Enter the first day of your last period, your average cycle length, and luteal phase length to see your fertile window and next few cycle predictions.
| Cycle | Period start | Fertile window | Ovulation |
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Ovulation typically occurs a fixed number of days — the luteal phase length, commonly 14 days — before your next period, not 14 days after your last one (that's only accurate for an exactly 28-day cycle). This calculator subtracts your luteal phase length from your cycle length to find ovulation day relative to your last period's start.
Sperm can survive up to about 5 days in the reproductive tract, while an egg is viable for roughly 24 hours after ovulation — so the fertile window spans about 5 days before ovulation through 1 day after, the period during which intercourse is most likely to result in conception.
Calendar methods are estimates based on average cycle behavior — actual ovulation timing varies cycle to cycle, especially with irregular cycles or luteal phases outside the typical 12–14 day range. Ovulation predictor kits (LH surge strips) or basal body temperature tracking give a more precise, cycle-specific read if you need higher confidence, particularly when trying to conceive.
The calculation works backwards from your next expected period, not forwards from your last one. The luteal phase — from ovulation to the next period — is relatively consistent at around 14 days for most people, while the follicular phase before ovulation varies considerably.
So on a 28-day cycle ovulation lands near day 14, but on a 35-day cycle it is nearer day 21, not day 17. Estimating from the midpoint of the cycle is the single most common error, and it grows worse the further your cycle is from 28 days.
Sperm survive up to about five days in the reproductive tract; an egg is viable for roughly 12-24 hours after release. The fertile window is therefore about six days — the five days before ovulation plus the day itself — and it is front-loaded. The days immediately before ovulation carry higher conception odds than the day after, which surprises people who focus on the ovulation date itself.
Cycle length varies with stress, illness, travel, sleep, weight change and age, and some cycles are anovulatory — no egg is released at all. Irregular cycles, PCOS, thyroid conditions and recently stopping hormonal contraception all reduce how much a date-based estimate is worth.
Treat this as a planning aid rather than contraception; calendar methods have high failure rates when used to avoid pregnancy. If you have been trying for 12 months without success — or 6 months if you are over 35 — that is the usual point to speak to a doctor. To estimate a conception date after the fact, see the Conception Calculator, and for a due date the Pregnancy Calculator.
Already expecting? See the Pregnancy Due Date Calculator. Working backwards from a known date instead? The Conception Calculator estimates when conception likely occurred. Want a calendar rather than a fertility window? The Period Calculator forecasts the next six cycles with start and end dates.