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Ovulation — when an ovary releases an egg — is the few-day hinge the whole menstrual cycle turns on. Timing intercourse to the days around it is the single biggest lever for conceiving, and knowing it also helps if you’re avoiding pregnancy or simply tracking your body.
Enter the first day of your last period and your typical cycle length. The tool estimates your ovulation day, the fertile window around it, and when your next period is due.
How is it calculated?
The luteal phase is the fixed part
A cycle has two phases. The first (follicular) phase — from your period to ovulation — varies in length from person to person and month to month. The second (luteal) phase, from ovulation to the next period, is remarkably stable at about 14 days. So ovulation is estimated by counting back, not forward:
| Step | Calculation |
|---|---|
| Ovulation day | LMP + (cycle length − 14) |
| Next period | LMP + cycle length |
For a 28-day cycle that lands on day 14; for a 32-day cycle, day 18; for a 24-day cycle, day 10.
The fertile window
You are not fertile for only one day. Sperm survive up to about five days in the reproductive tract, and the egg lives roughly 12–24 hours. That gives a fertile window of about six days:
| Window | Days relative to ovulation |
|---|---|
| Opens | ~5 days before |
| Peak | 1–2 days before and the day itself |
| Closes | ~1 day after |
Why it’s an estimate
This is calendar-based, so it assumes fairly regular cycles. Real ovulation shifts with stress, illness and natural variation. For a tighter signal, calendar timing is often combined with basal body temperature (which rises after ovulation) or ovulation predictor kits (which detect the LH surge 24–36 hours before).
Worked example
With a last period starting 1 March 2026 and a 28-day cycle, ovulation is estimated at day 14 — around 14 March. The fertile window opens about five days earlier (roughly 10 March) and closes a day after ovulation (about 15 March), and the next period is due around 29 March. If your cycles run 32 days, everything shifts later: ovulation lands near day 18 (around 18 March), because only the ~14-day luteal phase stays fixed.
FAQ
How is my ovulation day estimated?+
By counting back about 14 days from your next expected period, not forward from your last one. The luteal phase (ovulation to period) is fairly constant at ~14 days, so ovulation ≈ last period + (cycle length − 14). A 30-day cycle ovulates around day 16, not day 14.
What is the fertile window?+
The roughly six days when intercourse can lead to pregnancy: the five days before ovulation plus the day itself. It works because sperm can survive up to five days while waiting for the egg, which itself lives only about a day.
Can I use this as birth control?+
Not reliably on its own. Calendar estimates assume regular cycles and can’t detect a cycle where ovulation comes early or late. Fertility-awareness methods used for contraception combine several signals and training; a calendar alone has a high failure rate.
Why might my real ovulation differ from the estimate?+
Because ovulation timing varies with stress, illness, travel, weight change and simple natural variation. The calculator uses your average cycle; any month can differ. Basal body temperature or an ovulation predictor kit confirms the actual day.
Does a longer cycle mean later ovulation?+
Yes. Because the luteal phase stays near 14 days, extra cycle length is added to the first phase — so ovulation happens later. That’s why the tool asks for your cycle length rather than assuming 28 days.