Free Ovulation Calculator
Find your ovulation date and fertile window instantly. Enter your last period date and cycle length below.
Ovulation Calculator
* Results are estimates. Individual cycles vary. Not a substitute for medical advice.
For ongoing tracking, predictions, and reminders — use PeriodTracker.
Track My Cycle FreeUnderstanding Ovulation & Your Fertile Window
Ovulation is the release of a mature egg from the ovary, occurring once per menstrual cycle. The timing is driven by a surge in luteinizing hormone (LH), typically happening 24–36 hours before the egg is released.
Your fertile window spans approximately 6 days: the 5 days before ovulation and the day of ovulation itself. The highest probability of conception is in the 2–3 days immediately before and including ovulation day, because sperm can survive up to 5 days inside the fallopian tubes.
Key ovulation signs to watch for: egg-white cervical mucus (clearest, most slippery), mild one-sided pelvic pain (mittelschmerz), a slight drop then rise in basal body temperature, and a positive result on an ovulation predictor kit (OPK).
| Cycle Length | Estimated Ovulation Day | Fertile Window |
|---|---|---|
| 21 days | Day 7 | Days 2–8 |
| 24 days | Day 10 | Days 5–11 |
| 28 days | Day 14 | Days 9–15 |
| 30 days | Day 16 | Days 11–17 |
| 35 days | Day 21 | Days 16–22 |
Ovulation Calculator FAQs
How does the ovulation calculator work?
Our ovulation calculator uses your last period start date and average cycle length to estimate ovulation day (typically 14 days before your next period), your fertile window (5 days before ovulation to 1 day after), and your next expected period date.
What is the fertile window?
The fertile window is the 5–6 days per cycle when pregnancy is possible — the 5 days before ovulation and the day of ovulation itself. Sperm can survive 3–5 days inside the reproductive tract, so sex before ovulation can lead to conception.
When does ovulation occur in a typical cycle?
In a 28-day cycle, ovulation typically occurs around Day 14. However, ovulation always happens approximately 14 days before your next period — not necessarily on Day 14. If your cycle is 30 days, ovulation occurs around Day 16.
How accurate is this ovulation calculator?
This calculator provides an estimate based on average cycle patterns. For more precise ovulation detection, combine with basal body temperature (BBT) charting and ovulation predictor kits (OPKs) which detect the LH surge directly.
Can I use this calculator to avoid pregnancy?
The calendar-based fertile window estimate is not a reliable method of contraception. Cycle length varies month to month, making calendar-only methods unreliable. Use this calculator for family planning awareness, not as a sole contraceptive method.
What if my cycle length is irregular?
If your cycle varies by more than 7 days month to month, calendar-based ovulation prediction becomes less reliable. Consider using OPKs or basal body temperature tracking alongside this calculator, and speak with a healthcare provider about irregular cycles.