Ovulation Calculator
HealthEnter the first day of your last period and your average cycle length. The calculator estimates your ovulation day, your fertile window and when your next period should start, and lists the same dates for the next six cycles.
Step by step
September 29, 2026 + (28 - 14) days = October 13, 2026
- About 14 days separate ovulation from the next period. So ovulation comes 28 - 14 days after the first day of your last period: October 13, 2026.
- Fertile window: from 5 days before ovulation to 1 day after, October 8, 2026 to October 14, 2026.
- The next period is expected one cycle after the last: October 27, 2026.
Once every entry is valid, you’ll see each step of the calculation here.
Formula
- cycle length
- days from the first day of one period to the first day of the next
- 14
- average length of the luteal phase, from ovulation to the next period
How it works
Ovulation usually happens about 14 days before your next period starts. The time after ovulation, the luteal phase, is fairly constant from cycle to cycle, while the time before it varies. So the calculator takes the first day of your last period, adds your cycle length to find your next period, and counts back 14 days. In a 28-day cycle that is day 14; in a 32-day cycle, day 18.
The fertile window is the 5 days before ovulation, the day of ovulation and the day after. Sperm can survive in the body for up to about 5 days, while the egg lives for 12 to 24 hours after its release. Sex in the 2 to 3 days before ovulation gives the best chance of pregnancy. The table repeats the same cycle length for the next six cycles, so the dates assume your cycles stay regular.
This is the calendar method, an estimate. Cycles vary, and stress, illness or travel can move ovulation. Ovulation tests, which detect the hormone surge (LH) a day or two before ovulation, and tracking basal body temperature or cervical mucus give a more personal picture. Do not use the calendar method alone as birth control. Once pregnant, the pregnancy week calculator counts the weeks.
The formula
The 14 days are an average; the luteal phase lasts about 12 to 16 days in most women, so ovulation can fall a day or two either side of the date shown. That is one reason the fertile window spans 7 days.
If your cycle length varies, use the average of your last few cycles, or track the shortest and longest: the fertile window then runs from the earliest to the latest estimate.
Example
Last period on September 1, 2026, cycle of 28 days
- Ovulation: 28 - 14 = 14 days after the period started, so September 15, 2026.
- Fertile window: 5 days before to 1 day after, September 10, 2026 to September 16, 2026.
- The next period is expected on September 29, 2026.
You will most likely ovulate on September 15, 2026. Your fertile window runs from September 10, 2026 to September 16, 2026, and your next period is expected on September 29, 2026.
Load this example into the calculatorFrequently asked questions
How many days after my period do I ovulate?
Counted from the first day of your period, ovulation comes at about your cycle length minus 14 days. In a 28-day cycle that is day 14, in a 25-day cycle day 11 and in a 35-day cycle day 21. If your period lasts 5 days, that is about 9 days after it ends in a 28-day cycle.
What are my most fertile days?
The 2 to 3 days before ovulation and the day of ovulation itself. The full fertile window is about 6 to 7 days, because sperm can live up to 5 days while the egg survives only about a day.
What are the signs of ovulation?
Clear, slippery cervical mucus that increases just before ovulation, a rise in basal body temperature of about 0.5 to 1 degree Fahrenheit that stays up until your next period, mild cramping on one side of the pelvis, and a positive ovulation test, which picks up the LH surge about 24 to 36 hours before the egg is released. The temperature rise confirms ovulation only after it has happened, so use it to check the calendar estimate, not to time it.
When should I take an ovulation test?
Start 3 to 5 days before the predicted ovulation day, which is day 11 in a 28-day cycle, and test every day until the result is positive, because skipping a day can miss the surge. A positive test means the LH surge has begun and ovulation is likely within 24 to 36 hours, so that day and the next are the best days to try. With irregular cycles the surge is harder to catch, so ask your health care provider when to start testing.
How accurate is an ovulation calculator?
It gives a good estimate for regular cycles, but it cannot see your actual ovulation. Ovulation tests, basal body temperature and cervical mucus tracking confirm it. With irregular cycles, talk to your health care provider.
Can I get pregnant outside the fertile window?
It is unlikely but possible, because ovulation can come earlier or later than predicted. The calendar method alone is not a reliable form of birth control.
- Your Menstrual Cycle (Office on Women’s Health, U.S. Department of Health and Human Services)
- Fertility Awareness-Based Methods of Family Planning (American College of Obstetricians and Gynecologists)
- Ovulation home test (MedlinePlus, National Library of Medicine)
- Ovulation (Wikipedia)