Ovulation Calculator
Enter the first day of your last period and your cycle length to see your estimated ovulation day, fertile window and next period on a calendar.
If you are not sure, leave it at 28, the most common length. You can adjust it between 21 and 45 days.
Next estimated ovulation day
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| Cycle | Fertile window | Estimated ovulation | Next period |
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These results are statistical estimates based on an average cycle, for reference only. Your actual ovulation day can change from month to month with stress, illness and other factors. For decisions about trying to conceive or avoiding pregnancy, talk to a doctor. This tool is not a method of birth control.
How to use
How the ovulation calculator works
Ovulation is the moment an ovary releases a mature egg. It usually happens about 14 days before your next period starts, not 14 days after your last one. That distinction matters when your cycle is longer or shorter than 28 days. This ovulation calculator therefore counts backward: it adds your cycle length to the first day of your last period to estimate when the next period will start, then subtracts 14 days to find your likely ovulation day. It marks your fertile window, from five days before ovulation through one day after, and shows the dates for your current cycle and the next two, together with a monthly calendar.
What is the fertile window?
You can only become pregnant during a short window each cycle. Sperm can survive in the reproductive tract for up to five days, while an egg survives for roughly 12 to 24 hours after it is released. So the six days from five days before ovulation to the day after make up the fertile window, and the odds are highest in the two or three days leading up to and including ovulation. A healthy couple trying to conceive has about a 20 to 25 percent chance of pregnancy in any single cycle, which is why it can take several months even when everything is working normally.
Ovulation day = next period start − 14 days | Fertile window = ovulation − 5 days to ovulation + 1 day
Example: with a 28-day cycle starting on the 1st, the next period is expected on the 29th, ovulation on the 15th and the fertile window from the 10th through the 16th. With a 35-day cycle starting on the 1st, ovulation is on the 22nd and the window runs from the 17th through the 23rd.
How accurate is a calendar estimate?
The calendar method is a reasonable starting point if your cycles are regular, but it is an estimate. Typical cycles in adults last 21 to 35 days, and the number of days between ovulation and the next period (the luteal phase) is usually about 10 to 16, not exactly 14. Stress, illness, travel, breastfeeding, recent hormonal birth control and conditions such as polycystic ovary syndrome (PCOS) can all shift ovulation. If your cycle length varies by more than a few days from month to month, the dates here can be off by a week or more.
Other ways to pinpoint ovulation
- Ovulation predictor kits (OPKs) detect the surge of luteinizing hormone (LH) in urine, which comes about 24 to 36 hours before ovulation. They are the most popular home method.
- Cervical mucus becomes clear, slippery and stretchy, like raw egg white, in the days before ovulation.
- Basal body temperature rises by a few tenths of a degree (roughly 0.4 to 1.0°F or 0.2 to 0.5°C) after ovulation. It confirms that ovulation happened but does not predict it, so it is best used over several months.
- Fertility apps and wearables combine these signals. Use them to refine the estimate from this calculator.
This is not birth control
Avoiding intercourse on estimated fertile days is called a fertility awareness method, and when it relies on a calendar alone it has a much higher failure rate than hormonal methods, IUDs or condoms. Ovulation can come early, and sperm can survive for days. If you want to prevent pregnancy, talk to a health care provider about reliable contraception. If you are trying to conceive, this tool helps you time intercourse, not guarantee a result.
Tips for trying to conceive
Having intercourse every one to two days during the fertile window is generally as effective as timing it exactly. Take a daily prenatal vitamin with folic acid, which the CDC recommends for anyone who could become pregnant. Avoid smoking and limit alcohol. Many guidelines advise seeing a doctor if you have not conceived after 12 months of trying when you are under 35, or after six months when you are 35 or older, and sooner if you have irregular or absent periods or known fertility concerns.
The phases of a menstrual cycle
A cycle has four phases. In the menstrual phase the uterine lining sheds, which is your period. In the follicular phase, which overlaps the end of your period, follicles in the ovary mature and estrogen rises. Ovulation follows when the dominant follicle releases its egg. In the luteal phase the empty follicle produces progesterone to prepare the uterus for a possible pregnancy, and if no pregnancy occurs hormone levels fall and the next period begins. The follicular phase varies most between people and between cycles, which is why long or irregular cycles move ovulation, while the luteal phase tends to stay close to two weeks.
How to read the calendar
The calendar shows the month that contains your next estimated ovulation day. Pink days are the fertile window, the darkest pink day is estimated ovulation, light blue days are an expected period and the outlined day is today. The table below it lists the fertile window, ovulation day and next period for three consecutive cycles so you can plan ahead. Update the date each month, and adjust the cycle length if your recent cycles have been different.
Frequently asked questions
How do I calculate my ovulation day?
Enter the first day of your last period and your cycle length. The calculator estimates the start of your next period and subtracts 14 days to find ovulation. With a 28-day cycle, ovulation falls on day 15.
How long is the fertile window?
About six days: the five days before ovulation and the day of ovulation or the day after. Sperm can survive for up to five days, while the egg lasts about a day.
What if my cycle is irregular?
The estimate is less reliable. Enter your average cycle length, and consider ovulation predictor kits or tracking cervical mucus and temperature. A health care provider can also help if your cycles are very irregular or absent.
Can I get pregnant on any day of my cycle?
Pregnancy is possible only in the fertile window around ovulation. Because ovulation can shift, the safest assumption outside a reliable birth control method is that any day could carry some risk.
Does the calculator work after stopping birth control?
Cycles can take a few months to settle after stopping hormonal contraception, so early estimates may be off. Track a few cycles to see your true length.
What is the luteal phase?
It is the time between ovulation and the start of your next period, usually 10 to 16 days. The calculator assumes 14, the average. If yours is consistently shorter or longer, ovulation will be earlier or later than shown.
Is my data saved?
No. The dates stay in your browser and nothing is stored or sent.