Calculate when you're likely ovulating and which days you're most fertile, based on the first day of your last period and cycle length.
Ovulation normally happens about 14 days before the next period starts, no matter how long the cycle is:
Sperm can survive in the uterus for several days, while the egg released at ovulation can only be fertilized for about 12–24 hours. That's why the five days before ovulation, plus the ovulation day itself, are considered the fertile window where the chance of pregnancy is highest.
Ovulation always happens about 14 days before the next period, regardless of total cycle length. That's why the cycle length needs to be adjusted for each person for an accurate estimate.
The calculator gives a rougher estimate with an irregular cycle. Use your average cycle length from recent months, or combine it with an ovulation test for better precision.
The egg can normally be fertilized for 12–24 hours after ovulation. Sperm, on the other hand, can survive for up to 5 days, so intercourse in the days before ovulation can also lead to pregnancy.