Period Calculator
Enter your last period date and average cycle length to predict your next 6 periods, ovulation dates, and fertile windows.
Disclaimer: The results provided by this tool are estimates for informational purposes only and do not constitute medical advice. Please consult a healthcare professional for personal health decisions.
How the cycle prediction works
The period calculator starts from the first day of your last period and adds your average cycle length to project the next start date. Cycle length defaults to 28 days because that is the textbook average, but the field is adjustable — most people fall somewhere between 21 and 35 days, and the tool extrapolates across as many future cycles as you want to see. Subsequent periods are projected by stacking the same cycle length repeatedly, which means any drift in your actual cycles compounds over time and the prediction becomes looser the further forward you look.
Ovulation is estimated as cycle length minus 14 days, reflecting the relatively stable length of the luteal phase after ovulation. The fertile window is drawn as a five-day span ending on the ovulation day, because sperm can survive up to about five days in the reproductive tract while an egg remains viable for roughly 24 hours. These are population-based estimates — individual ovulation timing can shift due to stress, illness, or travel. Treat the results as a general guide; a healthcare provider or a medical-grade tracking method can give more accurate answers for family planning or contraception.
When the period calculator is most helpful
Cycle tracking is useful for packing, travel, and scheduling around events where cramps or heavier flow would be inconvenient. Many people use the calculator to pre-order supplies, plan exercise intensity, or anticipate PMS-related mood shifts. Others use it to notice when a period is unusually late or early, which can be a cue to take a pregnancy test or to review recent lifestyle changes like new medication, weight shifts, or unusual stress. Seeing several cycles laid out on a calendar also makes it easier to recognise patterns that were not obvious day to day.
Irregular cycles are the main edge case. If your cycles vary by more than a few days, enter the average length from your last three to six cycles rather than a single month, and treat the fertile window as wider than the tool shows. Conditions such as polycystic ovary syndrome, thyroid disorders, perimenopause, and recent pregnancy or breastfeeding can all disrupt cycle regularity. This calculator is an informational convenience, not a diagnostic or contraceptive device — consult a healthcare provider for medical guidance, especially if cycles are consistently unpredictable, very painful, or unusually heavy.