How to use
Pick the first day of your last period and enter your average cycle length (the number of days from the first day of one period to the day before the next one starts). The default is 28 days, and you can set anything from 20 to 45. You’ll see your next estimated ovulation date, your fertile window, your next expected period and which day of your cycle today is, followed by the expected dates for the next 3–6 cycles in a table and on a calendar. The calendar marks expected period days (●), the fertile window (○) and estimated ovulation (★) with both color and symbols. Period length is only used for the calendar.
Under More settings, you can change the luteal phase length, or, if your cycle length varies, enter your shortest and longest cycles to see the wider range in which your fertile window could fall. If more than one cycle has passed since the date you entered, the calculator starts from the estimated cycle containing today, but entering the first day of your most recent period gives a more accurate result.
How it works: luteal phase and fertile window
Ovulation divides the menstrual cycle into the follicular phase before it and the luteal phase after it. This calculator assumes a 14-day luteal phase and uses “estimated ovulation = next expected period − 14 days”. For example, if your last period started on September 1 and your cycle is 28 days, your next period is due on September 29 and your estimated ovulation date is September 15. The luteal phase also varies from person to person: in a study by Bull and colleagues (2019) that analyzed more than 600,000 cycles, the luteal phase averaged 12.4 days, with a 95% range of 7–17 days. This tool allows 10–17 days under More settings. This input range is not a medical definition of normal.
Based on Wilcox and colleagues (1995), this calculator marks estimated ovulation and the preceding 5 days as a six-day estimated fertile window. In the example above, this is September 10–15. These dates do not establish the limits of possible pregnancy. ACOG notes that sex on the day after ovulation can also lead to pregnancy.
Limits of calendar estimates
A calendar estimate assumes your past cycles will repeat exactly. In the study by Wilcox and colleagues (2000), which followed 221 women over 696 cycles, only about 30% had a fertile window that fell entirely within cycle days 10–17, the range often given in general advice, and on every day from cycle day 6 to day 21 there was at least a 10% chance of being in the fertile window. The researchers concluded that the fertile window can be highly unpredictable, even for people whose cycles are usually regular.
FIGO (the International Federation of Gynecology and Obstetrics) considers 24–38 days a normal cycle, and considers cycles regular when the difference between the shortest and longest cycle is 7 days or less (ages 26–41) or 9 days or less (ages 18–25 and 42–45). The NHS lists puberty, the start of menopause, some hormonal contraception (the progestogen-only pill, the contraceptive injection and the hormonal intrauterine system), large weight changes, stress and anxiety, too much exercise, and polycystic ovary syndrome or thyroid problems as causes of irregular cycles. With irregular cycles you may not ovulate regularly, so calendar estimates are even less accurate.
That’s why you shouldn’t use this calculator as birth control. The U.S. Centers for Disease Control and Prevention (CDC) puts typical-use failure rates of methods that avoid sex on fertile days (fertility awareness-based methods) at 2–23%, depending on the method.
More accurate ways to track ovulation
- Ovulation tests: Over-the-counter ovulation tests detect the surge in luteinizing hormone (LH) in your urine just before ovulation.
- Basal body temperature: Your temperature, taken right after waking and before getting up, rises slightly with ovulation. The rise is small, about 0.2–0.4°C (0.4–0.8°F), so you need a sensitive thermometer. A temperature rise mainly helps identify ovulation retrospectively; it does not pinpoint ovulation that day or guarantee future dates.
- Cervical mucus: Just before ovulation, mucus is at its most plentiful and becomes clear and slippery, like raw egg whites.
- Medical tests: A clinic can confirm the timing of ovulation more precisely with an ultrasound or blood tests.
The study by Bull and colleagues (2019) also found that tracking body signs such as basal body temperature, not just cycle length, is important for identifying the fertile window.
When to talk to a healthcare professional
If you’re trying to get pregnant, talk to a healthcare professional rather than relying on calendar estimates alone. The Office on Women’s Health (U.S. Department of Health and Human Services) advises seeing a doctor if you haven’t conceived after one year of sex without birth control when you’re under 35, or after six months when you’re 35 or older. It’s also worth getting advice if your cycles are shorter than 24 days or longer than 38 days, or if their length varies a lot from cycle to cycle.