Track your menstrual cycle, predict your next period, and identify your fertile window and ovulation day.
The uterine lining sheds. This is when bleeding occurs, typically lasting 3–7 days.
Hormones stimulate egg development in the ovaries. Estrogen rises, thickening the uterine lining.
A mature egg is released. This is the most fertile time — the egg survives ~24 hours.
Progesterone rises to support potential pregnancy. If no implantation occurs, hormone levels drop and the cycle restarts.
⚕️ Disclaimer: This calculator provides estimates based on average cycle data. It is not a substitute for medical advice and should not be used as a contraceptive method. Consult your healthcare provider for personalized guidance.
The Period Calculator tracks your menstrual cycle, predicts your next period, and estimates your fertile window and ovulation day using your cycle length and last period date, shown on a clear visual calendar.
This is the part that trips almost everyone up. The calculator does not count 14 days forward from your period. It counts 14 days backwards from the next one. On a 30-day cycle starting 1 March, your next period is due 31 March, so ovulation is estimated at 17 March — day 17, not day 14.
The reason is that the two halves of a cycle behave differently. The luteal phase, from ovulation to your next period, is remarkably consistent at around 14 days for most people. The follicular phase, from your period to ovulation, is the elastic part — it's what stretches when a cycle runs 33 days instead of 28. So counting back from the next period is the better estimate, and it's why a long cycle doesn't mean ovulating on day 14 and then waiting three weeks.
The fertile window runs from seven days before that estimate to one day after. The asymmetry is biological: sperm can survive around five days in the reproductive tract, while an egg is viable for roughly 24 hours. So the days that matter most are the two or three before ovulation, not the day itself — which is the opposite of what most people assume.
This is a projection from an average, not a measurement. Ovulation moves in response to illness, stress, travel, disrupted sleep and weight change, and it moves a great deal in the years after menarche, while breastfeeding, and approaching menopause. Conditions such as PCOS and thyroid disorders make calendar prediction unreliable in ways no arithmetic can fix.
Calendar prediction is not contraception. If you are trying to avoid pregnancy, use a method with a published failure rate and talk to a clinician. Irregular cycles, bleeding between periods, or a sudden change in pattern are all worth raising with a doctor rather than a calculator.
Everything you enter stays in your browser. The dates here are among the most sensitive things anyone types into a website, and there is no server behind this page to send them to.
If a projection turns into a positive test, the Due Date Calculator takes the same last-period date and carries it through to a due date and trimester milestones. After the birth, the Age Calculator takes over. The full chain, with worked dates, is in the guide on cycle tracking through to your baby's first birthday.
You enter the date your last period started and your average cycle length. The tool adds your cycle length to that date to estimate upcoming periods, and marks the likely fertile window and ovulation day.
Predictions are estimates based on average cycle length and are most accurate for regular cycles. Cycles naturally vary, so use the results as a guide, not a guarantee.
The fertile window is roughly the five days before ovulation plus ovulation day. For a typical 28-day cycle, ovulation is around day 14; the calculator adjusts this to your cycle length.
Yes. Everything is calculated in your browser — no cycle information is uploaded or stored on a server.