Ovulation and Fertile Window Calculator (Cycle-Day Method)
The expected day of ovulation and the six-day fertile window from your cycle length, with the days left until each.
Results
What this tool does
Ovulation happens about 14 days before the next period, not 14 days after the last one, so in a 35-day cycle it falls around day 21, not day 14. The luteal phase after ovulation is fairly constant, commonly 12–16 days, while the part before ovulation is what varies between cycles. In a 1995 study by Wilcox and colleagues, pregnancy was only possible when intercourse fell in a six-day window ending on the day of ovulation, with the highest chances in the two days before it. A calendar prediction is only as good as your cycle is regular: for irregular cycles, ovulation tests or temperature charting are more reliable, and this tool must not be used as contraception.
Formula
ovulation day ≈ cycle length − luteal phase · fertile window = the 5 days before ovulation plus the day itself
Variables
| Symbol | Meaning | Unit |
|---|---|---|
cy | Cycle length (first day of one period to the day before the next) | d |
lu | Luteal phase (ovulation to next period; about 14 days) | d |
td | Today's cycle day (day 1 = first day of bleeding) | — |
OV | Expected ovulation: cycle day | — |
FS | Fertile window opens: cycle day | — |
DW | Days until the fertile window opens | d |
DO | Days until ovulation | d |
DP | Days until the next period | d |
Worked example
- Cycle length (first day of one period to the day before the next)28 d
- Luteal phase (ovulation to next period; about 14 days)14 d
- Today's cycle day (day 1 = first day of bleeding)10
- Expected ovulation: cycle day14
- Fertile window opens: cycle day9
- Days until the fertile window opens0 d
- Days until ovulation4 d
- Days until the next period19 d
Limitations
- This tool is informational only. It does not diagnose, treat or replace a qualified health professional. Speak to a doctor before making decisions about your health.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.