Knowing roughly when your next period will start makes planning easier, whether that’s for travel, sports, or just not being caught off guard. This period calculator predicts your upcoming cycle from your last period date, your average cycle length, and your average period length, and keeps a running log so future predictions improve as your history builds up.

How to use this period calculator

1. Enter the First Day of Last Period using the date picker.
2. Set your Average Cycle Length (21 to 45 days) using the plus/minus buttons.
3. Set your Average Period Length (2 to 10 days) using the plus/minus buttons.
4. Click “Calculate My Cycle.”
5. Review your predicted next period date and cycle phase breakdown.
6. Log each actual period as it happens in the Period Tracker Log, saved on this device so future predictions improve as your history grows.

What this period calculator does

Rather than assuming a fixed textbook 28-day cycle for everyone, this calculator works from your own actual average cycle length and period length, since cycle length varies meaningfully from person to person and can range anywhere from 21 to 45 days and still be considered normal. Entering your specific numbers, rather than a generic default, is what makes the prediction relevant to your actual cycle rather than a population average that may not match you at all.

The core prediction formula

This is straightforward date arithmetic, but its accuracy depends entirely on how consistent your actual cycle length is. Someone with a highly regular 28-day cycle will get a tighter, more reliable prediction than someone whose cycle naturally varies by a week or more from month to month, which is exactly why building up your Period Tracker Log over several cycles improves the calculator’s usefulness over time, more logged data reveals your actual pattern rather than relying on a single self-reported average.

Why the tracker log matters over time

A single calculation based on one entered cycle length is a reasonable starting estimate, but it can’t account for natural month-to-month variation the way an ongoing log can. Logging each actual period as it happens builds a real history specific to you, letting predictions become more grounded in your actual pattern rather than a single guessed average, particularly useful if your cycle isn’t perfectly regular.

What this calculator does not do

This tool predicts period timing based on average cycle length, it does not diagnose irregular cycles, hormonal conditions, or fertility issues. If your logged cycles show significant unpredictability, or if you’re tracking your cycle for a specific health concern, that pattern is worth discussing with a doctor rather than relying on a prediction calculator to interpret it.

Frequently asked questions