Period Calculator
Predict your next period, fertile window, and ovulation date based on your cycle length and last period.
Enter your last period date and cycle information.
How Does the Formula Work?
The period calculator predicts upcoming menstrual periods, ovulation dates, and fertile windows based on your average cycle length and last period start date. It generates a forward-looking calendar of the next 6 cycles, helping you plan ahead for periods, track fertility, or simply understand your body's rhythm.
Next Period Start = Last Period Start + Cycle Length
Ovulation = Next Period Start − 14 days (luteal phase)
Fertile Window = Ovulation − 5 days to Ovulation + 1 day
Period End = Period Start + Duration − 1
Example: Last period May 1, Cycle 28 days, Duration 5 days
Next Period: May 29 – Jun 2
Ovulation: May 15
Fertile Window: May 10 – May 16
How the Menstrual Cycle Works
The menstrual cycle has two main phases. The follicular phase begins on the first day of your period and ends at ovulation — its length varies between individuals and even cycle to cycle. The luteal phase follows ovulation and lasts until the next period — it is remarkably consistent at approximately 14 days for most people. This consistency is why ovulation can be estimated by subtracting 14 days from the expected next period, regardless of total cycle length. A 21-day cycle ovulates around day 7, a 28-day cycle around day 14, and a 35-day cycle around day 21.
The Fertile Window
The fertile window spans approximately 6 days — the 5 days before ovulation plus the day of ovulation itself. Sperm can survive in the reproductive tract for up to 5 days, while the released egg remains viable for 12 to 24 hours. This means pregnancy is possible from intercourse that occurs several days before ovulation, not just on the day of ovulation itself. The calculator marks this window to help with either conception planning or awareness of peak fertility days.
Cycle Variability
Natural cycle variation of 1 to 7 days is common and normal. Stress, illness, travel, significant weight changes, intense exercise, and hormonal fluctuations can all shift timing. The predictions from this calculator assume a consistent cycle length, but real cycles are rarely perfectly regular. Tracking your period over several months gives a better average. If your cycles are consistently shorter than 21 days, longer than 35 days, or highly irregular, consulting a gynecologist is recommended.
Important Limitations
This calculator provides estimates based on average cycle patterns. It should not be used as a contraceptive method — the calendar method alone has a high failure rate because ovulation timing can shift unpredictably. It also cannot detect pregnancy, hormonal disorders, or other medical conditions. For contraception, fertility planning, or any reproductive health concerns, consult a qualified healthcare provider who can offer methods and guidance appropriate to your individual situation.
Tips & Recommendations
The more cycles you track, the better your average cycle length estimate. Most apps need 3+ months of data for reliable predictions.
Ovulation occurs approximately 14 days before the next period, regardless of cycle length. This is because the luteal phase is consistent.
Sperm live up to 5 days, the egg about 24 hours. The fertile window is ovulation minus 5 days to ovulation plus 1 day.
Cycle predictions are estimates. Do not rely on this calculator as a birth control method. Consult your doctor for reliable contraception.
Frequently Asked Questions
How does the period calculator predict my next period?
It adds your average cycle length to your last period start date. If your last period started May 1 and your cycle is 28 days, your next period is predicted for May 29.
How is ovulation calculated?
Ovulation typically occurs 14 days before the next period starts (the luteal phase). For a 28-day cycle starting May 1, the next period is May 29 and ovulation is around May 15.
What is the fertile window?
The 6-day window from 5 days before ovulation to 1 day after. Sperm can survive up to 5 days, and the egg is viable for about 24 hours after release.
How accurate are these predictions?
Predictions are estimates based on your average cycle. Natural variation of 1–5 days is normal. Stress, illness, travel, and hormonal changes can all affect timing. This tool should not be used as a contraceptive method.
What is a normal cycle length?
Cycles between 21 and 35 days are considered normal by the American College of Obstetricians and Gynecologists. The average is 28 days, but many healthy cycles are shorter or longer.
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