Ovulation Calculator
Estimate your fertile window.
Sat, May 23, 2026
Fertile window: Mon, May 18, 2026 to Sun, May 24, 2026
| Event | Date |
|---|---|
| Last period started | Sat, May 9, 2026 |
| Fertile window starts | Mon, May 18, 2026 |
| Ovulation day | Sat, May 23, 2026 |
| Fertile window ends | Sun, May 24, 2026 |
| Next period due | Sat, Jun 6, 2026 |
Fertile days at a glance
Mon
May 18
Tue
May 19
Wed
May 20
Thu
May 21
Fri
May 22
Sat
May 23
Sun
May 24
Uses the standard 14-day luteal phase rule: ovulation = cycle length minus 14, counting from the first day of your last period. Sperm survive about five days, so the fertile window starts five days before ovulation. Cycles vary, so this is an estimate, not a guarantee.
About
Ovulation usually happens 14 days before your next period, not 14 days after the last one. The fertile window is the 5 days before ovulation plus the day of. This calculator uses your cycle length to estimate.
How to use
- Enter the first day of your last period.
- Enter your average cycle length.
- See the predicted ovulation date and fertile window.
FAQ
How accurate is this?+
Calendar-based prediction is rough. Cycle length varies. For more precision, track basal body temperature or use ovulation predictor kits (LH strips).
Is the math reliable for irregular cycles?+
Less so. If your cycle varies by more than a few days, the predicted window can be off by a week. Tracking sticks works better.