How it is calculated
Ovulation usually happens about 14 days before the next period — the relatively fixed "luteal phase":
Ovulation = LMP + cycle length − 14The fertile window is about six days: the five days before ovulation plus ovulation day, because sperm can survive up to five days while the egg lives about one day.
If you know your exact luteal phase length (for example from ovulation tests), you can adjust it.
Worked example
Last period started 1 October with a 30-day cycle: ovulation = 1 Oct + 30 − 14 = 17 October, fertile days 12–18 October, next period expected 31 October.
Frequently asked questions
Can I use this as contraception?
No. Calendar methods are not reliable for avoiding pregnancy because ovulation can shift from month to month.
What is the best day to conceive?
Chances are highest in the two days before ovulation and on ovulation day itself.
How can I confirm ovulation?
With home LH ovulation tests, basal body temperature tracking, cervical mucus changes, or ultrasound monitoring.
My cycles are irregular — does this help?
It will be less accurate. Enter the average of your last few cycles and rely more on ovulation tests.