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Period Calculator

Predict your next period, track your menstrual cycle, and see your PMS window

🩺This tool uses generally available calculation formulas. Results are for informational purposes only — kindly consult a physician or cross-verify health information with a qualified healthcare provider.

About this tool

Predict your next period date and track your menstrual cycle. See a 3-cycle projection with PMS window and fertile window indicators.

Predictnext period date from last period start date
Seedays until your next period with live countdown
Viewa 3-cycle projection of upcoming periods
TrackPMS window with symptom timing alerts
Seefertile window and ovulation day for each cycle
Linkto the Ovulation Calculator for detailed fertility tracking
Customizecycle length and period length for accurate predictions

How to use it

Quick steps to get the most out of this utility.

  1. 1

    Enter last period date

    Select the first day of your last menstrual period.

  2. 2

    Set cycle and period length

    Adjust the sliders to match your average cycle length (default 28 days) and period length (default 5 days).

  3. 3

    View your prediction

    See your next period date, days until it starts, and the PMS window.

  4. 4

    Review 3-cycle projection

    See period predictions for the next 3 months to plan ahead.

Frequently asked questions

How does a period calculator work?+

A period calculator predicts when your next period will start based on the first day of your last period and your average cycle length. It assumes a consistent cycle and uses the formula: next period = last period start + cycle length.

What is a normal menstrual cycle length?+

A normal menstrual cycle typically ranges from 21 to 35 days, with 28 days being the most common average. Your personal cycle length may vary, so using your own average (calculated from the last 3–6 months) will give more accurate predictions.

What is the PMS window?+

The PMS (premenstrual syndrome) window is the 3–5 days before your period starts when hormonal changes can cause symptoms like mood swings, bloating, fatigue, food cravings, and irritability. This calculator highlights those dates for planning purposes.

How is the fertile window calculated?+

The fertile window is the 6 days leading up to and including ovulation. Ovulation typically occurs 14 days before your next period (assuming a 14-day luteal phase). Sperm can survive for up to 5 days, so the full fertile window spans 5 days before ovulation through ovulation day.

Can I use this as a method of contraception?+

No. This calculator provides estimates based on averages and should not be used as a form of birth control or family planning to avoid pregnancy. Many factors can affect actual cycle timing. Consult a healthcare provider for personalized guidance.

Why does my cycle length sometimes vary?+

Cycle lengths vary due to factors like stress, illness, travel, weight changes, hormonal fluctuations, medications, and sleep patterns. Using a rolling 3–6 month average rather than a single cycle gives a more reliable prediction.

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