Period Calculator

A period calculator helps you predict when your next menstrual period will start based on the first day of your last period and your average cycle length. Knowing your cycle pattern helps you plan ahead, understand your fertile window, and spot irregularities that may be worth discussing with a healthcare provider. This tool projects your period start, ovulation estimate, and fertile window across the next six cycles.

Quick Answer

The average menstrual cycle is 28 days, meaning most people get their period roughly once a month. Ovulation typically occurs around day 14 of a 28-day cycle — about 14 days before the next period starts.

These results are estimates based on general formulas and are not a substitute for professional medical advice. Consult a healthcare provider before making health decisions.

How the Formula Works

Calculate when your next period starts from your last period date and cycle length.
Next Period Start = Last Period Date + Cycle Length
Estimate when your current period ends based on your period duration.
Period End Date = Last Period Date + Period Duration
Estimate ovulation by counting back 14 days from the expected next period.
Ovulation Date = Last Period Date + (Cycle Length - 14)
Define the fertile window around ovulation, accounting for sperm survival time.
Fertile Window = Ovulation Date - 5 days to Ovulation Date + 1 day

Methodology & Sources

Reviewed and updated April 5, 2026 · Prepared by GetHealthyCalculators Editorial Team

This calculator uses the calendar method, assuming a consistent luteal phase of 14 days. Cycle length variability, stress, illness, and hormonal fluctuations can shift actual dates by several days. The calendar method alone should not be relied upon for contraception or fertility treatment decisions.

References

Menstrual Cycle: What Is Normal? · Mayo Clinic
The Normal Menstrual Cycle and the Control of Ovulation · Endotext / NCBI Bookshelf
Variability in Menstrual Cycle Length and Early Pregnancy Outcomes · BJOG: An International Journal of Obstetrics and Gynaecology

Limitations

This calculator assumes a regular cycle length and a consistent 14-day luteal phase, which may not reflect your individual pattern.
Stress, illness, significant weight changes, and hormonal conditions can shift ovulation and period timing by several days.
People with irregular cycles (varying by more than 7 days) will find calendar-based predictions less reliable.
This tool is not a substitute for medical advice and should not be used as the sole method for contraception or fertility planning.

Frequently Asked Questions

How do I calculate when my next period is?

Add your average cycle length to the first day of your last period. For example, if your last period started on January 1 and your cycle is 28 days long, your next period is expected around January 29. This calculator does that math and projects it across six future cycles.

What counts as a normal menstrual cycle length?

A normal menstrual cycle ranges from 21 to 35 days, with 28 days being the commonly cited average. A period (menstrual flow) typically lasts 3 to 7 days. Cycles outside this range or that vary significantly from month to month may be worth discussing with a healthcare provider.

Why does my period come at different times each month?

Cycle length naturally varies a few days from month to month. Stress, changes in sleep, travel, illness, significant weight changes, and hormonal fluctuations can all shift your period timing. Tracking your cycles over several months gives you a more reliable average to use in predictions.

Can I use this calculator to track my fertile window?

Yes, this calculator estimates your fertile window as the five days before ovulation through one day after ovulation. However, ovulation timing can vary, so calendar-based estimates are best used alongside other tracking methods such as basal body temperature charting or ovulation predictor kits for greater accuracy.

What should I do if my cycles are very irregular?

If your cycles vary by more than 7 days, are consistently shorter than 21 days or longer than 35 days, or you are missing periods, it is worth speaking to a healthcare provider. Conditions such as PCOS, thyroid disorders, and other hormonal imbalances can cause irregular cycles and are treatable.

How is ovulation timing related to cycle length?

Ovulation is estimated by counting back 14 days from the expected start of your next period. This means that with a 21-day cycle, ovulation occurs around day 7, while with a 35-day cycle it occurs around day 21. The variation in cycle length comes primarily from the follicular phase before ovulation, not the luteal phase after.

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