Find your fertile window
Enter your last period start date and cycle length to estimate your next ovulation date and fertile window.
Ovulation day by cycle length
Estimated ovulation day (counting from day 1 of your period) for common cycle lengths, assuming a standard 14-day luteal phase.
| Cycle length | Estimated ovulation day |
|---|---|
| 21 days | Day 7 |
| 24 days | Day 10 |
| 26 days | Day 12 |
| 28 days | Day 14 |
| 30 days | Day 16 |
| 32 days | Day 18 |
| 35 days | Day 21 |
Want your exact fertile window and next period date? Use the ovulation calculator above.
About the ovulation calculator
This calculator estimates when you're likely to ovulate and your fertile window based on the first day of your last period and your average cycle length. It uses the standard luteal-phase assumption that ovulation happens about 14 days before your next period starts — so for a typical 28-day cycle, that puts ovulation around day 14, but for shorter or longer cycles the calculator adjusts the estimated day accordingly.
The fertile window shown spans six days: the five days leading up to ovulation, plus the day of ovulation itself. This range exists because sperm can survive in the reproductive tract for up to about five days, while the egg itself is typically viable for only 12 to 24 hours after release — so intercourse anytime in that window has a realistic chance of resulting in conception, with the days right before and on ovulation generally considered the most fertile.
The calculator also estimates your next expected period by adding your average cycle length to the start date of your last one. This is useful both for fertility planning and simply for anticipating your next cycle, though it becomes less reliable the more your cycle length varies from month to month.
Cycle length is counted from the first day of one period to the first day of the next, and "normal" covers a wider range than many people expect — anywhere from about 21 to 35 days is considered typical for adults. Because this calculator relies on an average, tracking your cycles over several months (or using ovulation predictor kits or basal body temperature charting) will give you a more personalized and accurate picture than a single estimate.
Disclaimer: these are general estimates based on average cycle lengths and standard luteal-phase timing, not a substitute for advice from a doctor or fertility specialist. Actual ovulation timing varies by individual and can shift from cycle to cycle due to stress, illness, travel, and many other factors.
Ovulation by cycle length
Estimated ovulation day and fertile window for common average cycle lengths.
Ovulation calculator questions
How ovulation, the fertile window, and next period are calculated.
How is ovulation day calculated?
The calculator subtracts 14 days (the typical luteal phase length) from your cycle length, then adds that number of days to the start of your last period. For a 28-day cycle, that means ovulation is estimated around day 14.
What is the fertile window?
It's the six-day span made up of the five days before estimated ovulation plus the day of ovulation itself. Sperm can survive up to about five days, so intercourse anytime in this window has a realistic chance of resulting in conception.
Why does cycle length matter so much?
Ovulation timing shifts with cycle length because the luteal phase (the time between ovulation and your next period) tends to stay fairly constant at about 14 days, while the follicular phase (before ovulation) is what varies. Longer cycles generally mean later ovulation, and shorter cycles mean earlier ovulation.
How is my next period predicted?
The calculator adds your average cycle length to the first day of your last period. It's most accurate if your cycles are fairly regular; if they vary a lot month to month, treat the predicted date as a rough estimate.
What if my cycle length varies from month to month?
Use your average cycle length over the last several months for the most realistic estimate. If your cycles are very irregular, ovulation predictor kits, basal body temperature tracking, or cervical mucus monitoring will give more reliable real-time signals than date-based prediction alone.
Can I use this calculator to avoid pregnancy?
This tool is designed to estimate fertile days, not to serve as a method of contraception. Ovulation timing can shift unexpectedly, so it should not be relied on as a birth control method on its own.
What cycle length range does this calculator support?
You can enter any average cycle length between 21 and 35 days, which covers the typical range for most adults. Cycles consistently outside that range are worth discussing with a doctor.
Is this ovulation calculator a substitute for medical advice?
No. These are general estimates based on average cycle lengths and standard luteal-phase timing. They are not a substitute for advice from a doctor or fertility specialist, and actual ovulation dates vary by individual.