Menstrual Cycle Calculator
Enter the first day of your last period and your average cycle length to predict your next period, ovulation date, and fertile window. View predictions for up to 6 upcoming cycles. Everything runs in your browser — nothing is stored or sent anywhere.
Next Period
Aug 8
in 14 days
Ovulation
Jul 25
in 0 days
Fertile Window
Jul 20 – Jul 26
Highest chance of conception
Current cycle day: 15 of 28
Upcoming cycles
Cycle 1
Period
Aug 8–Aug 12
Ovulation
Sat, Jul 25, 2026
Fertile Window
Jul 20–Jul 26
Cycle 2
Period
Sep 5–Sep 9
Ovulation
Sat, Aug 22, 2026
Fertile Window
Aug 17–Aug 23
Cycle 3
Period
Oct 3–Oct 7
Ovulation
Sat, Sep 19, 2026
Fertile Window
Sep 14–Sep 20
How the menstrual cycle works
The menstrual cycle is counted from the first day of one period to the first day of the next. A typical cycle is 28 days, but normal cycles range from 21 to 35 days. The cycle has two main phases: the follicular phase (day 1 to ovulation) and the luteal phase (ovulation to the next period).
Ovulation — the release of an egg — typically occurs 14 days before the next expected period, regardless of cycle length. For a 28-day cycle, ovulation is around day 14. For a 35-day cycle, it is around day 21. This is why predicting ovulation from the start of the cycle requires knowing the cycle length.
Understanding the fertile window
An egg is viable for 12–24 hours after ovulation. However, sperm can survive in the female reproductive tract for up to 5 days. This means that unprotected sex in the 5 days before ovulation and on the day of ovulation can result in pregnancy.
This calculator estimates the fertile window as the 5 days before ovulation plus the day after. The probability of conception is highest on the 2 days before ovulation and on the day of ovulation itself.
Frequently asked questions
How accurate is this calculator for predicting ovulation?
This calculator uses the Ogino-Knaus method (calendar method), which assumes consistent cycle lengths and fixed ovulation timing relative to the next period. It is a reasonable estimate for women with regular cycles. Cycle length can vary month to month due to stress, illness, travel, weight changes, and hormonal fluctuations. For high-stakes decisions about fertility or contraception, use additional methods such as basal body temperature tracking or ovulation predictor kits (LH tests).
Can I use this as a form of contraception?
The calendar method alone has a high failure rate as contraception (around 25% per year with typical use) due to cycle variability. It is not recommended as a reliable standalone method. Modern fertility awareness-based methods (FABMs) that combine cycle tracking with BBT and cervical mucus observation are more effective but still less reliable than hormonal methods or long-acting reversible contraception.
What if my cycle is irregular?
This calculator works best for regular cycles. If your cycle length varies by more than a few days each month, the predictions will be less accurate. Irregularity can be caused by hormonal conditions (PCOS, thyroid issues), high stress, significant weight changes, or perimenopause. A healthcare provider can investigate causes and advise on tracking methods suited to irregular cycles.
