Your cycle
An estimate from calendar arithmetic. Not a contraceptive method, and not medical advice.
Counted backwards from the next period
Ovulation is roughly fourteen days before the next period starts, not fourteen days after the last one began. The luteal phase — the second half — is the stable part; the first half is what varies. On a 32-day cycle ovulation falls near day 18, not day 14.
The window opens before ovulation, not after
Sperm survive several days in the reproductive tract; an egg lives about a day. That makes the fertile window the five days before ovulation plus the day itself, which is why timing something for the day after is usually too late.
There is no safe period
Cycles shift with illness, stress, travel and sleep, and ovulation moves with them. Calendar methods have a high failure rate as contraception. Use this to understand your cycle, not to avoid pregnancy.
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Frequently asked questions
Q. How accurate is it if my cycles are irregular?
Much less. The calculation assumes the length you enter repeats. If your cycles vary by more than a few days, ovulation tests or basal temperature tracking tell you far more than a calendar can.
Q. Where does the cycle length come from?
Count from the first day of one period to the day before the next starts. Averaging three cycles gives a steadier number than the most recent one.
Q. Can I use this to avoid pregnancy?
No. Calendar-based prediction is unreliable for that. Speak to a healthcare provider about methods that work.