How to Calculate Your Fertile Window
How the fertile window is calculated from your cycle length and luteal phase, why it's roughly 6 days long, and how to combine it with other fertility signals.
Your fertile window is the stretch of days in a cycle when pregnancy is actually possible — and it's shorter, and positioned differently, than most people assume.
The formula
- Estimate your ovulation date: next expected period date minus your luteal phase length (14 days is the common average).
- Count back 5 days from ovulation — that's the start of your fertile window.
- Ovulation day itself is the last day of the fertile window.
That gives a 6-day window in total: 5 days before ovulation, plus ovulation day. It's not centered on your whole cycle and it isn't a fixed calendar range — it moves earlier or later depending on your actual cycle length that month.
Why it's 5 days before, not more or less
- Sperm can survive in the reproductive tract for up to about 5 days under favorable conditions.
- The egg itself is viable for only about 12-24 hours after ovulation.
- Together, that means intercourse any day in the 5 days before ovulation — not just ovulation day itself — can result in pregnancy.
Enter your last period and cycle length into the Ovulation Calculator to get your specific fertile window dates, plus a forecast for the next 3 cycles.