Why is ovulation not always on day 14?
Because day 14 is only correct for a 28-day cycle. The reliable part of the cycle is the luteal phase, the stretch between ovulation and the next period, which averages about 14 days. The follicular phase before ovulation is what varies. So the calculation counts backwards from the expected next period: on a 32-day cycle ovulation is estimated around day 18, and on a 24-day cycle around day 10.
How accurate is a calendar estimate?
It is the least accurate of the common methods. It assumes your cycle length is stable and that your luteal phase is exactly 14 days, and neither is reliably true: luteal phases range from roughly 10 to 16 days, and stress, illness, and travel move ovulation around. If you need to know when you are actually ovulating, an LH urine test or basal body temperature tracking will tell you far more than any calculator can.
Can I use this to avoid pregnancy?
No. It is designed to identify the days you are most likely to conceive, and it should never be used as contraception. Calendar-based methods have a typical-use failure rate in the region of 12 to 24 percent per year, and this tool has no knowledge of your cycle history, only the two numbers you typed. Use a real contraceptive method.
Why does the fertile window start five days before ovulation?
Because sperm can remain viable in the reproductive tract for up to about five days, so intercourse several days before the egg is released can still result in conception. The egg itself only survives around 12 to 24 hours, which is why the window closes quickly after ovulation and why the days immediately before it are the most fertile.
Is my data stored anywhere?
No. The date and the cycle length stay in the page's memory and the arithmetic runs in JavaScript on your device. Nothing is transmitted, there is no account, and reloading the page clears everything. That matters here more than for most tools, because reproductive-health data is exactly the category you should not be handing to a website.