How the period calculator works

This tool predicts your upcoming menstrual cycle from two simple pieces of information: the first day of your last period and your average cycle length. It adds your cycle length to your last start date to estimate when your next period will begin, then works back roughly 14 days to estimate ovulation and your fertile window. Tracking these dates helps with planning, spotting irregularities, and understanding your body.

Understanding your menstrual cycle

A menstrual cycle is counted from the first day of one period to the first day of the next. The average is 28 days, but anything from 21 to 35 days is considered normal for adults. Ovulation — when an egg is released — typically happens about 14 days before the next period starts. The “fertile window” is the roughly six days ending on ovulation day, when conception is most likely.

How accurate are the predictions?

Predictions are most reliable if your cycles are regular. Many factors — stress, illness, travel, exercise, and hormonal changes — can shift your cycle, so treat the dates as estimates. Tracking several cycles improves accuracy because you can use your true average cycle length rather than a default of 28 days.

Frequently asked questions

Can I use this for contraception?

No. Cycle-based predictions are not a reliable form of birth control because ovulation timing varies. Speak to a healthcare provider about effective contraception.

What if my cycle is irregular?

Predictions will be less precise. If your cycles are very irregular, very long, or very short, it is worth discussing with a doctor, as it can sometimes point to an underlying issue.

Disclaimer: This calculator provides estimates for informational purposes only and does not constitute medical advice or a method of contraception. Consult a qualified healthcare professional for personalised guidance.