How it works
Ovulation day = first day of last period + cycle length − luteal phase (14 days by default). The fertile window runs from 5 days before to 1 day after ovulation.
If your cycles vary, ovulation predictor kits, cervical mucus and basal body temperature tracking give a more precise picture.
Frequently asked questions
Can I use this as birth control?
No. Calendar estimates are not reliable enough to prevent pregnancy.
What if my cycle is irregular?
Use your average cycle length, and treat the results as a rough guide.
What is the luteal phase?
The time between ovulation and your next period, usually 12–16 days.