Period Calculator

Tracking your menstrual cycle can help you plan ahead, understand your body's natural rhythm, and identify your most fertile days. This free period calculator predicts upcoming period dates, ovulation days, fertile windows, and PMS windows for the next six months, all displayed on an easy-to-read color-coded calendar. Everything runs locally in your browser, so your sensitive health data never leaves your device.

Step 1: Enter Your Cycle Information

Start by selecting the first day of your most recent period using the date picker. Then set your average cycle length, which is the number of days from the start of one period to the start of the next. The default is 28 days, but cycles anywhere from 21 to 40 days are considered normal. Finally, enter your average period duration in days, typically between 3 and 7 days. If you are unsure of your exact cycle length, start with 28 and refine it later using the cycle history feature.

Step 2: Review Your Predictions

After clicking Calculate, the tool shows your next predicted period date, ovulation day, fertile window dates, and a countdown of days until your next period. Below the summary, a detailed table lists predictions for the next six cycles, including period start and end dates, ovulation day, and the fertile window for each cycle. This makes it easy to plan months ahead.

Step 3: Explore the Calendar View

The color-coded calendar provides a visual overview of your cycle patterns across six months. Period days are shown in red, PMS days in pink, fertile window days in green, and the ovulation day is marked with a blue dot. Today's date is highlighted with a border so you can see where you are in your current cycle at a glance. This visual approach makes it much easier to understand your cycle compared to raw dates alone.

Step 4: Log Past Periods for Better Accuracy

For more accurate predictions, use the cycle history section to log past period start dates. The calculator computes your average cycle length from your logged data. Once you have two or more entries, you can apply the calculated average as your cycle length with one click. Your history is stored in your browser's localStorage, so it persists between visits but never leaves your device. You can clear your history at any time.

Understanding Your Cycle Phases

A typical menstrual cycle has several phases. Menstruation is the period itself. The follicular phase follows, during which the body prepares for ovulation. Ovulation occurs roughly 14 days before the next period, which is when an egg is released. The luteal phase follows ovulation and lasts until the next period begins. The fertile window spans the five days before ovulation plus the ovulation day, as sperm can survive up to five days. The PMS window covers the seven days before menstruation, when many people experience premenstrual symptoms.