Period Calculator
Predict your next several menstrual period start dates from your last period and average cycle length.
About the Period Calculator
This projects your next several period start dates by repeatedly adding your average cycle length to the first day of your last period — a simple forward projection rather than anything that accounts for the natural variation most cycles have from month to month.
The further out a prediction sits, the less reliable it is. Cycle length isn't perfectly constant for most people, so while the very next predicted date is usually reasonably close, predictions three, six, or twelve cycles out can drift noticeably from what actually happens.
It's a quick way to get a rough calendar of upcoming period dates for planning purposes — travel, events, or just knowing roughly when to expect the next one — without maintaining a dedicated cycle-tracking app.
Menstrual cycle data is sensitive by any measure, so this runs entirely client-side: nothing about your cycle history or predicted dates is sent off your device or stored anywhere.
Frequently asked questions
How does this calculator predict future period dates?
It adds your average cycle length to the start date of your last period repeatedly, once for each cycle you ask it to predict, producing a simple straight-line projection rather than one that adapts to natural month-to-month variation.
Why might my actual period differ from the predicted date?
Cycle length naturally varies for most people due to stress, illness, travel, hormonal changes, and other factors this calculator has no way to account for — it only projects forward using one fixed average length.
How many cycles ahead should I trust these predictions?
Treat the nearest one or two predictions as the most reliable, since small variations in actual cycle length compound with each cycle projected further into the future.
What if my cycle length varies a lot from month to month?
This calculator's single-average approach will be less useful for you — consider tracking a few recent cycles' actual lengths and using your typical range rather than relying heavily on far-out predictions.
Is this the same as an ovulation prediction?
No — this predicts period start dates specifically. For estimated ovulation and fertile-window dates, the ovulation calculator applies a different calculation based on a typical luteal phase length.
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