Ovulation & Fertility Calculator
Find your fertile window and most likely ovulation day from your last period and cycle length.
Written by Rahman · Last reviewed
Quick answer
Ovulation occurs roughly 14 days before the next period starts, so on a 28-day cycle that falls around day 14 and on a 32-day cycle around day 18. The fertile window covers the five days before ovulation plus the day itself, because sperm can survive several days while the egg remains viable for under 24 hours.
Cycles vary, including from month to month in the same person. The fertile window is the six days ending on ovulation, because sperm survive several days while an egg lives about one. For accurate timing, combine this estimate with ovulation test kits or basal body temperature tracking. This is not a contraceptive method.
Results are general screening estimates, not medical advice or a diagnosis. They can read inaccurately for athletes, older adults, children and during pregnancy. Speak to a doctor or registered dietitian before changing your diet, exercise or medication.
About the Ovulation & Fertility Calculator
Ovulation typically occurs about 14 days before the start of your next period. For a regular 28-day cycle, that places it around day 14. For longer or shorter cycles, the ovulation day shifts accordingly.
The "fertile window" is the six-day period ending on ovulation day. Sperm can survive in the reproductive tract for up to five days, so conception is possible from any unprotected intercourse in this window. This calculator estimates both ovulation day and the fertile window from your last period and average cycle length.
Ovulation day = LMP + (cycle length − 14) days. Fertile window = 5 days before ovulation through ovulation day.
Cycle lengths vary even in the same person. Use the calculator as a guide, not a guarantee.
Why the fertile window is wider than ovulation itself
Ovulation is a single event, but the window in which intercourse can result in pregnancy is several days long. That asymmetry comes from how long each cell survives.
- Sperm can remain viable in the reproductive tract for up to about five days in favourable conditions.
- An egg is viable for roughly 12 to 24 hours after release.
- The result is a fertile window of about six days: the five days before ovulation plus the day of ovulation.
- Conception probability climbs across the window rather than sitting flat: in the study that defined it, intercourse five days before ovulation carried about a 0.10 chance of conception, rising to about 0.33 on the day of ovulation itself.
Calendar prediction assumes regular cycles and past cycles that resemble future ones. Cycle length varies with stress, illness, travel and age, so the estimate is a guide rather than a measurement. Ovulation predictor kits and basal body temperature tracking observe the current cycle instead of inferring from previous ones.
Frequently Asked Questions
How accurate is this for irregular cycles?
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Less so, and the caveat is wider than irregular cycles alone. The calculator assumes a consistent cycle length, but a prospective study of 696 cycles found the fertile window fell entirely within the textbook days 10 to 17 for only about 30% of women, and concluded that its timing can be highly unpredictable even in women whose cycles are usually regular. If your cycles vary, track several months to find an average, or use ovulation predictor kits (LH tests) and basal body temperature charting for sharper timing.
What is the most fertile day?
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The last two or three days of the fertile window: the day of ovulation and the couple of days before it. In the Wilcox study cited below, the per-day chance of conception rose from about 10% five days before ovulation to about 33% on ovulation day. Estimates of the single peak day differ between analyses of the same data, so treat the closing days of the window as a group rather than betting on one.
Can I get pregnant outside the fertile window?
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Yes. Cycle length varies, and so does the timing of ovulation within it. In the same study, women had at least a 10% chance of being in their fertile window on every day from day 6 to day 21, and a few remained potentially fertile into the fifth week of a cycle. The calculator gives the most likely window, not the only possible one.
Does the calculator work for trying to avoid pregnancy?
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Not reliably. This tool estimates a window; it is not a contraceptive method. The NHS puts fertility awareness at 91% to 99% effective when followed correctly every time, but 76% with typical use, meaning 24 in 100 women using it for a year become pregnant. For contraception, combine awareness methods with barrier or hormonal methods, or consult a healthcare provider.
Sources
- Wilcox, Weinberg & Baird (1995), New England Journal of Medicine — Timing of sexual intercourse in relation to ovulation — The study that defined the six-day fertile window: across 625 cycles, conception only followed intercourse in the six days ending on the day of ovulation. Also the source of the per-day probabilities quoted above, rising from about 0.10 five days before ovulation to about 0.33 on the day itself.
- Wilcox, Dunson & Baird (2000), BMJ — The timing of the "fertile window" in the menstrual cycle — Day-specific estimates across 696 cycles. Backs the unpredictability caveats: the window sat entirely within the textbook days 10 to 17 for only about 30% of women, every day from 6 to 21 carried at least a 10% chance of being fertile, and the authors conclude timing is highly unpredictable even when cycles are usually regular.
- NHS — Natural family planning (fertility awareness) — Backs the contraception caveat only: 91% to 99% effective with perfect use, 76% with typical use. Cited for the failure rate, not for the timing method itself.