Fertility Window Calculator
Find your real fertile window — based on your actual cycle, not the generic day 14 rule. Private, free, no account needed.
The day 14 rule is wrong for most women. Ovulation happens 12–16 days before your next period — which means a 32-day cycle ovulates around day 18, not day 14. This calculator uses your real cycle length to find your actual window.
Phase 1 — Your Cycle · Question 1 of 5
When did your last period start?
Every date in your fertile window is calculated from this anchor. Without it, we can only give you generic ranges — not your actual dates.
Why most fertility apps get your window wrong
Most apps use a fixed day 14 ovulation date regardless of your cycle length. This is the Knaus-Ogino calendar method — developed in the 1930s — and it only applies accurately to a perfect 28-day cycle. A woman with a 32-day cycle ovulates around day 18. A woman with a 35-day cycle ovulates around day 21. Using a fixed window means millions of women are timing sex on the wrong days.
Your fertile window is 6 days, not 1. Sperm survive up to 5 days in the fallopian tubes. The egg lives 12–24 hours. So your fertile window is the 5 days before ovulation plus ovulation day itself — and it shifts with every day your cycle length changes.
Cycle length variability makes app predictions significantly less reliable. Research published in peer-reviewed literature found that up to 46% of app-calculated fertile windows are inaccurate when cycle length varies by more than 3 days month to month. If your cycle shifts, your window shifts — and this calculator accounts for that, flags it, and tells you what to do instead.