Calculate your fertile window, the best days for sex, and exactly when to start OPK (ovulation predictor kit) testing — twice daily so you don't miss the LH surge. No data is stored or transmitted.
Choose your starting point
📅 Last Period & Cycle Length
🥚 Known Ovulation Date
Enter the first day of your most recent period.
28d
Range: 18–42 days. Most cycles are 21–35 days.
Use a confirmed date from OPK, ultrasound, basal body temperature, or cycle tracking app.
Please enter a valid date.
Start OPK Testing
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Twice daily
Fertile Window
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Best days for sex
Ovulation
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Peak fertility
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Start OPK Testing
Begin testing twice daily (morning and early afternoon) to avoid missing the LH surge. The surge can appear and peak within 24 hours.
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💚Recommended Days for Sex — Ranked by Conception Probability
📆 Day-by-Day Calendar
📋OPK Testing Protocol
Understanding Your Fertile Window
How long is the fertile window?
A woman can become pregnant from sex that occurs within a window of approximately 6 days: the 5 days before ovulation and the day of ovulation itself. The egg survives only 12–24 hours after release, but sperm can survive in the fallopian tubes for up to 5 days. The highest chance of conception is from sex on the 2 days before ovulation and the day of ovulation.
What is an OPK and how does it work?
An OPK (Ovulation Predictor Kit) detects the surge of luteinizing hormone (LH) in urine. LH surges 24–36 hours before ovulation. A positive OPK means ovulation is likely within the next 24–36 hours — this is your signal to have sex within the next 1–2 days. OPKs do not confirm that ovulation actually occurred, only that LH has surged.
Why test twice a day?
The LH surge can be brief — sometimes lasting less than 24 hours. Testing only once a day risks missing the surge entirely. Testing in the morning and again in the early afternoon (not first morning urine, which can dilute results) catches the surge at its peak. Many women only detect the surge on one of two daily tests.
Can I conceive the day after ovulation?
Conception is substantially less likely the day after ovulation because the egg's viability drops sharply after 12–24 hours. However, it is not impossible, particularly if ovulation occurred later in the day. This calculator includes the day after ovulation as a low-probability option but the window effectively closes after ovulation day.
Important:These calculations are estimates based on average cycle timing. Cycles vary. If you have irregular cycles, PCOS, or other fertility concerns, consult your healthcare provider. All calculations are performed in your browser — no data is sent or stored.
