Ovulation Calculator
Fertile window from cycle length and LMP.
Inputs
This calculator gives a general estimate and isn't medical advice. It doesn't account for your individual health history — talk to a doctor before making decisions based on it.
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Formula
Ovulation ≈ LMP + (cycle length − 14); fertile window = ovulation − 5 days through ovulation + 1 day
The luteal phase after ovulation is assumed to last 14 days, so ovulation is placed that many days before the next expected period. Sperm survival motivates a 6-day fertile window.
Worked example
With a 28-day cycle starting 1 August 2026, estimated ovulation is 14 August, and the fertile window runs 9 August through 15 August.
Where this can give the wrong answer
- Cycle lengths outside 21–45 days are rejected — shorter or longer cycles need clinical context this simple model doesn't provide.
- Stress, illness, travel, and hormonal conditions can shift ovulation day-to-day despite a regular average cycle length.
- The fertile window is an estimate for planning, not a guarantee of conception timing — LH tests and basal body temperature track ovulation more directly.
FAQ
- The model assumes ovulation occurs 14 days before the next period regardless of total cycle length. A 32-day cycle ovulates around day 18; a 26-day cycle around day 12.