Ovulation Calculator
Estimated fertile window from cycle length.
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The calendar assumes ovulation about 14 days before the next period. Real cycles vary. This is an estimate, not birth control.
How to use this tool
- Enter your measurements. Use the units shown on the labels. Do not send medical data anywhere.
- Read the estimate. These formulas are public rules of thumb, not a diagnosis.
- Talk to a clinician for decisions. Do not change medicine, diet, or training from this page alone.
Do not rely on this to prevent pregnancy
Calendar methods fail often. Use contraception advice from a clinician. Irregular cycles make the window even less reliable.
Frequently asked
Can I log basal temperature here?
No. This is a cycle-length estimate only.
What if my cycle is 21 or 40 days?
You can type that length. Accuracy falls as cycles vary.
Is this a pregnancy test?
No.
Can I use this window as contraception?
No. Calendar fertile days are an estimate and not contraceptive advice.
Does it predict twins?
No.