Fertile Window Calculator (Day-by-Day Odds)
The six fertile days ranked by published per-day conception probability — not just a highlighted range.
Conception odds peak the two days *before* ovulation, not on it — the egg lives ~24 hours while sperm wait up to five days, so being early beats being exact. After ovulation day the window slams shut. Every-other-day intercourse through the window matches daily timing in outcomes with less stress.
Formula
For general information only. Pregnancy and infant care decisions must be guided by your treating clinician or midwife — always defer to their assessment.
The six fertile days ranked by published per-day conception probability — not just a highlighted range. The Fertile Window Calculator (Day-by-Day Odds) is a free, private fertile window tool — every result computes instantly in your browser with no sign-up and no data upload.
About Fertile Window Calculator (Day-by-Day Odds)
Conception odds peak the two days *before* ovulation, not on it — the egg lives ~24 hours while sperm wait up to five days, so being early beats being exact. It applies per-day probabilities from Wilcox NEJM 1995 prospective cohort. Use the Fertile Window Calculator (Day-by-Day Odds) to get an instant, clearly-explained result with the working shown step by step — free, private and with the source method cited.
How to use Fertile Window Calculator (Day-by-Day Odds)
- 1Enter your details in the Fertile Window Calculator (Day-by-Day Odds) input fields above.
- 2The result updates instantly with the working and reference bands shown.
- 3Adjust any value to explore how it changes the outcome — it's free and unlimited.
Why use Fertile Window Calculator (Day-by-Day Odds)?
- ✓Instant fertile window result that recomputes as you type — no waiting, no page reloads
- ✓100% client-side: your health data never leaves your browser
- ✓Shows the actual formula and your numbers substituted in, so you can see exactly how the result is reached
- ✓Based on published, citable sources (Wilcox AJ et al., NEJM 1995;333:1517-21)
- ✓Free forever with no sign-up, account or app install
Frequently asked questions
How does the Fertile Window Calculator (Day-by-Day Odds) work?+
Conception odds peak the two days *before* ovulation, not on it — the egg lives ~24 hours while sperm wait up to five days, so being early beats being exact. The calculation uses the formula: per-day probabilities from Wilcox NEJM 1995 prospective cohort. Everything runs instantly in your browser as you type — your inputs are never uploaded.
Is the Fertile Window Calculator (Day-by-Day Odds) accurate, and what is it based on?+
The method is traceable to authoritative sources: Wilcox AJ et al., NEJM 1995;333:1517-21. Results are estimates — individual variation always applies, so treat the output as a well-grounded starting point.
Is the Fertile Window Calculator (Day-by-Day Odds) free and private?+
Yes. It's completely free with no sign-up, and all computation happens locally in your browser, so none of your health data ever leaves your device.
Can I use this for medical decisions?+
This tool is informational only. Always defer to your treating clinician or midwife for pregnancy and infant care decisions.
What can I use the Fertile Window Calculator (Day-by-Day Odds) for?+
It's commonly used for fertile window, best days to conceive, conception probability. The six fertile days ranked by published per-day conception probability — not just a highlighted range.
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