How is the result calculated?
Ovulation is calculated by counting backward from the next period date, because the "luteal phase" (from ovulation to the start of the period) is nearly fixed at ~14 days for most women, unlike the first phase of the cycle, which varies in length. So ovulation day = last period date + (cycle length − 14).
Why is the fertile window 6 days and not just one day?
Because sperm can survive inside the reproductive tract for up to 5 days before ovulation, while the egg remains viable for fertilization for about 24 hours after. So the fertile window includes the five days before ovulation plus one day after.
What is the "luteal phase" and why enter it if I know it?
It's the period from ovulation to the start of the next period. Although 14 days is the most common average, it varies from woman to woman (and stays roughly fixed for the same woman from cycle to cycle). If you know your actual length (from previous tracking or a medical test), entering it gives a more accurate result than the general assumption.
Is this calculator accurate if my cycle is irregular?
Accuracy decreases the more irregular the cycle is, since the calculation assumes a fixed cycle length. In this case, physical ovulation symptoms (like increased clear discharge, a slight rise in body temperature) or home ovulation test strips are more accurate than any purely calendar-based calculation.
Can this calculator be used to avoid pregnancy?
This isn't recommended. Although it identifies the highest-fertility days, the chance of pregnancy outside this window isn't zero (especially with irregular cycles or early/late ovulation), so it's a pregnancy planning tool, not a reliable contraception method.
