Ovulation Calculator
Estimate your ovulation date and fertile window from your last period and cycle length
Estimated ovulation
Thu, September 17, 2026
Fertile window
The ~6 days ending on ovulation day
Next period expected
Thu, October 1, 2026
Next three cycles (estimates)
| Cycle | Fertile window | Ovulation | Period expected |
|---|---|---|---|
| 1 | Sep 12 – Sep 17 | Sep 17 | Oct 1 |
| 2 | Oct 10 – Oct 15 | Oct 15 | Oct 29 |
| 3 | Nov 7 – Nov 12 | Nov 12 | Nov 26 |
Calendar estimates only — actual ovulation timing varies between people and between cycles, especially with irregular periods. Ovulation predictor kits, basal body temperature, or cervical mucus tracking give more reliable signals. This tool is not medical advice or a form of contraception.
How it works
The calculator projects your next period from your last period start date plus your average cycle length, then counts back 14 days for estimated ovulation — the luteal phase is relatively constant at about 14 days. The fertile window is the five days before ovulation plus ovulation day itself.
Calendar method
ovulation ≈ next period start − 14 days fertile window = ovulation − 5 days … ovulation
- next period
- last period start + cycle length
- 14 days
- typical luteal phase length
Worked example
- Last period started July 1
- Cycle length 30 days
- Next period ≈ July 31
- Ovulation ≈ July 31 − 14 = July 17
Fertile window ≈ July 12-17.
Good to know
- These are calendar estimates — cycles vary, and this tool is not medical advice or contraception.
- Ovulation predictor kits detect the LH surge about 24-36 hours before ovulation and are more direct than any calendar.
- Trying to conceive: the two to three days before ovulation are the highest-probability days.
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Frequently Asked Questions
How is the ovulation date estimated?
By the calendar method: ovulation typically happens about 14 days BEFORE your next period starts, because the luteal phase (ovulation to period) is relatively constant while the first half of the cycle varies. With a 28-day cycle that is around day 14; with a 32-day cycle, around day 18.
What is the fertile window?
The roughly six days ending on ovulation day. Sperm can survive up to about five days in fertile cervical mucus, while the egg lives only 12-24 hours after release — so the days just before ovulation matter most.
How accurate is an ovulation calculator?
It is an estimate. Ovulation timing varies between people and between cycles, and irregular cycles make calendar estimates unreliable. Ovulation predictor kits (LH tests), basal body temperature, and cervical mucus changes are more direct signals.
Can I use this to avoid pregnancy?
No — calendar estimates are not a reliable form of contraception. Cycle-based methods have substantially higher failure rates than other contraception; talk to a healthcare provider about options.
What if my cycles are irregular?
If your cycle length varies by more than a few days, a single estimate is not meaningful. Track several cycles to find your range, and lean on physical signs (LH kits, temperature, mucus). Persistent irregularity is worth discussing with a doctor.