Ovulation calculator
It is a common assumption that ovulation happens on day 14 of every cycle. It does not. What stays roughly constant is the luteal phase — the stretch from ovulation to the next period, usually 12 to 16 days. On a 35-day cycle that puts ovulation nearer day 21, not day 14. Counting backwards from the expected period is therefore far more reliable than counting forwards from the last one.
A calendar estimate only. Cycles vary, ovulation can shift by several days, and this is not a contraceptive method. Ovulation tests and basal temperature tracking are considerably more reliable.
Ovulation happens roughly one luteal phase before the next period — cycle length minus about 14 days. On a 28-day cycle that is day 14; on a 35-day cycle it is nearer day 21. The fertile window is the five days before ovulation plus the day itself.
How to find your fertile window
Sperm survive up to five days in the reproductive tract while an egg is viable for about twenty-four hours, which is why the fertile window opens well before ovulation and closes almost immediately after. That asymmetry means intercourse in the days leading up to ovulation matters more than on the day itself. Calendar prediction is the weakest of the available methods — an ovulation predictor kit detecting the luteinising hormone surge gives about a day and a half of warning, and basal body temperature confirms afterwards that ovulation happened at all.
Questions
About 14 days before your next period, not 14 days after the last one. On a 35-day cycle that is day 21.