How this ovulation calculator works
The calculator uses the calendar method. You enter the first day of your last period and your average cycle length, and it estimates when you ovulate by counting back from your next period.
Next period = first day of last period + cycle length
Ovulation day = next period − 14 days
The time from ovulation to the next period is the steadiest part of the cycle, at about 14 days, while the days before ovulation vary with cycle length. That is why a 28-day cycle puts ovulation on day 14 and a 35-day cycle puts it on day 21.
What the results show
- Estimated ovulation day. 14 days before your next period is expected.
- Likely ovulation window. Two days either side of that day, since the exact day shifts from cycle to cycle.
- Days you can conceive. From five days before the estimated ovulation day, because sperm can survive that long, through the end of the ovulation window. The true fertile window is shorter, about six days ending around ovulation; this range is wider because the ovulation day itself is only known to within a couple of days.
- Earliest pregnancy test. Nine days after the estimated ovulation day. This is a rough earliest date, and a test this early can read negative even if you are pregnant. Testing on or after the day your next period is due is more reliable.
- Next period. One cycle length after the first day of your last period.
- Due date if you conceive. 266 days after the estimated ovulation day.
The ovulation calendar marks these days for the current cycle, and the table repeats the ovulation window and due date for the next six cycles.
Frequently asked questions
When do you ovulate?
About 14 days before your next period starts. Subtract 14 from your cycle length to get the cycle day: day 14 of a 28-day cycle, day 16 of a 30-day cycle, day 12 of a 26-day cycle.
How many days after my period do I ovulate?
It depends on your cycle length, not on when bleeding stops. Count from the first day of your period: with a 28-day cycle, ovulation falls about two weeks after that first day.
How long does ovulation last?
Ovulation itself is brief: the egg can be fertilized for about 24 hours after it is released. The fertile window is longer, about six days, because sperm that arrive in the days before ovulation can still be alive when the egg is released. The calculator shows a wider range than six days because it allows for the estimated ovulation day being off by up to two days.
When is the best time to get pregnant?
During the fertile window, the few days before ovulation and the day of ovulation itself. An egg survives about a day after it is released, while sperm can survive up to five days, so the days leading up to ovulation count the most.
Does this work with irregular periods?
Less well. The calendar method assumes every cycle is the same length. If your cycles vary by more than a few days, ovulation test kits, basal body temperature, or cervical mucus tracking will tell you more than a calendar estimate.
Limitations
These dates are estimates from averages. Stress, illness, travel, breastfeeding, and conditions such as PCOS can move ovulation earlier or later, and the luteal phase is not exactly 14 days for everyone. Do not use this ovulation calculator as birth control. It is not medical advice; see a doctor if you have been trying to conceive for a year, or six months if you are 35 or older.
What's changed
- Launched.
References
- ACOG, Fertility Awareness-Based Methods of Family Planning
ovulation about 14 days before the next period; pregnancy possible from 5 days before ovulation until 1 day after
- Wilcox, Weinberg and Baird (1995), Timing of sexual intercourse in relation to ovulation
nearly all pregnancies come from a six-day window ending on the day of ovulation
- ACOG Committee Opinion No. 700, Methods for Estimating the Due Date
due date 280 days after the last period, with ovulation on day 14; the 266 days used here is 280 − 14