Period Calculator
Predict your next periods and cycle dates from your average cycle length.
From the first day of one period to the first day of the next (typically 28).
Next Ovulation
Next Fertile Window
Predicted Periods
| Cycle | Start Date | End Date |
|---|
These predictions assume regular cycles. Actual dates may vary, especially if your cycle length changes from month to month.
Understanding the Menstrual Cycle
The menstrual cycle is the monthly series of hormonal changes that prepare the body for a possible pregnancy. It is counted from the first day of one period (menstrual bleeding) to the first day of the next. A typical cycle lasts about 28 days, but anything from 21 to 35 days is considered normal in adults. The cycle has several phases: menstruation, the follicular phase, ovulation, and the luteal phase.
How This Calculator Works
Starting from the first day of your last period (LMP), each future period start date is estimated by adding multiples of your average cycle length. The bleeding end date is the start date plus the period duration minus one day. Ovulation is estimated to occur about 14 days before the next period begins, and the fertile window spans roughly five days before ovulation through one day after it.
Period start (cycle i) = LMP + i × cycle length
Period end = start + (duration − 1)
Ovulation = next period start − 14 days
Fertile window = ovulation − 5 days … ovulation + 1 day
Cycle Length Variability
Very few people have a cycle that is exactly the same length every month. Research shows that cycle length naturally varies by several days from one month to the next for most people. Because of this, calendar based predictions are estimates rather than guarantees. Factors that can shift your cycle include:
- Stress, illness, and significant changes in sleep
- Travel across time zones and major schedule changes
- Large changes in weight, diet, or exercise intensity
- Hormonal contraception, perimenopause, and certain medical conditions such as PCOS or thyroid disorders
For the most reliable predictions, track your cycle for several months and use your personal average cycle length rather than a fixed value of 28 days. The further into the future a prediction is, the more uncertainty it carries.
Frequently Asked Questions
Can I rely on this for contraception?
No. Calendar based predictions are not a reliable form of birth control on their own. Sperm can survive for several days, ovulation timing varies, and the fertile window shown here is an estimate. If you wish to avoid or achieve pregnancy, speak with a healthcare provider about appropriate methods.
What if my cycles are irregular?
If your cycle length varies widely or you frequently skip periods, calendar based estimates become much less accurate. Tracking basal body temperature or using ovulation predictor kits can give more personalized insight. Persistent irregularity is worth discussing with a doctor.
Why is ovulation 14 days before the next period?
The luteal phase, the time between ovulation and the next period, is relatively stable at about 14 days for most people. The follicular phase before ovulation is what varies most. That is why ovulation is estimated by counting back from the predicted next period rather than forward from the last one.
Note: This calculator provides estimates based on the calendar method and assumes regular cycles. It is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Consult a healthcare professional for questions about your menstrual health, fertility, or contraception.
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