Ovulation Calculator
Estimate ovulation as cycle start plus (cycle length minus 14 days). Also lists a simple fertile window and the next period if the cycle stays constant. Informational, not medical advice. Not contraception.
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How to estimate ovulation
- Enter the first day of the period as YYYY-MM-DD.
- Enter cycle length in days from 20 to 45. The default is 28.
- Read estimated ovulation as start plus (cycle minus 14).
- Note: informational, not medical advice, and not contraception. Nothing is uploaded.
Luteal-length ovulation estimate
Start + (cycle - 14)
| Cycle start | First day of menstrual bleeding |
|---|---|
| Cycle length | Days from start to the next start, 20 to 45 on this page |
| Luteal assumption | 14 days after ovulation until the next period |
| Ovulation day | Start plus (cycle - 14) |
| Fertile window | About 5 days before ovulation through 1 day after |
| Advice | Not contraception. Not medical advice. |
What this ovulation estimate is
The page assumes a 14-day luteal phase. Ovulation = cycle start + (cycle length - 14). For 1 January 2026 and 28 days, ovulation is 15 January 2026. A fertile window is listed as 5 days before through 1 day after that date.
Cycle start, luteal phase, and fertile window
Cycle start is day 1 of bleeding. The luteal phase is the span after ovulation. Many cycles are not 28 days and not 14 days luteal. Pregnancy dating from LMP is a different job on the due date page.
How this page adds days
Dates are civil calendar dates in your local timezone, not UTC day counts. Cycle length must be from 20 to 45. The next period row assumes the same length repeats.
Note
Informational, not medical advice. Not contraception. Cycles vary. Do not use this page to prevent pregnancy. See the disclaimer. CZNull does not receive the dates.