Menstrual Period Calculator

Enter Days Since Last Period Started (how many days ago your period began), Average Cycle Length (the typical span from one period's start to the next), and Period Duration (how many days your bleeding usually lasts), and this calculator projects your next period, ovulation window, and current cycle phase from those three numbers. Next Period In is Average Cycle Length minus Days Since Last Period Started -- if that figure would come out zero or negative (meaning your predicted period date has already passed), the calculator adds a full cycle back on so the result always reads as days remaining, not days overdue. Ovulation In works the same way, built from the standard 14-day luteal-phase assumption: ovulation is estimated to fall 14 days before the next period, and the Fertile Window spans five days before that ovulation estimate through one day after it, reflecting how long sperm can survive and the egg's viability window.

Current Cycle Day counts forward from day 1 of your last period, and Current Phase reports Menstrual, Follicular, Fertile Window, Ovulation, or Luteal based on where that day falls relative to Period Duration and the estimated ovulation day. Average Cycle Length has the largest effect on Next Period In and Ovulation In because it directly sets where both land; Period Duration only affects Current Phase, deciding whether today counts as the Menstrual phase, and otherwise leaves the day-count predictions unchanged. This is a statistical estimate based on average cycle timing, not a measurement of your actual hormone levels or ovulation -- it cannot account for cycle irregularity, PCOS, recent hormonal birth control use, stress, illness, or any factor that shifts an individual cycle away from its historical average.