Ovulation Calculator — Fertile Window for the Next 6 Cycles
Estimate your ovulation day and six-day fertile window from your last period and cycle length, with a wider range for irregular cycles and a calendar for your next six cycles.
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Your Next 6 Cycles
Estimated fertile window, ovulation day and next period for each cycle.
| Cycle Starts | Fertile Window | Ovulation | Next Period |
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How this calculator works
- Ovulation ≈ next period − luteal phase. The luteal phase (after ovulation) is usually about 14 days, so ovulation ≈ cycle day (cycle length − 14). If you know your luteal phase from tracking, enter it.
- Fertile window = the 5 days before ovulation plus ovulation day. Sperm can survive up to about 5 days, while the egg survives about a day. In Wilcox et al.'s study (NEJM, 1995), pregnancies only occurred from intercourse within this 6-day window, with the highest chances on the 2 days before and the day of ovulation.
- Varying cycles: the classic calendar rule (Ogino-Knaus) marks days from (shortest cycle − 18) to (longest cycle − 11) as potentially fertile.
By Suhaib Hassan · Reviewed 28 September 2026 against the Wilcox studies and ACOG guidance · How CalculatePilot verifies formulas →
A worked example
A last period starting 1 September with a 28-day cycle: next period ≈ 29 September, so ovulation ≈ 15 September (cycle day 15), and the fertile window runs 10–15 September. With cycles varying from 26 to 32 days, the wider calendar window is cycle days 8–21 (8–21 September).
How reliable is this estimate?
Calendar estimates work best for regular cycles. Even then, ovulation timing varies: a follow-up study (Wilcox et al., BMJ 2000) found only about 30% of women had their fertile window entirely within days 10–17 of the cycle. Ovulation predictor kits (which detect the LH surge), basal body temperature and cervical mucus tracking give more precise, cycle-by-cycle signals.
Not for contraception on its own. Calendar-based estimates are much less reliable than other birth control methods. If you want to use fertility awareness to avoid pregnancy, learn a full method with a healthcare professional.
Sources
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–1521.
- Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ 2000;321:1259–1262.
- Munro MG, Critchley HOD, Fraser IS; FIGO Menstrual Disorders Committee. The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions. Int J Gynaecol Obstet 2018;143:393–408.
- ACOG — Fertility Awareness-Based Methods of Family Planning
Frequently asked questions
How many days is the fertile window?
About six days: the five days before ovulation and the day of ovulation. The chance of conception is highest on the two days before ovulation and on ovulation day itself.
Does ovulation always happen on day 14?
Not necessarily. Ovulation is often described as “around day 14”, but it depends on your cycle length. This calculator counts back 14 days from your next period, which puts ovulation on cycle day 15 of a 28-day cycle and around day 19 of a 32-day cycle. Timing can also shift from cycle to cycle.
How can I confirm ovulation?
Ovulation predictor kits detect the hormone surge 1–2 days before ovulation, and a sustained rise in basal body temperature confirms it has happened. Tracking over several cycles also shows your personal pattern.
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