Ovulation + Fertile Window Calculator
Estimate ovulation day and the 6-day fertile window from last menstrual period and cycle length, using the luteal-phase fixed-length method.
Method: ovulation = next-period date − 14 days (fixed luteal phase). Fertile window from Wilcox AJ et al., NEJM 1995;333:1517–21 (5 days before ovulation + ovulation day). Peak per ASRM 2017 Practice Committee. Calendar method least accurate for cycles > 8 days variable — combine with LH urine tests or BBT charting.
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About this tool
Ovulation occurs approximately 14 days before the next menstrual period — the luteal (post-ovulation) phase is much more consistent than the follicular phase across cycles. The standard estimate: ovulation day = (first day of next period) − 14. The 6-day fertile window per Wilcox et al., New England Journal of Medicine 1995, spans the 5 days BEFORE ovulation + ovulation day itself, because sperm survive 3–5 days in cervical mucus while the ovum is viable only ~12–24 hours. Per the American Society for Reproductive Medicine (ASRM 2017 Practice Committee) and ACOG Committee Opinion 781, peak fertility falls on the 2 days immediately before ovulation. This calendar method is least accurate for irregular cycles (variation > 8 days) where fertility-awareness apps using basal body temperature or LH urine testing are more reliable. For cycles under 21 days or over 35 days, ovulation timing is harder to predict and basal body temperature charting is recommended.
How to use it
- Enter the first day of your last menstrual period (LMP).
- Enter your typical cycle length (28 days is average; 21–35 is the normal range).
- Read your estimated ovulation day, the 6-day fertile window, and predicted next-period date.
- For irregular cycles, also use basal body temperature or LH ovulation test strips.
Frequently asked questions
- Why 14 days before the next period?
- The luteal phase (corpus luteum lifespan) is 12–16 days, fairly fixed across cycles. Working back from the next period is more accurate than counting forward from the last one.
- What is the "fertile window"?
- Wilcox 1995 found pregnancy can result from intercourse on any of the 5 days before ovulation through ovulation day itself. Peak pregnancy rates: the 2 days before ovulation.
- What about irregular cycles?
- If your cycle length varies more than 8 days, calendar methods alone are unreliable. Use basal body temperature charting, cervical mucus tracking, or LH-detection urine strips (which signal ovulation 24–36 hours in advance).
- Can I use this for contraception?
- The calendar method alone has a typical-use failure rate of 24% per year (CDC contraceptive efficacy table). For natural family planning use Symptothermal or Standard Days methods with proper training.
- Is data sent anywhere?
- No. All math runs in your browser.