How the math works
Across most regular menstrual cycles, the luteal phase (the time between ovulation and the next period) is roughly fixed at ~14 days. The follicular phase — the time between menses and ovulation — is what varies between cycles and people. So the most reliable way to estimate when ovulation will happen this cycle is to count back 14 days from your *next* expected period, not forward from your last one.
The fertile window
The combined viability of sperm (up to ~5 days in the reproductive tract) and egg (~24 hours after release) gives a six-day window where intercourse can lead to pregnancy. The two days before ovulation are typically the most fertile, with conception probability dropping rapidly after ovulation day plus one.
Not a contraceptive method
The calendar method (predicting ovulation from cycle history) and the rhythm method (avoiding intercourse during the predicted fertile window) have typical-use failure rates around 24% per year — much higher than hormonal contraception, IUDs, or condoms. If you’re trying to avoid pregnancy, this calculator is the wrong tool. Talk to a Canadian regulated healthcare provider about evidence-based options.