How it is calculated

Ovulation usually happens about 14 days before your next period, so the calculator subtracts 14 days from your cycle length (day 14 in a 28-day cycle, day 21 in a 35-day cycle). Sperm can survive up to 5 days and an egg about 24 hours, so your fertile window runs from 5 days before ovulation to 1 day after. If your cycles vary, use the average of your last 3–6 cycles. Ovulation tests or tracking your temperature give a more exact date.