Period Calculator

Knowing when your next period is due helps with planning — trips, events, or just having supplies ready — and makes it easier to spot when something changes. Enter the first day of your last period, your usual cycle length, and how long your period lasts, and this calculator forecasts your next several periods along with each cycle’s fertile window.

How the prediction works

Your cycle is counted from the first day of one period to the first day of the next. To find your next period, add your cycle length to the day your last period started:

Next period = first day of last period + cycle length.

The calculator repeats this to project several cycles ahead, and uses your period length to show the expected end date of each one. That simple sum is all there is to how to calculate your next period date — the value is in seeing several months laid out at once.

What’s a normal cycle?

• Cycle length: usually 21–35 days in adults (a little longer for teenagers).
• Period length: usually 2–7 days.
• Blood loss: typically light to moderate; soaking a pad or tampon every hour for several hours is not typical and is worth checking.

Cycles are naturally more variable in the first years after your periods start and during perimenopause.

The phases of your cycle

Your cycle — the menses cycle — moves through four phases, and knowing them makes the predictions above make sense:

• Menstrual (about days 1–5): the period itself, when the uterine lining sheds. Day 1 is the first day of full flow, and it’s where every calculation starts.
• Follicular (about days 1–13): overlapping with the period, the body ripens an egg. This phase varies in length, which is what makes cycles longer or shorter.
• Ovulation (around day 14): an egg is released — the fertile peak. In a 28-day cycle this lands mid-cycle; in longer cycles it comes later.
• Luteal (about days 15–28): the roughly two weeks after ovulation, which stays fairly constant at about 14 days. If no pregnancy occurs, the next period begins and the cycle restarts.

Because the luteal phase is stable while the follicular phase flexes, most of the variation between cycles comes from the first half — which is why the calculator counts forward from your last period rather than assuming a fixed ovulation day.

Ovulation and the fertile window

Each cycle also has a fertile window. Ovulation is estimated about 14 days before the next period, and the fertile window spans the five days before ovulation through the day after. The calculator shows this cycle’s ovulation day and fertile window — useful whether you’re trying to conceive or simply understanding your body. For a fuller view of your fertile days across upcoming cycles, use the ovulation calculator; and if a period is late and pregnancy is possible, the pregnancy calculator can date things once you know.

When to see a clinician

Predictions assume fairly regular cycles. Talk to a doctor if you notice:

• Periods that stop for three or more cycles (and you’re not pregnant or in menopause)
• Cycles consistently shorter than 21 days or longer than 35 days
• Bleeding between periods or after sex
• Periods that are much heavier or more painful than usual

This tool is for general information and planning, not a diagnosis or a method of contraception.