About Ovulation Calculator

Estimate ovulation date, fertile window, and next period based on the first day of your last menstrual period, cycle length (21–40 days), and period duration (3–10 days). The calculation uses the standard luteal phase method—ovulation occurs approximately 14 days before the next period, and the fertile window spans from 5 days before ovulation through 1 day after. A color-coded fertility calendar maps each cycle day to its phase: menstrual, follicular, fertile window, or luteal.

• Ovulation estimated at cycle length minus 14 days (standard luteal phase assumption)
• Fertile window: 5 days before through 1 day after ovulation
• Current fertility status based on today’s position in the cycle (menstrual, follicular, fertile, luteal)
• Visual calendar color-codes every cycle day by phase
• Displays days until ovulation (or days since, if past)
• Export calculation results as JSON

Frequently Asked Questions

How reliable is the 14-day luteal phase assumption?

It’s a clinical average. Luteal phases typically range from 12–16 days and are relatively consistent for an individual but vary between people. This calculator provides estimates, not medical-grade predictions. For conception planning, combine this with basal body temperature or ovulation test strips.

Can irregular cycles affect the accuracy of these estimates?

Significantly. The calculator assumes a consistent cycle length and a 14-day luteal phase. If your cycles vary by more than a few days, the fertile window estimate can be off by a week or more. Track several cycles to get a reliable average, or use basal body temperature for better precision.