Calculating ovulation: how to find your fertile window
Ovulation is the moment a mature egg leaves the ovary. Around that moment you're fertile, typically about 6 days per cycle: the 5 days before ovulation (sperm survive several days) plus the day of ovulation itself.
The simple calculation
The luteal phase (ovulation to period) is fairly stable in most people: 12 to 14 days. The follicular phase (period to ovulation) varies a lot. So you count back:
Ovulation ≈ first day of next period minus 14 days.
- 28-day cycle: around day 14.
- 30-day cycle: around day 16.
- 24-day cycle: around day 10.
Your fertile window runs roughly from 5 days before to 1 day after that estimated ovulation day.
Why the formula alone isn't enough
It only works if your cycle is regular. Stress, illness, travel, short sleep or hormonal shifts can move ovulation by days. For a clear picture, combine the math with your own body's signals.
Signs around ovulation
- Cervical mucus turns clear, slippery and stretchy (like raw egg white), the strongest sign.
- Mild one-sided pelvic twinge (Mittelschmerz) for some.
- Slight rise in libido.
- Basal body temperature rise after ovulation (confirms, doesn't predict), see basal body temperature.
- Positive LH test 12-36 hours before ovulation.
Quick examples
Example 1. Average 29-day cycle, last period started June 1. Next period expected June 30. Estimated ovulation: around June 16. Fertile window: roughly June 11–17.
Example 2. 26-day cycle, period on June 3. Next on June 29. Estimated ovulation: around June 15.
This isn't medical advice. Trying to conceive for a while or have an irregular cycle? Talk to your doctor.
What if your cycle is irregular?
With variable cycles the formula gets unreliable. Cervical mucus, basal temperature and LH strips are much more useful. Read: irregular periods: causes and recognizing ovulation symptoms.
And the pill?
On the combined pill you don't truly ovulate, so the fertile window above doesn't apply. Just stopped? Ovulation can happen within weeks, sometimes before your first period. Read: stopping the pill.
Logging it in an app helps, as long as it stays yours
A cycle app calculates your fertile window automatically and keeps all your signals in one place. PERIOD. does it encrypted on EU servers, with no ad trackers. Read: a cycle app that doesn't sell data.
Want to understand your cycle better?
