Ovulation Calculator
Find your fertile window and ovulation date based on your menstrual cycle.
This is an estimate based on average cycle patterns. Actual ovulation varies. Not a substitute for medical advice.
How ovulation is calculated
An ovulation calculator works on a simple principle: ovulation occurs roughly 14 days before your next period starts, regardless of how long your cycle is. The fertile window then spans the 5 days before ovulation plus the day of ovulation itself, because sperm can survive up to 5 days in the reproductive tract while the egg is viable for about 12–24 hours.
Ovulation day by cycle length
This reference table shows the approximate ovulation day and fertile window for common cycle lengths, counting from the first day of your last period (day 1). Use it as a quick guide alongside the calculator above.
| Cycle length | Approx. ovulation day | Fertile window (cycle days) |
|---|---|---|
| 24 days | Day 10 | Days 6–10 |
| 25 days | Day 11 | Days 7–11 |
| 26 days | Day 12 | Days 8–12 |
| 27 days | Day 13 | Days 9–13 |
| 28 days | Day 14 | Days 10–14 |
| 29 days | Day 15 | Days 11–15 |
| 30 days | Day 16 | Days 12–16 |
| 31 days | Day 17 | Days 13–17 |
| 32 days | Day 18 | Days 14–18 |
| 33 days | Day 19 | Days 15–19 |
| 34 days | Day 20 | Days 16–20 |
| 35 days | Day 21 | Days 17–21 |
The four cycle phases
Day 1–5 (average). Uterine lining sheds. Hormone levels are at their lowest.
Day 1–13. Estrogen rises, follicles develop in the ovary, uterine lining thickens.
Around day 14. LH surge triggers egg release. Peak fertility. Lasts 12–24 hours.
Day 15–28. Progesterone rises to prepare uterus for implantation. Drops if no pregnancy.
Frequently asked questions
When do I ovulate?
You ovulate about 14 days before your next period is due. For an average 28-day cycle that is around day 14 (counting the first day of your period as day 1); for a 30-day cycle it is around day 16. Because the luteal phase after ovulation stays close to 14 days, longer cycles simply push ovulation later.
How do I calculate ovulation?
Take your cycle length and subtract 14 to get your ovulation day, then count from the first day of your last period. For example, a 29-day cycle gives 29 − 14 = day 15. Enter your last period date and cycle length in the calculator above to get the exact dates and a calendar view.
What is the fertile window?
The fertile window is the 5–6 days when conception is most likely: the 5 days before ovulation plus the day of ovulation itself. Sperm can survive up to 5 days in the reproductive tract, so the days leading up to ovulation are fertile even though the egg is only released on ovulation day.
How accurate is this ovulation calculator?
This calculator gives an estimate based on average cycle patterns. It assumes a consistent cycle length and that ovulation occurs 14 days before the next period. In reality, ovulation can vary by several days due to stress, illness, travel, or hormonal changes. For precise tracking, use ovulation predictor kits (OPKs) or basal body temperature monitoring.
What is the fertile window?
The fertile window is the 5–6 days during which pregnancy is possible: the 5 days before ovulation and the day of ovulation itself. An egg survives 12–24 hours after ovulation, but sperm can live up to 5 days in the reproductive tract, so conception can result from intimacy in the days before ovulation.
My cycle is irregular. Can I still use this?
This calculator works best for regular cycles between 21–35 days. If your cycles vary significantly, use the average of your last 3–6 cycles as input. For highly irregular cycles (PCOS, perimenopause), consult a gynaecologist for personalised tracking methods.
What cycle length should I enter?
Count from the first day of one period to the first day of the next period. A typical range is 21–35 days, with 28 days being average. Track 2–3 cycles to get your personal average.
Can I use this to avoid pregnancy?
This calculator is intended as an educational tool, not as contraception. The fertile window prediction has significant uncertainty. For reliable birth control, consult a healthcare provider about evidence-based contraceptive methods.