Predict your next period and track your cycle — private, instant, free
This calculator predicts your next period based on a regular cycle length. The average cycle is 28 days, but normal cycles range from 21–35 days. Your fertile window is typically Days 10–17 of your cycle (around ovulation at Day 14 for a 28-day cycle).
This tool is for general tracking only. For medical advice, speak with your healthcare provider.
The Period Calculator predicts your next period start date, ovulation window, and fertile days based on the date of your last period and your average cycle length. Enter the first day of your most recent period and your typical cycle length (most commonly 28 days, though anything from 21 to 35 days is considered normal), and the tool instantly maps out your next several cycles. The results include the predicted start of each period, the estimated ovulation day, and a fertility window for each month ahead. All calculations happen entirely in your browser — nothing is stored or transmitted.
Understanding your menstrual cycle can help you plan around important events, manage symptoms more proactively, and gain confidence about what is happening in your body each month. The calculator uses the standard method of subtracting 14 days from your cycle length to estimate ovulation (since the luteal phase — the time from ovulation to your period — is relatively consistent at around 14 days for most people). Your fertile window is typically the five days before ovulation plus the day of ovulation itself, as sperm can survive in the reproductive tract for up to five days. The day after ovulation is generally the last fertile day of the cycle.
Cycle tracking is useful for more than pregnancy planning. Many people use period predictions to schedule holidays, sports events, or medical appointments around their cycle. Knowing when to expect heavier flow, mood changes, or energy dips lets you prepare rather than be caught off guard. The calculator can project up to twelve months ahead so you can plan well in advance. For people with irregular cycles, averaging a few recent cycle lengths before entering them will give the most accurate forecast.
For people with regular cycles, predictions are typically accurate within one to two days. Accuracy is lower for irregular cycles — if your cycle length varies significantly month to month, use an average of your last three to six cycles for a better estimate. This tool is for general tracking only and should not be used as a sole method of contraception.
A typical menstrual cycle lasts between 21 and 35 days, with 28 days being the average most commonly cited. The length of the cycle is counted from the first day of one period to the first day of the next. Cycles can vary slightly from month to month, especially during stress, illness, or significant lifestyle changes.
Ovulation usually occurs about 14 days before your next period. For a 28-day cycle that means around day 14; for a 35-day cycle it would be closer to day 21. The Period Calculator estimates this automatically based on your cycle length.
Your fertile window covers the five days before ovulation plus ovulation day itself — roughly six days per cycle. Conception is most likely in the two to three days immediately before ovulation. Sperm can survive for up to five days, while an egg is only viable for 12–24 hours after release.
Yes. All calculations run locally in your browser. No period dates, cycle information, or personal data is ever sent to a server or stored anywhere outside your own device.
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