Next period calculator
Published On: 05/03/2026
Next Period Calculator
Estimate your next period start date using the first day of your last period and your average cycle length. You can also estimate a simple fertile window and ovulation day (approximate).
Inputs explained:
• First day of last period: the date your last menstrual bleeding started.
• Average cycle length: number of days from one period start to the next (commonly 21–35).
• Average period length: how many days bleeding usually lasts (commonly 3–7). Used only to show an estimated end date.
These are estimates and may vary month to month. If you have severe pain, very irregular cycles, or think you may be pregnant, consult a clinician.
• First day of last period: the date your last menstrual bleeding started.
• Average cycle length: number of days from one period start to the next (commonly 21–35).
• Average period length: how many days bleeding usually lasts (commonly 3–7). Used only to show an estimated end date.
These are estimates and may vary month to month. If you have severe pain, very irregular cycles, or think you may be pregnant, consult a clinician.
Used for ovulation estimate: ovulation ≈ next period − luteal.
Tip: If you track basal body temperature (BBT) or LH tests, you’ll get a more accurate ovulation estimate than a calendar calculation.
Next Period Calculator — Description
The Next Period Calculator estimates your next menstrual period start date based on the first day of your last period and your average cycle length. It can also provide a basic estimate of your ovulation day and fertile window using a standard luteal phase assumption (commonly 14 days). Results are for informational purposes and may vary from cycle to cycle.
If your cycles are irregular, you’ve recently stopped/started hormonal birth control, or you’re tracking fertility, consider using additional methods such as LH tests or basal body temperature for more accurate ovulation timing.