Period Calculator – Estimate Your Next Period Date
Predict your next periods using cycle length.
About the Period Calculator
What is the Period Calculator?
A period calculator predicts your next menstrual periods using your average cycle length.
Why use this calculator?
Forecasting when your period will come helps with planning travel, supplies, symptoms and any health checks that depend on cycle timing.
How to use it
- Enter the first day of your last period.
- Adjust cycle and period length to your average.
- Read the next start date and the four-cycle forecast table.
Formula
Next period = last period + cycle length.
Cycles vary. Stress, illness and hormonal shifts all shift the date; treat forecasts as approximate.
Frequently Asked Questions
What cycle length is normal?
21 to 35 days is a typical range. Consistent irregularity beyond that is worth discussing with a doctor.
Why is my cycle different every month?
Hormonal balance shifts with sleep, stress, exercise and weight changes. Tracking several months reveals your real average.
Does it account for leap years?
Yes. The date logic follows the Gregorian calendar, including leap years and differing month lengths.
Are weekends and holidays excluded?
Only if the calculator says so. By default, results count every calendar day; use a business-day option where available.
Which time zone is used?
Calculations use whole calendar dates and are time-zone independent unless a time-of-day is involved.
Is this a medical diagnosis?
No. This tool provides general wellness information only. It is not a substitute for evaluation by a qualified healthcare professional.
Why do results differ from my doctor's measurements?
Clinical measurements use calibrated equipment and professional technique. Home estimates rely on formulas and self-reported values, so small differences are normal.
How often should I recalculate?
Recheck after meaningful changes such as weight gain or loss, a new workout routine, or every few months to track trends over time.
Do these formulas apply to everyone?
Most population formulas are less accurate for athletes, pregnant people, very young children and the elderly. Use them as a general guide.
Is this calculator free to use?
Yes. Every calculator on this site is completely free, with no hidden fees, no subscriptions and no limits on how many times you can use it.