Give it the first day of your last period and your average cycle length, and it works out your estimated ovulation day, your seven-day fertile window, and your next period dates for three cycles ahead. It also shows which phase of the cycle today falls in, and the due date if you conceive in that cycle.
Ovulation is calculated as a fixed number of days *before* your next period, not at the midpoint of the cycle. What stretches on a longer cycle is the phase before ovulation — so on a 35-day cycle you ovulate around day 21, not day 17.
Sperm can survive up to about five days in the reproductive tract, while an egg lives roughly 24 hours after release. That asymmetry is why the window runs from five days before ovulation to the day after.
As a rough guide, yes, but the margin widens. The calculation assumes your average cycle repeats. With irregular cycles, ovulation predictor kits or basal body temperature tracking will pinpoint the actual day far better.
From the day your period is due. Implantation happens 6–12 days after ovulation and hCG only starts rising after that, so testing earlier often gives a negative even when you are pregnant.
No. Ovulation shifts month to month with stress, sleep, illness and medication, so calendar-only methods have a high failure rate.