Ovulation Calculator
Estimate your ovulation day, fertile window, and next period.
An estimate based on average cycle math, not a contraception method or medical advice — actual ovulation timing varies cycle to cycle, and this doesn't account for irregular cycles.
Estimated ovulation day
Tue, Sep 15
Fertile window
Thu, Sep 10 – Wed, Sep 16
Next period (estimated)
Tue, Sep 29
For informational and educational purposes only — not medical advice, and not a substitute for consulting a qualified professional about your specific situation. TrueMeasureKit is not liable for decisions made based on these results. See our Terms of Service.
Why ovulation is calculated backward from the next period
The luteal phase — the time from ovulation to the next period — is far more consistent across cycles and across women (typically 12–14 days) than the time leading up to ovulation, which is why this calculator works backward from the predicted next period rather than forward from the last one. The fertile window spans roughly 5 days before ovulation to 1 day after, reflecting how long sperm can survive in the body versus the shorter window the egg itself remains viable. For a due date estimate if you're already expecting, see the pregnancy due date calculator.
Frequently asked questions
How is ovulation day estimated?
Typically as roughly 14 days before the start of your next expected period, based on your average cycle length — the fertile window is then estimated as the several days leading up to and including that day, since sperm can survive a few days awaiting ovulation.
How reliable is a calendar-based estimate?
It works best for people with regular, predictable cycles. Cycle length naturally varies month to month for many people, so methods like ovulation predictor kits or tracking basal body temperature can add more precision than calendar math alone.