Period Calculator
Estimate your next menstrual period and several upcoming cycles using the first day of your last period, average cycle length and typical period duration. Menstrual cycles naturally vary, so these dates are planning estimates rather than predictions of exactly when bleeding will begin.
How to use the Period Calculator
- Enter the first day of your most recent menstrual period.
- Enter your usual cycle length, measured from the first day of one period to the first day of the next.
- Enter the number of days your period usually lasts.
- The calculator adds your average cycle length to the previous start date to estimate the next period.
- The same interval is repeated to show several future estimated periods.
- Irregular cycles, illness, stress, pregnancy, breastfeeding, perimenopause and other factors can shift actual cycle timing.
Examples
28-day cycle example
If a period begins on June 1 and the average cycle is 28 days, the next estimated period begins approximately 28 days later.
Longer cycle example
A person with an average 32-day cycle will receive later estimated dates than someone whose average cycle is 28 days.
Frequently asked questions
How is the next period date estimated?
The calculator adds the entered average cycle length to the first day of the previous period.
Is a period calculator exact?
No. Menstrual cycles can vary between months, so calendar calculations provide estimates rather than guaranteed dates.
What is menstrual cycle length?
Cycle length is counted from the first day of one menstrual period to the first day of the next period.
Why might my actual period arrive earlier or later?
Cycle timing can change because of natural variation, stress, illness, pregnancy, breastfeeding, medications, hormonal changes and other factors.
Can this calculator diagnose irregular periods?
No. It is a calendar estimation tool and does not diagnose menstrual or hormonal conditions.
Can I use the estimates for birth control?
The dates should not be relied upon alone to prevent pregnancy because ovulation and cycle timing can vary.
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