Ovulation Calculator
Estimate your ovulation date, fertile window, and next period. Free, instant, private.
How this estimate works
This calculator uses the calendar method, a common way to estimate ovulation. Count forward from the first day of your last period by your average cycle length. That gives your estimated next period date.
Then count back 14 days. Those 14 days are your luteal phase, the stretch between ovulation and your next period. Phzse calls the peak days around ovulation your Inner Summer: short, high-energy, and usually the most consistent marker in your cycle.
The luteal phase tends to hold a steady length, even when the days leading up to ovulation shift around. That is why counting back from your next period gives a more reliable estimate than counting forward from your last one.
Planning further out? The cycle calculator projects your upcoming periods and fertile windows across several months.
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Frequently asked questions
How is ovulation calculated?
This calculator uses the calendar method. It counts forward from the first day of your last period by your average cycle length to estimate your next period, then counts back 14 days. Those 14 days represent the luteal phase, the time between ovulation and your next period. The luteal phase is usually more consistent in length than the days leading up to ovulation, which is why counting backward from your next period gives a more reliable estimate.
How accurate is this ovulation calculator?
This is an estimate, not a measurement. It works best for people with regular, predictable cycles. Stress, illness, travel, weight changes, and many other factors can shift the actual day of ovulation earlier or later than the calendar method predicts. For a closer estimate, track ovulation signs like cervical mucus changes or use an ovulation predictor kit alongside this calculator.
What if my cycles are irregular?
The calendar method is less reliable for irregular cycles, since it assumes a fairly steady cycle length. If your cycles vary by more than a few days each month, treat this estimate as a rough starting point rather than a precise date. Tracking your cycle over a few months, whether with Phzse or another method, gives you a clearer picture than a single calculation.
How long is the fertile window?
Your fertile window is about 6 days: the 5 days before your estimated ovulation date and the day of ovulation itself. This range accounts for how long sperm can survive in the body and the short lifespan of the egg after it is released. Fertility is generally highest in the day or two right before ovulation.
When should I test?
If you are using an ovulation predictor kit, start testing a few days before your estimated ovulation date to catch the LH surge. If you are testing for pregnancy, most home tests are more reliable starting around the day of your expected next period, since testing earlier can miss low hormone levels. This calculator gives estimates only. It is not a substitute for a pregnancy test, an ovulation test, or medical advice.