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Track your menstrual cycle, predict your next period start date, calculate your fertile window & peak ovulation days — 100% free and private.
Enter the first day your last menstrual period (LMP) started.
Default is 28 days cycle length and 5 days period duration (adjust to match your body).
Get your fertile window, next period countdown, and 1-click Google/Apple calendar exports.
Tracking your menstrual cycle and predicting your ovulation window helps you understand your body’s reproductive patterns, plan or avoid pregnancy, and monitor overall hormonal health. This calculator uses standardized gynecological algorithms to forecast cycle patterns.
The cycle begins on the first day of menstrual bleeding. In this phase, the body sheds the thickened lining of the uterus (endometrium) because pregnancy did not occur. Estrogen and progesterone levels are low.
This phase overlaps with your period and lasts until ovulation. Follicle-Stimulating Hormone (FSH) prompts the ovaries to grow follicles, which contain eggs. As follicles mature, they release estrogen, thickening the uterine lining again.
Triggered by a surge in Luteinizing Hormone (LH), the mature follicle ruptures, releasing a ripe egg into the fallopian tube. Ovulation marks the peak of fertility. The released egg remains viable for 12 to 24 hours.
After ovulation, the ruptured follicle transforms into the corpus luteum, producing progesterone. Progesterone maintains the uterine lining, preparing it for an embryo. If fertilization doesn't occur, hormones drop, triggering the next period.
The fertile window lasts about 6 days: the 5 days before ovulation plus the day of ovulation itself. This is because human sperm can survive in the female reproductive tract for up to 5 days, whereas the egg lives for 12-24 hours. Having intercourse during this window—especially the 2 days before ovulation and ovulation day—maximizes the probability of conception.
Days outside your fertile window are considered 'low fertility' or 'safe' days where pregnancy is highly unlikely. However, because menstrual cycle lengths can shift due to stress, diet, or hormone fluctuations, tracking safe days is not 100% effective as a standalone birth control method.
Frequently asked questions
The calculator estimates your next period date, ovulation day, and fertile window based on the first day of your last period (LMP) and your average cycle length (typically 28 days). Peak ovulation occurs approximately 14 days before your next expected period.
Your fertile window includes the 5 days leading up to ovulation plus ovulation day itself. Because sperm can survive in the reproductive tract for up to 5 days, intercourse during this 6-day window carries the highest likelihood of conception.
The menstrual cycle consists of 4 main phases: 1) Menstrual Phase (bleeding), 2) Follicular Phase (egg maturation), 3) Ovulation Phase (release of mature egg), and 4) Luteal Phase (preparation for pregnancy or next period).
Ovulation typically occurs midway through your cycle, approximately 14 days before your next period begins (assuming a standard 14-day luteal phase). For a 28-day cycle, ovulation happens around Day 14.
Yes, 100%! Go Age Calculator runs entirely in your browser using client-side JavaScript. Your dates and cycle information are never sent to a server, never sold, and require no account creation. Profiles are stored locally in your browser.
Yes! Below your results, click "Google Calendar" or "Apple / Outlook (.ics)" to add 1-click reminders for your upcoming period or fertile window.
If your cycle varies (e.g. 26 to 32 days), calculate using your average cycle length over the past 3 to 6 months. For irregular cycles, consult your healthcare provider for clinical tracking.
No. Calendar predictions are estimates based on averages and should not be used as a reliable primary form of birth control or natural family planning. Always consult a healthcare professional for contraception guidance.
The follicular phase begins on Day 1 of your period and ends at ovulation (variable length). The luteal phase begins after ovulation and lasts until your next period (typically constant at 12–16 days, average 14 days).
High stress, severe illness, rapid weight changes, or jet lag can disrupt hormonal signals (GnRH/LH), delaying ovulation or causing anovulatory cycles, which shifts your expected next period date.