Ovulation Calculator
Enter the first day of your last period and your average cycle length to predict ovulation dates and fertile windows for the next 3 cycles.
Result
How It Works
Ovulation occurs about 14 days before your next period, because the luteal phase — the time between ovulation and the next period — is fairly constant at around 14 days. So: next period = first day of last period + cycle length; ovulation day = next period − 14 days. Sperm can survive in the body for up to about 5 days, while an egg lives for about 1 day after release, so the fertile window runs from 5 days before ovulation to 1 day after, with ovulation day being the best day to conceive.
Cycle Length vs. Ovulation Day
| Cycle length (days) | Ovulation day (from first day of period) |
|---|---|
| 21 | Day 7 |
| 22 | Day 8 |
| 23 | Day 9 |
| 24 | Day 10 |
| 25 | Day 11 |
| 26 | Day 12 |
| 27 | Day 13 |
| 28 | Day 14 |
| 29 | Day 15 |
| 30 | Day 16 |
| 31 | Day 17 |
| 32 | Day 18 |
| 33 | Day 19 |
| 34 | Day 20 |
| 35 | Day 21 |
Things to Know
This method estimates based on an average, regular cycle. If your cycles are irregular, ovulation timing can vary widely and this method may be inaccurate — consider confirming with basal body temperature tracking or ovulation test kits.
Contraception warning: the calendar (rhythm) method has a high failure rate — over 20% per year with typical use. Do not rely on this tool's results for contraception.
Disclaimer
This tool is for reference only and does not constitute medical advice. Please consult a healthcare professional for questions about conception, contraception, or reproductive health.