Period Calculator

Enter your last period start date, period duration, and average cycle length to predict your next period date, ovulation window, and fertile days for the next 6 months. The Period Calculator maps out your upcoming cycle dates so you can plan ahead with confidence.

Disclaimer: This tool is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making any health-related decisions.

Enter the date your most recent period started.

days

How many days does your period typically last?

days

Count from the first day of one period to the first day of the next. Average is 28 days.

Results

Next Period Start Date

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Next Period End Date

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Estimated Ovulation Date

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Fertile Window

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Days Until Next Period

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Results Table

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Frequently Asked Questions

How do I use a period calculator?

Enter the first day of your last period, how many days your period typically lasts, and your average cycle length. The calculator then predicts your next period date, ovulation day, and fertile window for the next 6 months. For best results, use an average cycle length based on your last 3–6 cycles.

How do I calculate my next period date?

Your next period is estimated by adding your average cycle length to the first day of your last period. For example, if your last period started on January 1st and your cycle is 28 days, your next period is predicted to start around January 29th.

What is the average cycle length?

The average menstrual cycle is 28 days, but a healthy cycle can range from 21 to 35 days. Cycle length is counted from the first day of one period to the first day of the next. It's normal for your cycle to vary slightly from month to month.

How do I know if I have irregular periods?

Periods are generally considered irregular if the difference between your shortest and longest cycles over several months is more than 8 days. Factors like stress, illness, significant weight changes, and hormonal conditions can all contribute to irregular cycles. If you're concerned, speak with a healthcare provider.

Why might my period be late?

A late period can be caused by many factors including stress, illness, changes in diet or exercise, travel, hormonal imbalances, or pregnancy. A single late period is usually not a cause for concern, but if your period is consistently irregular or more than a few weeks late, it's worth consulting a doctor.

What are some possible menstruation symptoms?

Common symptoms in the days before and during a period include cramps (dysmenorrhea), bloating, breast tenderness, fatigue, headaches, and mood changes. Symptoms vary widely between individuals — some experience mild discomfort while others have more significant symptoms.

How do I understand period calculator results?

The calculator shows your predicted period start and end dates, estimated ovulation date (typically 14 days before your next period), and your fertile window (the 5 days before ovulation plus ovulation day). These are statistical estimates based on average cycle patterns and may not match your exact cycle.

How do I calculate when my last menstrual period (LMP) was?

If you've lost track, count backwards from today by your average cycle length to estimate when your last period likely started. For example, if today is day 15 of your cycle and your cycle is 28 days, your last period started approximately 15 days ago. Tracking apps can help you log and recall past dates.