Ovulation Calculator
Ovulation is counted backwards from the next period, not forwards from this one.
Reading your result
Fill in every field and the steps will be spelled out here.
How it is calculated
Ovulation is counted backwards from the next period. Cycle length varies between people and between months, but the luteal phase after ovulation is close to fixed at 14 days. On a 35-day cycle that puts ovulation near day 21, not day 17 — halving the cycle gives the wrong answer.
The fertile window is six days rather than one because sperm survive up to five days in the reproductive tract, so intercourse before ovulation can still result in pregnancy. The egg is the short-lived half, viable for 12 to 24 hours.
An estimate is all this can be. Stress, illness, travel and poor sleep all delay ovulation by days. Where precision matters, ovulation tests or basal body temperature tracking are the better instruments.
Frequently asked questions
What if my cycles are irregular?
Use an average of the last three to six months and read the result as a range. Where variation is large, an ovulation test is far more reliable.
Is pregnancy impossible outside the window?
Less likely, not impossible. Ovulation shifts, and the window shifts with it. This calculation is not a contraceptive method.
How does basal temperature help?
Progesterone raises resting temperature by a few tenths of a degree after ovulation — which confirms it happened rather than predicting it.
An estimate assuming reasonably regular cycles; actual ovulation commonly differs. Do not rely on it as the sole basis for contraception or conception planning.