Ovulation Calculator
Find your estimated ovulation day and fertile window from your last period and cycle length, plus the next three cycles.
Quick answer: Enter the first day of your last period and your average cycle length, and we estimate your ovulation day (about 14 days before your next period) and your fertile window — the five days before ovulation plus ovulation day itself — for the next three cycles.
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Frequently asked questions
- How is the ovulation day estimated?
- By counting back ~14 days from your projected next period, since the luteal phase (ovulation to period) averages 14 days and varies less than the first half of the cycle. Your next period is projected from your last period plus your cycle length.
- What is the fertile window?
- The roughly six days when conception is possible: the five days before ovulation plus ovulation day itself. Sperm can survive up to ~5 days, while the egg lives about 12–24 hours after release.
- How accurate is this?
- It's a calendar estimate that assumes regular cycles and a typical 14-day luteal phase. Actual ovulation shifts with stress, illness and hormonal changes. For precision, combine it with ovulation predictor kits or basal body temperature tracking.
- What if my cycles are irregular?
- Calendar estimates are less reliable with irregular cycles. Use your average length as a rough guide, but lean on ovulation tests or fertility monitoring for irregular cycles.
- Can this help me avoid pregnancy?
- Not reliably on its own — calendar-based methods have a meaningful failure rate. Don't use this as your only contraception; talk to a healthcare provider about effective options.
- Why do you show the next three cycles?
- So you can plan ahead — the table projects ovulation days and fertile windows for the coming months based on your average cycle length.
- What cycle length should I enter?
- The number of days from the first day of one period to the first day of the next, averaged over a few months. Typical cycles run 21–35 days; we accept that range.
- Is my data uploaded?
- No — all calculations run locally in your browser. Your dates and cycle information never leave your device.
- Does the fertile window change with cycle length?
- The window is always tied to ovulation, which we place 14 days before your next period — so a longer cycle pushes both ovulation and the fertile window later.
- Is this medical advice?
- No — it's an informational estimate. For fertility concerns or family planning, consult a doctor or fertility specialist.