Period CalculatorThe menstrual cycle is a recurring hormonal process that prepares the body for pregnancy each month. It begins on the first day of your period and ends the day before your next period starts. While the average cycle is 28 days, normal cycles range from 21 to 35 days.
Next Period Calculator
Predict your next 3–6 menstrual cycles with our next period calculator, including accurate ovulation dates and fertile windows. This global period calculator works for users worldwide and offers a smart monthly period cycle calculator with an instant, color-coded calendar view — no sign-up required.
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How to Use This Period Calculator
1
Enter your last period start date
This is the first day of your most recent period — the day bleeding began, not spotting.
2
Set your average cycle length
Count from the first day of one period to the first day of the next. Most women fall in the 25–35 day range.
3
Enter your period duration
How many days does your period typically last? The average is 3–7 days.
4
Click Calculate
You'll see your next 3 cycles with period dates, fertile windows, and a color-coded calendar.
What Is a Menstrual Cycle?
Menstrual Phase (Days 1–5)
The uterine lining sheds. Estrogen and progesterone are at their lowest. Cramping, bloating, and fatigue are common.
Follicular Phase (Days 1–13)
FSH triggers follicle growth. Estrogen rises, rebuilding the uterine lining. Energy and mood tend to improve.
Ovulation (Day ~14)
An LH surge causes egg release. This is your most fertile time. Ovulation typically occurs 14 days before your next period.
Luteal Phase (Days 15–28)
Progesterone rises to prepare for implantation. If no pregnancy occurs, hormone levels fall and your period begins.
Factors That Affect Your Cycle
Stress
High cortisol levels can suppress GnRH production, disrupting ovulation timing and causing missed or delayed periods.
Diet and Exercise
Extreme weight loss, high-intensity training, or poor nutrition can cause hypothalamic amenorrhea — your brain stops signaling ovulation.
Age and Perimenopause
Cycle length naturally becomes less predictable in the years preceding menopause (typically 40s–50s), as ovarian reserve declines.
Hormonal Contraceptives
Coming off hormonal birth control can cause irregular cycles for several months as your body's natural hormonal rhythm reestablishes itself.
Warning: If your periods are consistently absent for 3+ months and you are not pregnant or menopausal, consult a healthcare provider.
Frequently Asked Questions
Why did my period come early or late?
Stress, illness, travel, changes in sleep, strenuous exercise, or hormonal fluctuations can all shift your cycle by a few days. Variations of 1–7 days are generally considered normal.
How do I know if my cycle is regular?
Track your period start dates for 3 months. If the variation between cycles is less than 7–9 days, your cycle is considered regular. Our Irregular Period Analyzer can help assess your data.
Can the period calculator tell me if I'm pregnant?
No. This calculator cannot determine pregnancy. If your period is more than a week late, take a home pregnancy test or consult your healthcare provider.
What if my cycle length changes every month?
Use your average cycle length over the past 3–6 months for the most accurate prediction. You can use our Cycle Length Tracker and Irregular Period Analyzer for a deeper analysis.