Ovulation Calculator
Estimate your most fertile days and next ovulation date from the first day of your last period and your cycle length. For planning guidance only.
Assumes a 14-day luteal phase — actual timing varies.
The fertile window contains 6 days: the 5 days before ovulation plus ovulation day. The following day is shown separately as low probability.
This calendar method cannot be used for contraception.
Next 3 cycles
Relative chance: High = 1–2 days before ovulation; Medium = 3–4 days before or ovulation day; Low = 5 days before. The day after is outside the window.
Daily timing
Due date later? Due date calculator · cycle length: Date difference · sleep recovery: Sleep calculator.
Using your fertile window
The six-day fertile window includes the five days before ovulation and ovulation day, with the highest relative chance 1–2 days before ovulation. If cycles vary, enter the shortest and longest lengths to see a broad possible range rather than a single predicted day.
Estimate your fertile window from the first day of your last period (LMP) and average cycle length. With LMP = August 1 and a 28-day cycle, the calculator shows ovulation ≈ August 15, a fertile window of August 10–15, and the next period ≈ August 29 — all computed locally in your browser.
The method, stated openly: ovulation is estimated as cycle length minus 14 days (the luteal phase is the most stable part of the cycle at roughly 14 days). The fertile window is the 5 days before ovulation plus ovulation day itself — sperm survive up to ~5 days, the egg about 24 hours. A 35-day cycle therefore shifts ovulation to day 21 (August 22 for an August 1 LMP), not day 14.
Irregular cycles: enter your shortest and longest recent cycle lengths and the tool shows a possible ovulation range (each bound computed as that cycle length − 14) plus a broad fertile range, instead of pretending to know a single day. Contradictory inputs (shortest longer than longest) are flagged inline rather than silently swapped.
Boundaries that matter medically: calendar math cannot confirm ovulation — stress, travel, illness and conditions like PCOS shift it. Ovulation predictor kits (LH tests) and basal body temperature give direct signals; a clinician gives answers. And the boundary this page states in bold: the calendar method is not reliable contraception. This tool is an estimator, not medical advice.
Published test vectors — each validated against the live tool: LMP 2026-08-01, 28-day cycle → ovulation Aug 15, window Aug 10–15, next period Aug 29 · 35-day cycle → ovulation Aug 22, next period Sep 5 · irregular 25–32 days → ovulation range Aug 12–19 · shortest > longest is rejected with a field-level error. Last verified: 2026-08-12.
FAQ
How is the ovulation day actually estimated?
Cycle length minus 14: the luteal phase (ovulation to next period) is the most stable segment at ~14 days, while the pre-ovulation phase varies. So a 28-day cycle ovulates around day 14, a 35-day cycle around day 21 — not everyone ovulates mid-cycle.
Why is the fertile window six days long?
Sperm can survive in the reproductive tract for up to about five days, and the egg is viable for roughly 24 hours after ovulation. The highest-probability days are the two days immediately before ovulation.
My cycles are irregular — how should I use this?
Use the irregular mode: enter your shortest and longest recent cycles. You get a possible ovulation range (each bound = cycle − 14) and a broad fertile range. For direct confirmation, LH test strips or basal body temperature tracking beat any calendar.
Can I use this as contraception?
No. Calendar-based estimates shift with stress, illness, travel and normal variation; the page states this explicitly. If preventing pregnancy matters, use a method with clinical evidence — talk to a healthcare provider.
Is my cycle data stored or uploaded?
Dates are processed locally in your browser and never uploaded. Nothing about your cycle is sent to a server — you can verify in the network tab: the calculation triggers no requests.
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