🌸 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.
Fill in the fields above, results appear instantly.
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.