This is a general estimate based on average cycle timing — actual ovulation varies, especially with irregular cycles. Ovulation predictor kits or tracking basal body temperature give a more precise picture.
How this is calculated
Ovulation typically happens about 14 days before your next period starts, regardless of how long your total cycle is — it's the days before ovulation that vary, not the days after. This calculator works backward from your entered cycle length to estimate that 14-day mark, then adds a 5-day fertile window before it, since sperm can survive in the body for up to five days waiting for an egg to be released.
Why this is an estimate, not a guarantee
Cycle length can vary month to month even for people who consider their cycles "regular," and stress, illness, travel, and other factors can shift ovulation earlier or later than predicted. Tracking methods like ovulation predictor kits (which detect the LH hormone surge) or basal body temperature charting can narrow the window considerably if you're trying to conceive with more precision.