Fertile Window Calculator
Calculate your most fertile days with conception probability by day, optimized for trying to conceive.
Results
Visualization
How It Works
The fertile window is the ~6-day period ending on ovulation day. Conception probability is highest 1-2 days before ovulation, not on ovulation day itself.
The Formula
Cumulative Probability = 1 - (1 - Monthly Rate)months
Variables
- O-1 — One day before ovulation — the single most fertile day (~28% chance)
- O-2 — Two days before ovulation — second most fertile (~25% chance)
Worked Example
Age 30, regular 28-day cycle: ~20% monthly chance. After 6 months: 74% cumulative. After 12 months: 93% cumulative.
Practical Tips
- Having intercourse every other day during the fertile window is sufficient.
- The day before ovulation (O-1) has the highest single-day probability.
- If cycles are irregular, widen your window and use OPKs for confirmation.
- Most couples under 35 conceive within 12 months — see a doctor if not.
- Lifestyle factors (sleep, diet, stress) can shift ovulation unpredictably.