Predict your ovulation day and most fertile days from your cycle.
28-day cycle, period 1 June
Most fertile on 14–15 June.
Ovulate ~15 June
Irregular 35-day cycle
Longer cycles push ovulation later; luteal phase stays ~14 days.
Ovulate ~day 21
How the figures above are worked out — the rules, rates and reasoning behind every line.
Ovulation usually happens about 14 days before your next period. From your last period and cycle length, this estimates your ovulation day and fertile window.
We assume a fixed ~14-day luteal phase, so ovulation falls (cycle length − 14) days after your last period started. The fertile window is the five days before ovulation plus ovulation day.
Cycles naturally vary, so combine this with ovulation predictor kits or basal body temperature if you're tracking closely.
The two days before ovulation and ovulation day itself are the most fertile. The full window spans about six days.
Yes. Because the luteal phase is fairly fixed at ~14 days, a longer cycle pushes ovulation later and a shorter cycle earlier.
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