Ovulation Calculator

How the ovulation calculator estimates your fertile days

Ovulation typically happens about 12–16 days before your next period starts. Calculator.net models this by subtracting your luteal phase length (the time between ovulation and your next period) from your full cycle length. We use the first day of your last menstrual period (LMP) as day one, then count forward to estimate when ovulation occurs.

Input details that matter most

• Last menstrual period: Use the first day of bleeding, not the last day.
• Cycle length: Count days from the start of one period to the start of the next. If it varies, use an average from the last 3–6 cycles.
• Luteal phase length: Optional, but helpful if you track ovulation with tests or basal body temperature. If unknown, 14 days is a common average.

Understanding the fertile window

Sperm can survive for up to five days in the reproductive tract, while an egg is typically viable for 12–24 hours. That is why the fertile window covers the five days before ovulation through the day after ovulation. Planning intercourse during this range increases the likelihood of conception.

Tips for better accuracy

• Track at least three cycles to find a reliable average length.
• Pair the estimate with ovulation predictor kits or basal body temperature changes.
• Adjust your luteal phase length if you consistently ovulate earlier or later than expected.

Use BiteCheck to log cycle data alongside nutrition, sleep, and stress trends. That context can help you identify patterns that affect your cycle regularity and overall well-being.

Frequently asked questions