--- title: "Calculating ovulation: how to find your fertile window" description: "When exactly is your ovulation? Calculate it from your cycle length, and learn why your body is often a better predictor than any formula." url: https://myperiod.app/en/blog/eisprong-berekenen lang: en date: 2026-06-17 publisher: PERIOD. B.V., Amsterdam --- [Back to blog](https://myperiod.app/blog) [Your cycle](https://myperiod.app/blog/categorie/cyclus) · 5 min read # Calculating ovulation: how to find your fertile window [Lees dit artikel in het Nederlands](https://myperiod.app/blog/eisprong-berekenen) PERIOD. team · June 17, 2026 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](https://myperiod.app/blog/basale-lichaamstemperatuur-cyclus). - **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](https://myperiod.app/blog/onregelmatige-menstruatie-oorzaken) and [recognizing ovulation symptoms](https://myperiod.app/blog/ovulatie-symptomen-herkennen). ## 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](https://myperiod.app/blog/stoppen-met-de-pil-cyclus). ## 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](https://myperiod.app/blog/cyclus-app-zonder-data-verkopen). [Download PERIOD. on the App Store](https://apps.apple.com/us/app/period-cycle-period-tracker/id6757947793) · [via Google Play](https://play.google.com/store/apps/details?id=app.myperiod.android). Want to understand your cycle better? ## Keep reading [Best app for period and ovulation tracking (2026)Tracking your period and ovulation in one app: what an app can and cannot know, which measurements really help, and how Flo, Clue, Natural Cycles and PERIOD. approach it.Read more](https://myperiod.app/blog/best-app-period-and-ovulation-tracking)[Trying to conceive: when do you switch from a cycle app to a pregnancy app?Short answer: preferably never. The moment you switch is exactly when your cycle data is worth the most. What you lose, what you need, and how it fits in one app.Read more](https://myperiod.app/en/blog/van-cyclus-app-naar-zwangerschapsapp-wanneer-overstappen)[Cycle app on the pill: is tracking still worth it when you use contraception?Short answer: yes, but for a different reason than you think. On the pill you do not predict ovulation, but you do track complaints, bleeding and what happens when you stop.Read more](https://myperiod.app/en/blog/cyclus-app-met-de-pil-heeft-het-zin)