About this calculator
Ovulation — the release of an egg — happens once per cycle, and the days around it are the only days pregnancy can begin. The egg survives about a day, but sperm survive up to five days, so the fertile window is roughly six days: the five days before ovulation and ovulation day itself, with the two to three days right before ovulation the most fertile of all.
The calculator works backward from your next expected period, because ovulation is tied to the end of the cycle, not the start: it occurs about 14 days before the next period regardless of cycle length. That is why a 35-day cycle ovulates around day 21, not day 14 — the popular "day 14" rule only fits a textbook 28-day cycle.
How to use it
- Enter the first day of your most recent period.
- Enter your average cycle length (count from the first day of one period to the first day of the next, averaged over a few months if it varies).
- Aim for the fertile window shown, especially the two to three days ending on ovulation day.
The formula
The luteal phase — ovulation to the next period — is the most stable part of the cycle, about 14 days for most women. Subtracting it from the cycle length locates ovulation; sperm survival of up to five days opens the window before it, and the egg's short life closes it about a day after.
Worked example
Last period Oct 1 with a 28-day cycle: ovulation is estimated on Oct 15 (day 14), the fertile window runs Oct 10–16, and the next period is due Oct 29. With a 35-day cycle starting the same day, ovulation would fall on Oct 22 — a full week later — which is why cycle length changes everything.
Frequently asked questions
What are the chances of conceiving in the fertile window?
For couples in their late 20s and early 30s, studies estimate roughly a 20%–30% chance per cycle with well-timed intercourse, highest on the two days before ovulation. Chances decline with age — gradually through the early 30s, more steeply after 35. About 8 in 10 couples conceive within a year of trying.
My cycles are irregular. Can I still use this?
Only loosely. The estimate is only as good as the cycle-length guess, and with cycles varying by a week or more, the fertile window estimate can miss entirely. Ovulation predictor kits (urine LH tests) and basal body temperature charting detect your actual ovulation and are worth using if cycles are irregular or you have been trying for a while.
What signs indicate ovulation is happening?
Common ones: cervical mucus turning clear and stretchy (like raw egg white), a small rise in basal body temperature the day after ovulation, mild one-sided pelvic twinges (mittelschmerz), and a positive LH test about 24–36 hours before. No symptom is definitive on its own; the temperature shift confirms ovulation only in hindsight.
Can this calculator be used to avoid pregnancy?
Not reliably. Calendar estimates are the basis of the rhythm method, which has a typical-use failure rate around 24% per year because ovulation timing varies. If avoiding pregnancy matters, use a proven contraceptive method — this tool is designed for people trying to conceive.