Formula
Next period ≈ LMP + cycle length. Ovulation ≈ next period − 14 days. Fertile window ≈ ovulation − 5 days through ovulation day.
Ovulation Calculator
Estimate ovulation day, fertile window, and next period from your last menstrual period and average cycle length.
Your inputs
About this method
Best for
People with fairly regular cycles who want a planning estimate of fertile days and next period.
Limitations
Not contraception. Stress, illness, PCOS, and shift work can move ovulation. Educational use only.
Frequently asked questions
How does the ovulation calculator work?
It places ovulation about 14 days before the next expected period (standard luteal-phase assumption). The fertile window is roughly the five days before ovulation plus ovulation day, when sperm can survive and the egg can be fertilised.
What is a fertile window?
The days in a cycle when pregnancy is most likely. Peak chance is typically the day of ovulation and the day before.
Can I use this for contraception?
No. Calendar and app estimates are not reliable birth control. Speak with a clinician about contraception and fertility planning.
What if my cycles are irregular?
Estimates become less reliable. Track several cycles or use ovulation tests / clinical guidance for better timing.