Cycle Day Today Calculator
Enter the first day of your last period to see which cycle day you’re on today, the likely phase, and how many days until your next period.
How it works
cycle day = (days since current cycle start) + 1 · next period = current cycle start + cycle length · ovulation ≈ day (cycle − 14)
Cycle day one is the first day of bleeding, so today’s cycle day is just the number of days since your most recent period start, plus one. When more than one cycle length has passed, the calculator rolls forward to the current cycle so the count stays in range. Phases are estimated from the classic model — menstrual, follicular, a fertile window around ovulation near day (cycle − 14), then luteal — which is a guide rather than a precise clock, since ovulation timing shifts between cycles.
Worked example
If your last period began September 2, 2026 and today is September 16, that is 14 days later — cycle day 15 of a 28-day cycle. Ovulation is estimated around day 14, so this lands in the fertile ovulation window, with the next period due about September 30.
Frequently asked questions
What counts as cycle day 1?
The first day of full menstrual bleeding, not spotting. Everything is counted forward from there, so accurate tracking of that day is what makes the cycle-day number reliable.
How is the ovulation day estimated?
It uses the luteal phase of about 14 days, so ovulation is placed near day (cycle length − 14). For a 28-day cycle that is day 14; for a 32-day cycle it is day 18.
Can I rely on the phase for contraception?
No. Phase estimates are approximate and ovulation can move by several days, so this is not a reliable method to prevent or achieve pregnancy on its own.
Related calculators
Educational estimate — phase timing varies and this is not a contraception or fertility guarantee.