Period Calculator
Predict your next period and track your cycle for the next 12 months
How Does the Period Calculator Work?
This calculator predicts your future period dates by adding your cycle length to the first day of your last period. For example, if your last period started on 1 April and your cycle is 28 days, your next period is predicted to start on 29 April. The calculator repeats this for 12 cycles, giving you a full year of predicted period dates.
It also calculates your fertile window and ovulation date for each cycle, making it useful for both family planning and understanding your monthly rhythm. All calculations are based on the average cycle pattern — if your cycles are irregular, actual dates may vary by several days.
What is a Normal Cycle Length?
A normal menstrual cycle ranges from 21 to 35 days, with 28 days being the most commonly cited average. However research shows that most women do not have a textbook 28-day cycle — cycle lengths vary significantly between women and even from month to month in the same woman. Cycles outside the 21 to 35 day range, or cycles that vary by more than 7 to 9 days from month to month, are worth discussing with a doctor.
Factors That Affect Your Cycle
Many factors can cause your period to arrive earlier or later than predicted. Significant stress, illness, major changes in diet or exercise, travel across time zones, and hormonal changes can all shift your cycle. Certain medications including hormonal contraceptives obviously alter cycle timing. Perimenopause, which can begin years before the last period, often causes increasing cycle irregularity. If your periods are consistently irregular, a period tracking app combined with a visit to your doctor gives you the most accurate picture.
Period Calculator — Frequently Asked Questions
What is the average menstrual cycle length?
The average menstrual cycle is 28 days, but a normal cycle can range from 21 to 35 days. Cycle length is measured from the first day of one period to the first day of the next. Cycles shorter than 21 days or longer than 35 days may warrant discussion with a healthcare provider.
How long does a period last?
A typical period lasts 3–7 days. Flow is usually heaviest in the first 1–2 days and lightens toward the end. Periods lasting fewer than 2 days or longer than 7 days can sometimes indicate a hormonal imbalance worth discussing with a doctor.
Why is my period irregular?
Irregular periods can be caused by stress, significant weight changes, excessive exercise, thyroid disorders, polycystic ovary syndrome (PCOS), perimenopause or coming off hormonal contraception. Occasional irregularity is common, but consistently irregular cycles should be discussed with a healthcare provider.