Ovulation Calculator
Find your fertile window and the two days conception is most likely.
Your cycle
First day of one period to the first day of the next
Calendar prediction assumes your cycles are regular, and most people's vary by a few days month to month. Ovulation predictor kits, basal body temperature and cervical mucus tell you what is actually happening this cycle. If you have been trying for a year (six months over 35), see a doctor.
This estimates your ovulation date and fertile window from the first day of your last period and your usual cycle length.
It is an estimate based on cycle averages, not a confirmation that ovulation has occurred.
How ovulation timing works
Ovulation is the release of an egg from the ovary, and it happens once per cycle. The fertile window is the short period around it when conception is possible.
The fact that changes most people's timing: ovulation is about 14 days before your next period, not 14 days after your last one. The second half of the cycle, the luteal phase, is fairly consistent at around 14 days; the first half is what varies. On a 28-day cycle both give day 14, which is why the error hides. On a 32-day cycle ovulation is day 18, not day 14.
The second surprise is that the fertile window sits mostly before ovulation. Sperm can survive up to five days in the reproductive tract; the egg survives 12-24 hours. So the window is roughly the five days before ovulation plus ovulation day, and waiting for a positive test means missing most of it.
Cycle length minus about 14 days. Assuming day 14 on a 32-day cycle puts the fertile window four days too early, which misses it entirely.
What to enter
- First day of last period
- Day one is the first day of full flow, not spotting. Everything is counted from here.
- Average cycle length
- From day one of one period to day one of the next. Track a few cycles rather than assuming 28 days.
- Luteal phase length
- Usually 12-16 days and fairly consistent for an individual. If you know yours from tracking, use it.
Signs that ovulation is approaching
- Cervical mucus
- Becomes clear, slippery and stretchy, often compared to raw egg white. One of the most useful signs, and it appears before ovulation.
- Ovulation predictor kits
- Detect the LH surge, which precedes ovulation by 24-36 hours. Useful, but they only give you about a day of notice.
- Basal body temperature
- Rises slightly after ovulation. It confirms that ovulation happened; it cannot predict it, so it is for learning your pattern rather than timing this cycle.
- Mittelschmerz
- One-sided pelvic twinge around ovulation. Some people notice it consistently and many never do.
What this assumes
This assumes regular cycles and a typical luteal phase. Irregular cycles, PCOS, thyroid conditions, stress and illness all shift or prevent ovulation.
This is an estimate for planning, not a contraceptive method. Calendar-based methods have high failure rates for preventing pregnancy.
How to calculate your fertile window
Work backwards from your next expected period, not forwards from your last one.
- cycle length
- Day one of one period to day one of the next
- 14
- The typical luteal phase, which is the consistent half of the cycle
Track cycle length across several months. Count from day one to day one. If it varies by more than a few days, the estimate will be correspondingly rough.
Subtract 14 from your cycle length. That gives the approximate ovulation day. On a 30-day cycle that is day 16.
Count back five days for the window. The fertile window is those five days plus ovulation day. It sits almost entirely before ovulation.
Confirm with body signs. Cervical mucus changes give the earliest warning. An LH test gives 24-36 hours of notice. Neither replaces knowing your own cycle.
See a worked example: why the 28-day assumption misleads
- Cycle
- 32 days
- Last period started
- Day 1
Ovulation: 32 − 14 = day 18, not day 14.
Fertile window: days 13 through 18.
Someone assuming a 28-day cycle would target days 9-14, and would have finished before the window even opened.
The same arithmetic on a 24-day cycle gives ovulation on day 10, so the window is days 5-10, overlapping the end of the period.
Day 18, with a window of days 13-18
Frequently asked questions
About 14 days before your next period, which is your cycle length minus 14. On a 28-day cycle that is day 14; on a 32-day cycle it is day 18.
The luteal phase, from ovulation to the next period, is the consistent half. The follicular phase before ovulation is what varies, which is why counting forward from your last period is unreliable.
About six days: the five days before ovulation plus ovulation day itself.
The asymmetry is because sperm survive up to five days while the egg survives only 12-24 hours. Chances are highest in the two or three days immediately before ovulation.
They are useful. They detect the LH surge, which precedes ovulation by 24-36 hours, so a positive means the window is closing rather than opening.
Start testing a few days before your predicted ovulation. And do not wait for a positive to start trying, since by then most of the fertile window has already passed.
Calendar prediction becomes much less reliable, and body signs become more useful. Cervical mucus changes and LH testing track what is actually happening rather than what is expected.
Cycles consistently shorter than 21 days or longer than 35, or that vary widely, are worth discussing with a doctor. PCOS and thyroid conditions are common and treatable causes.
It is not reliable for that. Calendar-based methods have high typical-use failure rates, because ovulation timing shifts with stress, illness, travel and sleep.
Fertility awareness methods that combine temperature, mucus and calendar tracking are more effective, and they require proper instruction. For dependable contraception, talk to a healthcare provider.
No, it confirms it. The rise of about 0.5°F happens after ovulation, in response to progesterone.
That makes it valuable for learning your pattern over several cycles, and useless for timing the current one. Cervical mucus is the sign that comes early enough to act on.
Problems people actually run into
Counting 14 days forward from the last period
It only works on a 28-day cycle. On a 32-day cycle it puts ovulation four days early, and the whole fertile window is missed.
Always subtract 14 from your cycle length instead. It is the same arithmetic on a 28-day cycle and correct on every other one.
Starting only once a test turns positive
An LH surge means ovulation is 24-36 hours away, so a positive test signals the end of the fertile window rather than the start.
Most of the window is the five days beforehand. Use the test to confirm timing, not to decide when to begin.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Last updated: September 4, 2026