Predict ovulation and fertile window from LMP and cycle length. Includes history mode and biomarkers (LH, mucus, BBT) for refinement.
Estimated ovulation
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Standard fertile window
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Extended fertile window
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Forecast window
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Select a mode and enter your data.
LH testing is usually most reliable between 2 p.m. and 8 p.m. If the surge differs greatly from the baseline math, trust the biomarker.
Sperm can survive up to 3–5 days in the reproductive tract, while the egg remains viable for roughly 24 hours after ovulation.
How we estimate your fertile days
We estimate ovulation by subtracting the luteal phase from your average cycle length. The standard fertile window spans two days before and two after ovulation; the extended window covers five days before up to one day after.
If you add a recent cycle range we display a forecast window from the earliest fertile day (min − luteal − 2) to the latest (max − luteal + 1). Current-cycle biomarkers (LH, mucus or basal temperature) always take priority.
Positive LH test → ovulation expected 24–36 h later.
Fertile mucus peak → ovulation the same day or the following day.
Confirmed thermal shift → ovulation occurred 0–1 days before the rise.
Confidence indicators
We mark results as high confidence when a biomarker aligns with the baseline estimate within ±1 day. Without biomarkers the confidence is medium, and if the spread between your shortest and longest cycles exceeds 7 days it drops to low.
A typical luteal phase lasts about 14 days. Values outside 11–17 days merit clinical review.
Quick glossary
LMP
First day of your last menstrual period.
Luteal phase
Days between ovulation and the next period.
Fertile window
Span where conception is most likely.
BBT
Basal body temperature taken immediately after waking.
Tips for better tracking
Record each cycle consistently: take basal temperature at the same time every morning, note cervical mucus quality and keep the timing of LH tests. The more data you enter, the more reliable the estimate becomes.
This tool is informational only. It is not a contraceptive method and does not replace personalised medical advice. Seek medical guidance if you notice short luteal phases, unusual bleeding or difficulty conceiving after 12 months (6 months if you are over 35).
What do “standard” and “extended” mean?
The standard window includes the estimated ovulation day plus two days on either side. The extended window adds five days before and one day after to cover sperm and egg viability.
What if my cycles vary a lot?
Enter the shortest and longest cycles from the past months. If the spread exceeds 7 days and you have no biomarkers, confidence decreases and extra tracking methods are advised.
How should I interpret a thermal shift?
A sustained rise in basal temperature confirms ovulation already happened. Use it to validate previous cycles and refine future predictions.
Can I rely on this to avoid pregnancy?
Fertility awareness requires discipline and multiple indicators. This calculator provides guidance but is not a certified birth control method. Consult your healthcare provider before basing decisions solely on these dates.
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