🌸 Ovulation Calculator
Ovulation typically happens about 14 days before your next period — so for a 28-day cycle it's around day 14, and the fertile window spans the 5 days before ovulation plus the day itself. Enter your values below — results update instantly, entirely on your device.
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How the ovulation calculator works
Estimated ovulation = first day of last period + (cycle length − 14), because the luteal phase after ovulation is fairly constant at about 14 days. The fertile window is the 5 days before ovulation through 1 day after, since sperm survive several days and the egg about a day.
Example: Last period started June 1, 28-day cycle → ovulation about June 15, fertile window roughly June 10–16, next period about June 29.
This is an estimate based on a regular cycle and the calendar method — it is not a form of contraception and not medical advice. Real ovulation shifts with cycle variability, stress and health, so ovulation tests, temperature tracking or a clinician give more accuracy. Your dates are personal data; this tool keeps them on your device and uploads nothing.
Frequently asked questions
When am I most fertile?
In the roughly 6-day window ending on ovulation day — the 5 days before plus the day of ovulation — because sperm can survive up to about 5 days and the egg for about 24 hours. The calculator marks this window from your last period and cycle length.
How is the ovulation day estimated?
By counting back ~14 days from your expected next period (the luteal phase is relatively fixed). For a cycle of length C, ovulation is estimated at day C − 14 of the cycle.
How accurate is a calendar ovulation estimate?
It's a useful approximation for regular cycles but can be off by days if your cycle varies. For higher accuracy, combine it with ovulation predictor kits, basal body temperature, or cervical-mucus tracking.
Can I use this as birth control?
No. It is not a contraceptive method and doesn't account for cycle variability. If you're avoiding pregnancy, use a proven contraceptive method and speak with a healthcare provider.
Is this ovulation calculator accurate and private?
Yes. It uses the standard published formula, shows its working under every result, and computes locally in your browser — your inputs are never sent to a server, and the page works offline.