Ovulation Predictor

The Ovulation Predictor helps you identify your most fertile days and estimate your ovulation date based on your menstrual cycle data. It provides a visual fertility calendar with color-coded phases, a cycle phase timeline, and a 6-cycle forecast with estimated due dates if conception occurs. This tool is useful for family planning, whether you are trying to conceive or simply tracking your cycle.

How Ovulation Prediction Works

Ovulation prediction uses the calendar method, which calculates your likely ovulation date by subtracting the luteal phase length from your total cycle length. For example, with a 28-day cycle and 14-day luteal phase, ovulation is predicted on day 14.

The fertile window spans approximately 6 days: the 5 days before ovulation plus ovulation day itself. This is because sperm can survive up to 5 days in the female reproductive tract, while an egg remains viable for only 12–24 hours after release.

Understanding Your Cycle Phases

How to Use This Tool

1. Enter last period date: Select the first day of your most recent menstrual period.
2. Set cycle length: Choose your average cycle length (the number of days from one period to the next). Normal range is 21–35 days; the average is 28 days.
3. Set luteal phase: If known, select your luteal phase length (typically 12–16 days). Leave at 14 if unsure.
4. View results: Review your ovulation date, fertile window, cycle phase timeline, color-coded calendar, and 6-cycle forecast table.

Tips for Accuracy

Estimated Due Date

If conception occurs, the estimated due date is calculated using Naegele's rule: adding 280 days (40 weeks) from the first day of the last menstrual period. Only about 5% of babies are born on their exact due date — most arrive within a 2-week window around it.

Frequently Asked Questions

How does the ovulation predictor work?

The ovulation predictor estimates your ovulation date by subtracting your luteal phase length from your total cycle length. For a 28-day cycle with a 14-day luteal phase, ovulation is predicted around day 14. The fertile window spans 5 days before ovulation through 1 day after, since sperm can survive up to 5 days in the reproductive tract.

What is the fertile window?

The fertile window is the approximately 6-day period when conception is possible: the 5 days before ovulation and the day of ovulation itself. The 2 days before ovulation and ovulation day are the peak fertility days with the highest chance of conception.

What is the luteal phase?

The luteal phase is the second half of the menstrual cycle, starting after ovulation and ending when the next period begins. It typically lasts 12–16 days (average 14) and is relatively consistent for each individual. During this phase, progesterone prepares the uterine lining for potential implantation.

How accurate is a calendar-based ovulation predictor?

Calendar-based prediction provides a reasonable estimate but is not exact. Ovulation can vary by 1–2 days even with regular cycles due to stress, illness, or hormonal fluctuations. For more precise tracking, combine calendar prediction with BBT charting, cervical mucus monitoring, or OPK test strips.

How is the estimated due date calculated?

The estimated due date uses Naegele's rule: 280 days (40 weeks) from the first day of the last menstrual period. This assumes a standard 28-day cycle. Only about 5% of births occur on the exact due date.

Additional Resources

• Ovulation - Wikipedia
• Menstrual Cycle - Wikipedia
• Fertility Awareness - Wikipedia