# When is my next ovulation?

Estimates the ovulation day and the six-day fertile window from the first day of the last period and the cycle length: ovulation is about 14 days before the next period.

- Page: https://www.acalculator.org/health/ovulation-calculator
- JSON spec: https://www.acalculator.org/health/ovulation-calculator.json
- Version: aa5828dbad2e

## Default answer

Example with the default inputs (First day of your last period September 22, 2026, Cycle length (days) 28, Luteal phase (days) 14, Today’s date September 29, 2026) on the example date Tuesday, September 29, 2026: Your likely ovulation day is Tuesday, October 6, 2026, and your fertile window is October 1, 2026 to October 6, 2026.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| lmp | First day of your last period | The first day of your most recent menstrual period. |
| cycle | Cycle length (days) | Days from the first day of one period to the first day of the next. 28 if you are not sure. |
| luteal | Luteal phase (days) | Days from ovulation to the next period. 14 unless you have tracked your own. |
| on | Today’s date | The date to count from: the dates are for the first cycle whose ovulation day is on or after it. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| ovulation | Likely ovulation day | The estimated ovulation day: the next period minus the luteal phase. |
| cycleStart | Cycle starts | The first day of the cycle the dates are for: the last period, or a later predicted period when the last one was more than a cycle ago. |
| fertileStart | Fertile window from | The first day of the fertile window: 5 days before ovulation. |
| fertileEnd | Fertile window to | The last day of the fertile window: the ovulation day. |
| cycleDay | Ovulation on cycle day | The ovulation day as a day of the cycle, counting the cycle start as day 1. |
| nextPeriod | Next period starts | The first day of the period after this cycle: the cycle start plus the cycle length. |
| due | Due date if you conceive this cycle | The estimated due date for a pregnancy from this ovulation: ovulation + 266 days. |

## Method

Ovulation = cycle start + cycle length − luteal phase (14 days), where the cycle start is the last period moved on by whole cycles until ovulation is on or after today; the fertile window runs from 5 days before ovulation to the ovulation day.

## Assumptions

- Ovulation happens a luteal phase (14 days unless you change it) before the next period starts.
- The fertile window is the 6 days ending on ovulation day: sperm can live up to about 5 days, the egg about a day.
- Cycles are taken as regular. Real ovulation can move by several days from cycle to cycle.
- If this cycle’s ovulation day has passed, the dates are for the next cycle with the same length.
- This is an estimate, not medical advice, and not a way to prevent pregnancy.

## Worked examples

1. lmp = 2026-09-01, cycle = 28, luteal = 14 gives ovulation = 2026-09-15, fertileStart = 2026-09-10, fertileEnd = 2026-09-15, cycleDay = 15, nextPeriod = 2026-09-29, due = 2027-06-08. Source: Wilcox et al. 1995 (fertile window); ovulation 14 days before the next period; hand calculation in content.mdx.
2. lmp = 2026-02-10, cycle = 32, luteal = 14 gives ovulation = 2026-02-28, fertileStart = 2026-02-23, cycleDay = 19, nextPeriod = 2026-03-14. Source: hand calculation in content.mdx.
3. lmp = 2028-02-20, cycle = 26, luteal = 12 gives ovulation = 2028-03-05, fertileStart = 2028-02-29, nextPeriod = 2028-03-17, due = 2028-11-26. Source: Bull et al. 2019 (luteal phases vary, mean 12.4 days); hand calculation in content.mdx.
4. lmp = 2026-06-01, cycle = 28, luteal = 14, on = 2026-09-28 gives cycleStart = 2026-09-21, ovulation = 2026-10-05, fertileStart = 2026-09-30, cycleDay = 15, nextPeriod = 2026-10-19, due = 2027-06-28. Source: hand calculation in content.mdx: first ovulation June 15; 105 days to September 28; ⌈105 ÷ 28⌉ = 4 cycles.
5. lmp = 2026-09-10, cycle = 30, luteal = 14, on = 2026-09-28 gives cycleStart = 2026-10-10, ovulation = 2026-10-26, nextPeriod = 2026-11-09. Source: hand calculation in content.mdx: ovulation September 26 is 2 days before today; ⌈2 ÷ 30⌉ = 1 cycle.

## FAQ

### When do I ovulate?

Ovulation usually happens about 14 days before your next period starts. In a 28-day cycle that is day 15, counting the first day of your period as day 1. In a 32-day cycle it is about day 19. The calculator counts back from your next period.

### What is the fertile window?

It is the 6 days ending on the day of ovulation. In a study of 221 women trying to conceive, pregnancies came only from sex in the 5 days before ovulation or on the day of ovulation (Wilcox and others, 1995). Sperm can live in the body for up to about 5 days, and an egg lives for less than a day.

### When is the chance of pregnancy highest?

The chance rises through the fertile window. In the 1995 study it went from about 10% for sex 5 days before ovulation to about 33% for sex on the day of ovulation. The calculator shows the whole window, because your exact ovulation day is uncertain.

### How accurate is an ovulation calculator?

It is a guide. Ovulation moves from cycle to cycle, even in regular cycles. A study of more than 600,000 cycles found an average luteal phase (ovulation to next period) of 12.4 days, not 14. If you have tracked your own, enter it under More options. Ovulation tests or basal body temperature can pin down your own day.

### Can I use this to avoid pregnancy?

No. Calendar estimates are not reliable enough to prevent pregnancy, because the fertile window moves from cycle to cycle. Talk to a health care provider about birth control.

### What if my cycles are irregular?

Use your average cycle length, but the dates will be less certain. Very irregular cycles are worth discussing with a health care provider.

## Sources

- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation: effects on the probability of conception, survival of the pregnancy, and sex of the baby. N Engl J Med. 1995;333(23):1517-1521. https://doi.org/10.1056/NEJM199512073332301
- Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ. 2000;321(7271):1259-1262. https://doi.org/10.1136/bmj.321.7271.1259
- Bull JR, Rowland SP, Scherwitzl EB, Scherwitzl R, Danielsson KG, Harper J. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med. 2019;2:83. https://pmc.ncbi.nlm.nih.gov/articles/PMC6710244/
- MedlinePlus Medical Encyclopedia. Pregnancy - identifying fertile days (life span of sperm and egg). https://medlineplus.gov/ency/article/007015.htm
- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150-e154. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date