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Period Calculator

Predict your next 3 period dates and fertile windows based on your cycle.

Cycle TrackingFertile Window

Last Period Start Date

Cycle Length (days)

Period Duration (days)

About this tool

Predict your next 3 period dates and fertile windows based on your cycle. MyCalcTools calculators are designed for quick everyday estimates with clear inputs, instant results and no account required. Results are provided for general information and planning, not as professional financial, medical, legal or trade advice.

How to use it

Enter the values requested in the calculator fields.
Choose the option that best matches your situation, unit or goal.
Press the calculate button and review the result summary.
Adjust your inputs to compare different scenarios.

Common use cases

Checking a quick estimate before making a decision.
Comparing two or more everyday scenarios side by side.
Planning budgets, meals, projects, dates or personal routines.
Double-checking manual calculations with a simple online reference.

About the Period Calculator

The period calculator predicts your next several menstrual cycles based on the date of your last period and your average cycle length. It helps with planning around menstruation, understanding your typical cycle pattern, and identifying when to expect your period in the coming months.

How to Use It

Enter the date your last period started.
Enter your average cycle length — the number of days from the first day of one period to the first day of the next. The average is 28 days, but a normal range is 21–35 days.
Enter how many days your period typically lasts (average is 3–7 days).
Review the predicted dates for upcoming periods.

Understanding Your Cycle

The menstrual cycle is counted from the first day of one period to the day before the next. A cycle shorter than 21 days or longer than 35 days, or significant variation from cycle to cycle, can sometimes indicate hormonal imbalances or conditions such as polycystic ovary syndrome (PCOS) or thyroid issues, and is worth discussing with a GP. Tracking your cycle over several months gives you the most accurate picture of your personal pattern — period tracking apps on your phone can complement this calculator for long-term logging.

No. This tool is for general planning and awareness only. Cycle-based contraception (fertility awareness methods) requires careful daily tracking of multiple indicators — including basal body temperature and cervical mucus — over many months, and should be taught by a trained practitioner. Relying on predicted dates alone for contraception is not recommended and carries a high risk of unintended pregnancy.