Calculate your ovulation and fertile window to help with family planning. This tool processes all data locally in your browser. No information is ever sent to any server. Completely free, no registration required.
An Ovulation Calculator estimates your most fertile days — when you're most likely to conceive — by tracking your menstrual cycle and predicting ovulation. For couples trying to get pregnant, timing intercourse during the 'fertile window' (the 5-6 days leading up to and including ovulation) dramatically increases conception chances from near-zero to 20-35% per cycle. The calculator uses the standard 28-day cycle assumption as a starting point but can be adjusted for any cycle length (21-40 days). In 2026, with many women delaying pregnancy to their 30s, understanding and optimizing fertility timing is more important than ever.
Enter the first day of your last period and your average cycle length. Ovulation typically occurs about 14 days before your next period starts (not 14 days after the last one started — a common misconception). The calculator identifies: estimated ovulation day, the fertile window (5 days before ovulation + ovulation day), and the best days for conception. For irregular cycles, the calculator shows your date range based on your shortest and longest recent cycles.
Ovulation Day = First Day of Last Period + Cycle Length − 14\nFertile Window Start = Ovulation Day − 5\nFertile Window End = Ovulation Day + 1\nBest Conception Days = Ovulation Day − 2, −1, and Ovulation Day\n\nNext Period = First Day of Last Period + Cycle Length\nImplantation Window = Ovulation Day + 6 to +12\nPregnancy Test = First Day of Missed Period (or 14 days after ovulation)
For healthy couples under 35, about 80% conceive within 6 months of trying, and 85-90% within 12 months. Fertility starts declining more significantly after 35. If you're under 35 and haven't conceived after 12 months (or 6 months if over 35), consult a fertility specialist.
While less likely, yes — especially if you have a short cycle (21-23 days). Sperm can survive 5 days, so intercourse toward the end of your period could result in conception if you ovulate early in your cycle.
Use the average of your last 3-6 cycles for the calculator's estimate. For highly irregular cycles (varying by >7-10 days), ovulation prediction kits and BBT tracking are more reliable than calendar methods. Irregular cycles may indicate PCOS, thyroid issues, or other conditions worth discussing with a doctor.
Free online Ovulation Calculator — no signup, 100% client-side processing. All data stays in your browser.