Period Calculator
Your next three periods from your last start date and cycle length, with ACOG's normal ranges checked against your numbers. Uses your logged cycles if you keep them in the cycle tracker.
Next period starts
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| Period | Starts | Ends about |
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Common questions
What is a normal cycle length?
ACOG: typically 21 to 35 days, with a period lasting up to 7 days. The average is 28 days.
When are irregular periods worth a doctor visit?
ACOG lists cycles shorter than 21 or longer than 35 days, cycle length varying by more than 7 to 9 days, bleeding longer than 7 days, bleeding between periods or after sex, no period for 3 to 6 months, and any bleeding after menopause.
How accurate is the prediction?
It's arithmetic: last start plus your cycle length. Regular cycles predict well; variable cycles won't. If you log periods in the perimenopause cycle tracker on this device, the calculator uses your logged average instead of 28.
How it's computed
Day 1 is the first day of bleeding (ACOG's definition). Next start = last start + cycle length, repeated three times; end = start + period length − 1. The cycle length is flagged if outside 21–35 days, period length if over 7 days, and a logged history if cycle lengths vary by more than 7 days (ACOG's 7–9 day irregularity range). The calculator shares its date engine with the ovulation calculator and the perimenopause cycle tracker. Dates stay on your device.
Sources
- ACOG. Fertility Awareness-Based Methods of Family Planning (FAQ024, reviewed Feb 2025). www.acog.org
- ACOG. Abnormal Uterine Bleeding (FAQ095, reviewed Aug 2025). www.acog.org