure rate. Speak to a healthcare provider about evidence-based contraceptive options.
The luteal phase is the time from ovulation to the start of the next period — typically 12 to 14 days, and more consistent across individuals than the follicular phase. The default is 14 days. If you have tracked your own cycles and know your luteal phase differs, entering your actual value will give a more personalised estimate.
Because ovulation is estimated backward from the next expected period, a longer cycle pushes the estimated ovulation day later in the month — not earlier. For example, a 35-day cycle with a 14-day luteal phase places ovulation on day 21, whereas a 28-day cycle places it on day 14. Entering your real average cycle length is the single biggest factor in accuracy.
Ovulation is estimated as: Ovulation date = LMP + (cycle length − luteal length). With a 28-day cycle and 14-day luteal phase this places ovulation on day 14 of the cycle. The fertile window runs from five days before ovulation through the day after ovulation. Three cycles are projected by adding successive cycle-length multiples to the LMP. The method follows the calendar (rhythm) approach described by ACOG and the Cleveland Clinic (sources listed on this page). Because it relies on an assumed cycle regularity, it is informational only.
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