Fitness & Health · Live
Track your cycle,
predict your period.
Enter your last period date and average cycle length to instantly predict your next six periods, fertile windows, and ovulation days , with a visual calendar that keeps every phase in view.
Cycle inputs
When did your last period start?
Cycles typically range from 21 to 45 days; 28 is the common average. Ovulation is predicted 14 days before your next expected period, accounting for your unique cycle length.
Next period
In 14 daysCycle calendar
May 2026
Upcoming periods
Next 6 predicted periods
| # | Starts | Fertile window |
|---|---|---|
| 1 | May 29 | June 7 – June 12 |
| 2 | Jun 26 | July 5 – July 10 |
| 3 | Jul 24 | August 2 – August 7 |
| 4 | Aug 21 | August 30 – September 4 |
| 5 | Sep 18 | September 27 – October 2 |
| 6 | Oct 16 | October 25 – October 30 |
Field guide
How menstrual cycle prediction works.
A menstrual cycle is counted from the first day of one period to the first day of the next. The average is 28 days, but normal cycles range from 21 to 45 days. This calculator uses your last period date and average cycle length to predict your next periods, ovulation days, and fertile windows, across six cycles ahead so you can plan months in advance.
The two phases of your cycle
Every menstrual cycle has two hormonally distinct halves divided by ovulation:
- Follicular phase: from the first day of bleeding until ovulation. This phase varies in length and is the reason cycles differ from person to person. A person with a 24-day cycle has a shorter follicular phase than someone with a 35-day cycle.
- Luteal phase: from ovulation until the next period begins. This phase is biologically stable at approximately 14 days in the vast majority of healthy cycles. This consistency is the mathematical foundation of the prediction.
Because the luteal phase is constant, the most reliable way to predict ovulation is to count backward from the next expected period, not forward from the last. The formula is:
The fertile window
The fertile window spans six days: the five days before ovulation plus ovulation day itself. Sperm can survive in the reproductive tract for three to five days; the egg lives only 12 to 24 hours after release. This creates a window where unprotected sex can result in pregnancy, peaking on the day before and the day of ovulation.
This calculator highlights the full six-day fertile window on the calendar in green, with ovulation day marked solid. The two peak fertility days (ovulation day and the one before) are shown with deeper shading.
How accurate is period prediction?
Calendar-based prediction is most accurate for people with regular cycles. The method assumes your next cycle will be the same length as your average, which holds well when stress, travel, illness, and hormonal changes are absent. Sources of variability include:
- Cycle-to-cycle variation: even a "regular" cycle may vary by 1–3 days each month. Over longer horizons, predictions become less precise.
- Life events: significant stress, illness, rapid weight change, or travel across time zones can delay ovulation and shift the entire cycle.
- Perimenopause: cycles become progressively less predictable in the years leading up to menopause.
- Postpartum and breastfeeding: cycles may not return or may be irregular for months after giving birth.
For contraception, calendar methods alone are not considered reliable. For conception planning, they are a useful guide but should be combined with physical signs (basal body temperature, cervical mucus) or ovulation test strips.
What is an average cycle length?
A menstrual cycle of 28 days is often cited as the norm, but population data shows that the distribution is wide:
- 21–24 days: shorter cycles, still within the normal range
- 25–30 days, the most common cluster (majority of cycles)
- 31–35 days: longer but still regular
- 36–45 days: can be normal, especially in adolescents and perimenopause
Cycles shorter than 21 days or longer than 45 days persistently may warrant a conversation with a healthcare provider.
How long does a period last?
Most periods last three to seven days, with five days being a widely cited average. The first one or two days typically involve the heaviest flow; the final days are lighter spotting. The calculator uses your entered period duration to shade the full bleeding window on the calendar.
Period vs ovulation calculator: what is the difference?
This period calculator focuses on predicting multiple upcoming period start and end dates across six cycles, giving a long-range planning view. The ovulation calculator focuses on a single cycle, giving precise ovulation day, peak fertility days, and an estimated due date if conception occurs. Use this tool for general cycle tracking and scheduling; use the ovulation calculator when actively trying to conceive.
Tips for using this calculator
- Update your last period date each time your period starts. The longer you wait, the less accurate the predictions become.
- If your cycles are irregular, use the average of your last three to six cycles as the cycle length, not just the most recent one.
- The calendar highlights both the period days and the fertile window. Navigate forward with the arrow buttons to see all six predicted cycles.
- For family planning or contraception purposes, always consult a healthcare professional; calendar prediction alone is not medically sufficient.
Disclaimer
This calculator provides estimates for informational and planning purposes only. It is not a substitute for medical advice, diagnosis, or treatment. Period and ovulation predictions are based on averages and may not reflect individual variation. Always consult a qualified healthcare provider for personal reproductive health guidance.