Period Calculator

Never be caught off guard again. This period calculator predicts your next period date and tracks future cycles for up to 12 months based on your last period and average cycle length.

How to Use This Calculator

1. Enter the first day of your last period.
2. Enter your average cycle length (21–35 days; typically 28).
3. Enter your average period duration (typically 3–7 days).
4. Click Calculate to see your next period date, duration, and a 12-month calendar of predicted dates.

How Period Dates Are Predicted

Next period = Last period start date + Cycle length

This repeats for each subsequent cycle. The calculator shows start and end dates for each predicted period over 12 months.

What Is a Normal Cycle?

• Cycle length: 21–35 days (28 days is the textbook average, but significant variation is normal)
• Period duration: 2–7 days
• Flow: Ranges from light to heavy; typically heaviest in first 1–2 days
• Irregular cycles: Variations of >7–8 days month-to-month warrant a conversation with a gynecologist

Common Mistakes to Avoid

• Using predicted dates as absolute facts — Real periods are influenced by stress, illness, travel, weight changes, and hormonal shifts. Predictions are estimates, not certainties.
• Assuming irregular cycles are abnormal — Up to 20% of women have cycles outside the 25–31 day range. Many conditions (PCOS, thyroid issues, perimenopause) cause irregularity.
• Relying solely on a period app for contraception or conception planning — Calendar tracking has significant error margins. Use more reliable methods for critical decisions.

Frequently Asked Questions

Why is my period late?

Common causes: pregnancy, stress, significant weight changes, excessive exercise, illness, hormonal changes, PCOS, thyroid dysfunction, and perimenopause. A period more than 7 days late warrants a pregnancy test and/or doctor visit.

Can my cycle length change?

Yes — cycles naturally vary month to month. Significant changes (previously regular cycles becoming very irregular) can signal hormonal changes and should be evaluated by a healthcare provider.

What is PMS and when does it occur?

Premenstrual syndrome (PMS) symptoms typically begin in the luteal phase — 5–11 days before the period starts. Common symptoms include mood changes, bloating, breast tenderness, and fatigue.

How long after my period can I get pregnant?

Pregnancy is possible from just after your period ends (with short cycles and long-surviving sperm) until the fertile window closes (about 2–3 days after ovulation). See our Ovulation Calculator.

Conclusion

Tracking your menstrual cycle is a fundamental aspect of women's health awareness. Use this calculator to predict upcoming periods, plan events, and monitor cycle regularity over time.

Related: Ovulation Calculator | Pregnancy Calculator | Conception Calculator