Enter your last period date and cycle length to predict your next 6 periods, ovulation days, and fertile windows — instantly.
| # | Period starts | Period ends | Ovulation day | Fertile window |
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Your menstrual cycle starts on the first day of your period and ends the day before your next period begins. The length of the cycle varies from person to person — anywhere from 21 to 35 days is considered normal — and can also vary from cycle to cycle in the same person.
Knowing your cycle length helps you predict not just when your period will arrive, but when you'll be ovulating, when you're most fertile, and when you can expect hormonal shifts that affect energy, mood, and appetite.
The uterine lining sheds. Progesterone and estrogen are at their lowest. Most people experience period symptoms: cramping, bloating, fatigue, and mood changes are all normal during this phase.
Overlapping with menstruation, this phase starts on day 1. FSH stimulates follicles in the ovaries. Rising estrogen thickens the uterine lining. Energy and mood typically improve through this phase as estrogen rises.
An LH surge triggers the release of an egg. The egg is viable for 12–24 hours. The fertile window spans the 5 days before ovulation plus ovulation day — sperm can survive up to 5 days, so sex before ovulation is most effective for conception.
After ovulation, progesterone rises to prepare the uterine lining for a potential pregnancy. If the egg is not fertilised, progesterone drops, triggering the next period. PMS symptoms often appear in the second half of this phase.
A period calculator assumes a regular cycle, but real cycles aren't always that simple. Common reasons your period might arrive later than the calculator predicts:
Log the first day of every period, including spotting that turns into flow. Consistency over 3+ cycles gives you a reliable average that's more accurate than any default estimate.
If your cycle regularly varies by more than 7 days, you may have irregular cycles. Tracking these variations over time helps identify patterns — and helps your doctor if you ever need to discuss cycle health.
A period calculator estimates when ovulation occurs, but BBT charting confirms it. A sustained temperature rise of 0.2–0.5°C after ovulation tells you exactly when it happened, improving future predictions.
WomensPal predicts your cycle, logs symptoms, and tracks BBT and cervical mucus. No subscription. No paywall. No data selling.
Start tracking free →Add your average cycle length to the first day of your last period. If your period started July 1 and your cycle is 28 days, your next period is expected around July 29. This calculator does this automatically and predicts your next 6 periods at once.
The most common reasons: stress, weight changes, excessive exercise, illness, thyroid issues, or pregnancy. If you are sexually active and your period is more than a week late, take a pregnancy test. If periods are regularly late or irregular, speak with a doctor.
A cycle that varies by up to 7 days from your average is considered within the normal range. A period is typically considered late if it hasn't arrived within 5–7 days of when you expected it.
A normal cycle length ranges from 21 to 35 days, with the average being around 28 days. It's also normal for your cycle length to vary by a few days from month to month.
A typical period lasts 3 to 7 days. Bleeding that consistently lasts fewer than 2 days or longer than 7 days is worth discussing with a doctor.
Yes. Stress can delay ovulation by days or even a week or more, which in turn delays your period. This is because the hypothalamus — which regulates ovulation hormones — is sensitive to cortisol (the stress hormone).