How the menstrual cycle works
A cycle is counted from the first day of one period to the first day of the next. The average is around 28 days, but anywhere from about 21 to 35 days is common and normal. This calculator takes your last start date and your usual cycle length and projects the dates forward.
What the fertile window means
Ovulation — when an egg is released — typically happens about 14 days before your next period starts. The "fertile window" is roughly the five days before ovulation plus the day itself, because sperm can survive for several days. The estimates here are based on averages and regular cycles.
Why these are estimates
Real cycles vary from month to month, affected by stress, travel, illness, sleep and many other things. Even people with regular cycles see some drift. Treat every date here as a best guess, not a certainty.
An important note
This tool is for general information and planning only. It is not a reliable form of contraception and is not medical advice. If your cycles are very irregular, unusually painful, or you have concerns about your health or fertility, please speak with a doctor or gynaecologist.
Frequently asked questions
Can I use this to avoid pregnancy?
No. Fertility-awareness methods need proper training and daily tracking to be used safely. This estimate should never be relied on as birth control.
My cycle is irregular — is this accurate?
Less so. The more your cycle varies, the rougher the estimate. A period-tracking app that learns your pattern, or a chat with your doctor, will serve you better.
Is this calculator free?
Yes — free, no signup, and your dates are never stored.