Period calculator
Your next period is expected one cycle length after your last one started. Enter your last period and average cycle length to predict your next three periods on a calendar.
Predictions assume your recent cycles are typical for you. Stress, travel and illness can shift your period by several days.
🔒 Calculated locally — your dates never leave your device
The calendar shows your predicted period days, fertile window and estimated ovulation for the next two cycles, so you can plan ahead.
A variation of a few days is normal. If your period is more than 7 days late, take a pregnancy test first if you are sexually active; if cycles stay irregular for months, talk to a doctor.
Frequently asked questions
How late can a period be and still be normal?
Up to about 7 days of variation is common. If you are sexually active and more than a week late, take a pregnancy test; if delays repeat, see a doctor.
Why does my cycle length change?
Hormones respond to stress, sleep, weight changes, exercise and illness. Occasional shifts are normal; persistent irregularity deserves a check-up.
Can I rely on this calendar for contraception?
No. Cycle predictions are not a contraceptive method. Use reliable contraception if you want to avoid pregnancy.
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Related reading
References
- NHS – Periods and fertility in the menstrual cycle
- ACOG – Your First Period / Menstruation FAQs
This page will be reviewed by a named OB-GYN before launch. Educational content — not medical advice.
Medical disclaimer: This tool and content are for educational purposes only and are not a substitute for professional medical advice. If something feels wrong, see a doctor.