How do I calculate my next period?

Add your average cycle length to the first day of your most recent period. If you enter several period start dates, the calculator estimates the average from those intervals.

What cycle length is considered typical?

For many adults, 21 to 35 days is a common reference range, but normal patterns vary. Teens, postpartum people and perimenopause can be more variable.

Can this tell me if I am pregnant?

No. A period calculator cannot diagnose pregnancy. If your period is late and pregnancy is possible, use a pregnancy test and follow medical guidance.

Period Calculator

Estimate your next period, cycle day, cycle regularity and upcoming period dates from one or more recent period start dates.

Last period start dates

Enter 1 date for a basic forecast, or 2-6 period start dates to estimate your average cycle length and regularity.

Average cycle length, days

Period length, days

How period predictions are calculated

Period calculator intent is usually practical: enter the first day of your last period and average cycle length, then see the next expected period date. Multiple past dates can make the estimate less generic.

next period = last period start + average cycle length

Cycle length is counted from the first day of one period to the first day of the next.

average cycle = \frac{\sum cycle intervals}{n}

When several start dates are provided, the calculator averages valid cycle intervals.

Predictions are estimates

Stress, illness, travel, hormonal contraception, breastfeeding, perimenopause, PCOS and pregnancy can all change timing. A late or unusual period may need a pregnancy test or clinical advice.

Normal ranges and when to ask for care

Pattern	Common adult reference
Cycle length	About 21-35 days
Period length	Up to about 7 days
Teen cycles	Can be more variable after first periods
Concerning changes	Heavy bleeding, bleeding between periods, cycles outside usual range or missed periods

Frequently Asked Questions

Sources and References

Calculations are based on the listed reference sources. Links open in a new tab.

Updated: June 2, 2026

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