Period Calculator — Next Period, Ovulation & Fertile Window
Calculate your next period date, ovulation day, fertile window and PMS window. Free period tracker with 3-month calendar view and upcoming cycle schedule.
How to Use
- Enter the first day of your last period.
- Set your average cycle length (most women are 21–35 days, average is 28).
- Set how long your period usually lasts (typically 3–7 days).
- See your next period date, fertile window, ovulation day, PMS window and current cycle phase instantly.
- The calendar highlights all upcoming period days, fertile days and ovulation across 3 months.
Formula
Next Period = Last Period Date + Cycle Length
Ovulation Day = Next Period − 14 days (luteal phase is fixed at ~14 days)
Fertile Window = Ovulation − 5 days to Ovulation day (sperm survive 3–5 days)
PMS Window = Next Period − 7 days to Next Period − 1 day
Example Calculation
Example: Last period March 1, 2026 · 28-day cycle · 5-day duration
| Event | Date |
|---|---|
| Last Period Started | March 1, 2026 |
| Next Period | March 29, 2026 |
| Ovulation Day | March 15, 2026 |
| Fertile Window | March 10 – March 15, 2026 |
| PMS Window | March 22 – March 28, 2026 |
| Cycle Phase Today | Follicular (day 14) |
Frequently Asked Questions
Period calculators are estimates based on average cycle length. They are most accurate for women with regular cycles. Stress, illness, travel, weight changes, and hormonal conditions can all affect cycle timing.
The fertile window is the 6 days ending on ovulation day — sperm can survive up to 5 days inside the body, so intercourse on any of these days can lead to pregnancy. Ovulation typically occurs 14 days before your next period.
A normal menstrual cycle is 21–35 days. The average is 28 days. Cycles shorter than 21 or longer than 35 days may indicate a hormonal imbalance and a doctor should be consulted.
Menstrual (days 1–5): bleeding. Follicular (days 1–13): estrogen rises, energy increases. Ovulation (day 14): egg is released, peak fertility. Luteal (days 15–28): progesterone rises, PMS may occur if no pregnancy.
Cycle length varies due to stress, diet, exercise, illness, sleep disruption, travel (timezone changes), and hormonal fluctuations. Tracking your period over 3+ months gives a more accurate average cycle length.
This calculator assumes regular cycles. Women with PCOS or highly irregular cycles may find predictions less accurate. Consult a gynecologist and consider using an ovulation predictor kit (OPK) for more reliable fertility tracking.