Ovulation Calculator
Ovulation date and fertile window from the first day of your last period and your average cycle length, 14 to 40 days. A calendar estimate, not a test. How it works
Use the Ovulation Calculator
About the Ovulation Calculator
Ovulation is estimated at the first day of the last period plus the cycle length minus 14 days, and the fertile window is the five days before it plus the day itself. A last period that began January 1, 2026 with a 28-day cycle gives ovulation on January 15, 2026 and a window from January 10 to January 15.
Ovulation date by cycle length, for a last period that began January 1, 2026
| Average cycle | Ovulation is this many days after the last period began | Fertile window starts | Ovulation date |
|---|---|---|---|
| 21 days | 7 days | January 3 | January 8 |
| 24 days | 10 days | January 6 | January 11 |
| 26 days | 12 days | January 8 | January 13 |
| 28 days | 14 days | January 10 | January 15 |
| 30 days | 16 days | January 12 | January 17 |
| 32 days | 18 days | January 14 | January 19 |
| 35 days | 21 days | January 17 | January 22 |
| 40 days | 26 days | January 22 | January 27 |
The fertile window ends on the ovulation date. The tool accepts whole cycle lengths from 14 to 40 days.
Methodology
Ovulation = first day of the last period + (cycle length − 14) days. Fertile window start = ovulation − 5 days, and the window ends on the ovulation date. The 14 is the number of days assumed between ovulation and the start of the next period.
How to use this tool
- Enter the first day of your last period.
- Enter your average cycle length in days, a whole number from 14 to 40. A cycle runs from the first day of one period to the first day of the next, not just the days of bleeding; use the average of your last three.
- Read the fertile window start, the ovulation date and the fertile window end.
- Below the result, read the next six cycles: each period start, ovulation date and window, counted with the same cycle length.
Frequently asked questions
- How do I calculate my ovulation date?
- Add your cycle length minus 14 days to the first day of your last period. For a period that began January 1, 2026, a 28-day cycle adds 14 days and gives January 15, a 25-day cycle adds 11 days and gives January 12, and a 30-day cycle adds 16 days and gives January 17.
- When do I ovulate with a 35-day cycle?
- About 21 days after your last period began: January 22, 2026 for a period that began January 1, with the fertile window starting January 17. Counting a 28-day cycle instead would put ovulation a week too early.
- Is ovulation really 14 days before the next period?
- That is the rule this tool applies, and it is an average rather than a fact about your body. Cleveland Clinic gives ovulation as around day 14 of a 28-day cycle on average, with timing that varies from cycle to cycle. If your cycles change length from month to month, the real date moves with them. Cleveland Clinic also describes ovulation kits, which detect LH in urine, and tracking the small rise in body temperature around ovulation as other ways to spot it.
- How long is the fertile window?
- This tool shows six days: the five days before ovulation and the ovulation date. Cleveland Clinic puts the window from about 5 days before ovulation through 1 day after, so the day after the window end shown here can still count. It gives the egg 12 to 24 hours after ovulation and sperm up to 3 to 5 days.
- Can I use it to avoid pregnancy?
- No. It is a calendar estimate, not a test, and it does not prevent pregnancy. Ovulation timing varies from cycle to cycle, and the tool’s window ends a day earlier than the one Cleveland Clinic gives.
- What cycle lengths does it accept?
- Whole numbers from 14 to 40. An empty field, a decimal such as 28.5 or a number outside the range shows a message under the field and no dates. From 14 to 20 days the dates come with a warning that they are unreliable: Cleveland Clinic gives 21 to 35 days as the range that can still be normal, and a fixed 14-day rule puts ovulation within a week of the period starting in a cycle that short. The Period Calculator on this site takes 21 to 45 days.
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