Period Tracker
Log your cycle to predict your next period and fertile window. All data stays in your browser — nothing is sent to any server.
Your Cycle Details
Average is 28 days
Average is 3–7 days
Upcoming Predictions
Enter your last period start date to see predictions.
Frequently Asked Questions
Understanding your menstrual cycle
The menstrual cycle is a monthly series of hormonal changes that prepares the body for the possibility of pregnancy. The average cycle is 28 days long, but anywhere from 21 to 35 days is considered normal. Every cycle is divided into distinct phases, each governed by shifting levels of oestrogen and progesterone.
Cycle length is counted from the first day of one period (day 1) to the first day of the next. The follicular phase runs from day 1 to ovulation — the brain releases FSH (follicle-stimulating hormone), which causes follicles in the ovaries to develop. One dominant follicle matures and releases an egg at ovulation, triggered by a surge in LH (luteinising hormone).
After ovulation, the luteal phase begins. The empty follicle becomes the corpus luteum and secretes progesterone, thickening the uterine lining in preparation for implantation. If the egg is not fertilised, progesterone drops, the lining sheds, and the next period begins. Understanding this cycle helps you predict not just your period, but also ovulation, your fertile window, and why you may feel different at different points in the month.
The four phases of your cycle
The uterine lining sheds. Oestrogen and progesterone are at their lowest. Energy may be lower, and cramps are common. Iron-rich foods can help offset blood loss.
Oestrogen rises as follicles develop. Energy, mood, and cognitive sharpness tend to improve through this phase. A good time for demanding mental or physical work.
An egg is released from the dominant follicle. LH and oestrogen peak. Many people notice increased energy, libido, and social confidence. Lasts approximately 24 hours, though the fertile window spans 5–6 days around it.
Progesterone dominates. The body temperature rises slightly (~0.2–0.5°C). If pregnancy does not occur, hormones drop in the final days, which can trigger PMS symptoms — bloating, mood changes, and fatigue — before the next period begins.
What can cause an irregular cycle?
Stress
Chronic psychological stress raises cortisol, which suppresses the hormonal cascade needed for ovulation. Even a single stressful life event can shift your cycle by several days or cause a missed period.
Intense exercise
High training loads combined with low energy availability (under-eating relative to output) can disrupt the hypothalamic–pituitary–ovarian axis, delaying or stopping ovulation. Common in endurance athletes.
Weight changes
Both significant weight loss and weight gain can disrupt cycle regularity. Body fat plays a role in oestrogen production — too little or too much can both cause irregular cycles.
Hormonal contraception
Stopping hormonal birth control can cause cycles to be irregular for 1–3 months while the body re-establishes its natural rhythm. This is normal and typically resolves on its own.
Illness
Fever, infection, or significant illness — even something as brief as the flu — can delay ovulation. The period that follows may be later than expected as a result.
Underlying conditions
PCOS (polycystic ovary syndrome), thyroid disorders, and endometriosis can all cause irregular, painful, or very heavy periods. Persistent irregularity warrants a conversation with a healthcare provider.
Medical disclaimer: This tool is for general information and planning purposes only. It is not a substitute for medical advice, diagnosis, or contraception. If you have concerns about your cycle, fertility, or reproductive health, please consult a qualified healthcare provider.