How does the menstrual cycle work?
The menstrual cycle is the monthly process that prepares the body for a possible pregnancy. It is counted from the first day of bleeding until the day before the next period. A normal cycle lasts between 21 and 35 days, averaging 28 days, and this calculator estimates your next period, ovulation, and fertile window.
With a 28-day cycle that started today, estimated ovulation falls on day 14 and the next period would arrive 28 days later. The fertile window covers the 5 days before ovulation plus the day of ovulation.
Phases of the menstrual cycle
1. Menstrual phase (days 1-5)
The uterine lining sheds, causing menstrual bleeding. It typically lasts 3 to 7 days.
2. Follicular phase (days 1-13)
Ovarian follicles begin to mature and estrogen levels rise.
3. Ovulation (day ~14)
Ovulation day = Cycle length − 14 days
A mature egg is released from the ovary. This is the most fertile moment.
4. Luteal phase (days 15-28)
The body prepares for a possible pregnancy. If fertilization does not occur, a new cycle begins.
Step-by-step example (28-day cycle)
- First day of bleeding = day 1
- Estimated ovulation = 28 − 14 = day 14
- Fertile window = days 9 to 14 (sperm can live up to 5 days)
- Next period = day 29 (start of the next cycle)
Fertile window
The most fertile days are the 5 days before ovulation plus the day of ovulation. Sperm can survive up to 5 days in the reproductive tract, so intercourse up to 5 days before ovulation can result in pregnancy.
Scenario table (cycle length)
| Cycle | Ovulation (day) | Fertile window | Next period |
|---|---|---|---|
| 21 days | 7 | days 2-7 | day 22 |
| 28 days | 14 | days 9-14 | day 29 |
| 30 days | 16 | days 11-16 | day 31 |
| 35 days | 21 | days 16-21 | day 36 |
This calculator is only an estimate based on regular cycles. It must not be used as a contraceptive method. Consult your doctor for family planning or if you have very irregular cycles.
Frequently asked questions
Not necessarily. Any length between 21 and 35 days is considered normal. What matters is that it is relatively consistent month to month.
Stress, weight changes, excessive exercise, polycystic ovary syndrome (PCOS), thyroid problems, and certain medications can cause irregularities.
The fertile window is the 5 days before ovulation plus the day of ovulation. In a 28-day cycle, that is approximately days 9 to 14.
It is unlikely but possible, especially in short cycles: if you ovulate early (day 7-9), sperm can survive until ovulation. Use protection if you are not trying to conceive.
Related calculators
- Ovulation calculator: estimate your most fertile days
- Pregnancy probability: chances by cycle day
- Due date: calculate your estimated due date
- Pregnancy weeks: week-by-week tracking