Fertile Window Calculator — When Are You Most Likely to Conceive?

Your fertile window is the period of days each cycle when pregnancy is possible. Understanding it helps whether you're trying to conceive or trying to avoid pregnancy naturally.

What Is the Fertile Window?

The fertile window is the 6-day period ending on ovulation day. It exists because sperm can survive inside the body for up to 5 days, while the egg is only viable for 12–24 hours after release.

So pregnancy is possible from intercourse on any of the 5 days before ovulation or on ovulation day itself.

How to Calculate Your Fertile Window

Step 1: Find your average cycle length (count from Day 1 of one period to Day 1 of the next).

Step 2: Subtract 14 from your cycle length to get your estimated ovulation day. (Example: 30-day cycle → ovulation around day 16)

Step 3: Your fertile window starts 5 days before ovulation. (Example: Day 16 ovulation → fertile window Day 11–16)

Signs Your Fertile Window Is Approaching

• Cervical mucus: Changes from dry/sticky to clear and stretchy (like egg white) as ovulation approaches
• Basal body temperature: Drops slightly just before ovulation, then rises sharply after
• LH surge: Detectable with ovulation test strips 24–48 hours before ovulation
• Mittelschmerz: A mild one-sided pelvic ache some people feel at ovulation

How Accurate Are Calculators?

Calendar-based calculations are estimates. They work best for regular cycles. Factors that affect accuracy:

• Stress (can delay ovulation)
• Illness or travel
• Cycle irregularity
• Age (cycles can shift in perimenopause)

For higher accuracy, combine calendar tracking with daily BBT logging and cervical mucus observation — all of which Luna supports.

Luna's Fertile Window Feature

Luna automatically calculates your fertile window each cycle and marks it in green on your calendar. You'll also see your ovulation day, fertile window dates, and a day-count on your dashboard. Reminders are available for when your fertile window starts.

Track your fertile window free with Luna →