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A period calculator predicts when your next several periods are due from the first day of your last period and your average cycle length. Counting forward one cycle length gives each period start; ovulation falls about 14 days before the next period, and the fertile window is the five days before ovulation plus the day itself. It also lays out a schedule of upcoming cycles with their period, ovulation and fertile dates.

On a 28-day cycle, your next period is due 28 days after the last one began, the one after that at 56 days, and so on for as many cycles as you ask it to show. Ovulation for each cycle is placed 14 days before that period start, with the fertile window running the five days before it. Enter a longer or shorter average and every projected date shifts by the same amount.

Use it to know when to expect your period over the coming months, to plan around it, or to see your fertile days alongside each cycle. Set your usual period length so the schedule shows start and end dates. Real cycles vary by a few days month to month with stress, illness and hormones, so the predictions are estimates; a consistently missed period is worth a pregnancy test or a clinician’s advice.