Menstrual Cycle Calculator

Predict your next period.

Enter the first day of your last period and your average cycle length to predict your next period, fertile window, and estimated ovulation date.

First Day of Last Period

Average Cycle Length (days)

Average Period Duration (days)

⚠️ For informational purposes only. Not a substitute for medical advice.

How Period Prediction Works

The menstrual cycle begins on the first day of your period and ends the day before your next period starts. By tracking the number of days between your periods, you can estimate a pattern. This calculator adds your average cycle length to the first day of your last period to predict your next one. Keep in mind that stress, travel, diet changes, and illness can all affect your cycle timing. For persistent irregularities, speak with a healthcare provider.

Frequently Asked Questions

How is the next period date calculated?

Your next period is predicted by adding your average cycle length to the first day of your last period. For example, if your last period started on June 1 and your cycle is 28 days, your next period is predicted around June 29.

What is a normal menstrual cycle length?

A normal menstrual cycle ranges from 21 to 35 days, with 28 days being the most common average. The length can vary from person to person and even cycle to cycle.

When is the fertile window?

The fertile window is typically the 5 days before ovulation and the day of ovulation itself — about 6 days total. Ovulation usually occurs around 14 days before your next period, not 14 days after your last period.

How accurate is the period prediction?

Period predictions are estimates based on past cycle averages. Stress, illness, diet changes, and hormonal fluctuations can all shift your actual period date. This calculator provides a rough prediction, not a medical diagnosis.

What is the luteal phase?

The luteal phase is the second half of your cycle — the time between ovulation and your next period. It typically lasts 12–14 days. A shorter luteal phase (under 10 days) can sometimes affect fertility.