How it works
The second half of the cycle (after ovulation) is fairly constant at about 14 days. So the calculator counts back 14 days from your expected next period to estimate ovulation, and marks the 5 days before it plus ovulation day as your fertile window.
Improving accuracy
If your cycles vary, track them for a few months. Ovulation test strips (LH kits), changes in cervical mucus and basal body temperature can confirm ovulation more precisely.
When to get tested
If you have been trying to conceive for 12 months (or 6 months if you are over 35), or your cycles are very irregular, hormone tests such as FSH, LH, AMH, prolactin and thyroid can help your doctor find the cause.