Ovulation Calculator
Fertility is not spread evenly across a cycle: conception is realistically possible in a window of about six days. This calculator estimates ovulation day and that fertile window from your cycle length and last period, which is the practical starting point whether you are trying to conceive or avoid it.
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How the ovulation calculator works
Ovulation is estimated as cycle length minus 14 days from the first day of the last period, because the luteal phase is relatively fixed at 12–16 days.
The fertile window spans roughly five days before ovulation to one day after, since sperm survive up to five days and the egg about 12–24 hours.
Cycle length variation shifts the window; the follicular phase varies far more than the luteal phase, which is why counting back from the next period is more reliable than counting forward.
Worked example: last period started 3 June, 30-day cycle
- Ovulation: 3 June + (30 − 14) = 19 June.
- Fertile window: 14 June to 20 June.
- Most fertile days: 17 to 19 June, immediately before ovulation.
- Next period expected: 3 July.
- On a 26-day cycle, ovulation would fall on 15 June instead — a four-day shift from cycle length alone.
Common mistakes to avoid
Assuming day 14 for every cycle
Day 14 only applies to a 28-day cycle. On a 32-day cycle ovulation is nearer day 18, and timing intercourse for day 14 misses the window entirely.
Relying on calendar timing for contraception
Calendar methods have high failure rates because cycles vary. They are not a substitute for contraception.
Waiting for a positive ovulation test to start trying
Fertility peaks in the days before ovulation. By the time an LH surge is detected, much of the window has already opened.
Frequently asked questions
When am I most fertile?
The two to three days immediately before ovulation, with the wider window running from about five days before to one day after.
How do I know I am ovulating?
Ovulation predictor kits detect the LH surge; cervical mucus becomes clear and stretchy; basal body temperature rises slightly after ovulation.
Can I get pregnant during my period?
It is unlikely but possible with a short cycle, where ovulation can occur soon after bleeding ends and sperm survive up to five days.
How long does it usually take to conceive?
About 85% of couples conceive within a year of trying. Seeking advice earlier is reasonable after 35 or with known conditions.
Does cycle length change the accuracy?
Yes. Irregular cycles make calendar estimates unreliable; tracking with tests or temperature over a few months is much better.
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