Period Calculator

Q: What do the colors on the calendar mean?

Red marks your predicted period days. Orange shows your fertile window. Dark green is your estimated ovulation day. Dark pink marks the pre-period days right before your next period starts. White cells are standard days with no special phase.

Q: Can I use this calculator to prevent pregnancy?

No. This tool gives estimates only. It is not a birth control method. Cycle length can shift from month to month, so predictions may not be accurate. Talk to your doctor about reliable ways to prevent pregnancy.

Q: Can I print or save my cycle calendar?

Yes. Click the Print Tracker button to print your calendar. You can also click Save as PDF to create a file you can keep on your phone or computer. This is handy to share with your doctor.

Period Calculator

Updated Jun 19, 2026 By Jehan Wadia

Introduction

This free period calculator helps you track your menstrual cycle and predict when your next period will start. Just enter the first day of your last period, how long your period lasts, and your average cycle length. The tool then builds a color-coded calendar that shows your upcoming periods, fertile window, and estimated ovulation day for up to 12 cycles ahead.

Knowing when your period is due can help you plan ahead and feel more in control. The fertile window and ovulation dates shown here can also help if you are trying to get pregnant. For more precise ovulation tracking, try our dedicated ovulation calculator. Keep in mind that this calculator gives estimates based on average cycle length. It works best for people with regular cycles. It is not a form of birth control and should not be used to prevent pregnancy. If your cycle is irregular or you have concerns about your menstrual health, talk to your doctor.

How to Use Our Period Calculator

Enter a few details about your menstrual cycle below. The calculator will show you a calendar with your predicted period dates, fertile window, and ovulation day for upcoming cycles.

First Day of Last Period: Pick the date your most recent period started. You can also click "Use Today's Date" if your period started today. If you need help figuring out how many days have passed since a specific date, our date duration calculator can help.

Period Duration: Use the plus and minus buttons to set how many days your period usually lasts. Most periods last between 3 and 7 days.

Average Cycle Length: Set the number of days from the first day of one period to the first day of the next. A typical cycle is 28 days, but anywhere from 21 to 44 days is normal.

Cycles to Project: Choose how many future cycles you want to see on the calendar. You can pick 3, 6, 9, or 12 cycles.

Luteal Phase Length (Advanced): Open "Advanced Options" to change this number. The luteal phase is the time between ovulation and your next period. Most people can leave this set to 14 days.

Click Calculate to see your results. The color-coded calendar and timeline chart will show your predicted period days, fertile window, ovulation day, and pre-period days.

What Is a Period Calculator?

A period calculator helps you predict when your next period will start, when you are most fertile, and when you will ovulate. It uses the date of your last period, how long your period lasts, and the length of your menstrual cycle to estimate these dates for future months.

How the Menstrual Cycle Works

A menstrual cycle is the monthly process your body goes through to prepare for pregnancy. It starts on the first day of your period and ends the day before your next period begins. Most cycles last between 21 and 35 days, with 28 days being the most common length.

Each cycle has a few key phases:

Period (Menstruation): The lining of your uterus sheds and leaves your body as blood. This usually lasts 3 to 7 days.
Fertile Window: The days when you are most likely to get pregnant. This window is about 6 days long and includes the 5 days before ovulation and the day of ovulation itself. Our ovulation calculator can help you pinpoint this window more precisely.
Ovulation: The day one of your ovaries releases an egg. This typically happens about 14 days before your next period starts. The egg can be fertilized for about 12 to 24 hours after it is released.
Luteal Phase: The time between ovulation and the start of your next period. For most people, this phase is 12 to 16 days long.

Why Track Your Period?

Tracking your period helps you know what to expect each month. It can help you plan ahead, notice if something is off with your cycle, and understand your body better. If you are trying to get pregnant, knowing your fertile window and ovulation day is very important. Once you do conceive, you can use our conception calculator to estimate when conception occurred, or our due date calculator to find out when your baby may arrive. You can also track healthy weight changes during pregnancy with a pregnancy weight gain calculator. If you are not trying to get pregnant, this information can still help you stay aware of your health.

What to Know About Irregular Cycles

Not everyone has a cycle that is the same length every month. Stress, diet, exercise, age, and health conditions can all cause your cycle to be shorter or longer than usual. Maintaining a balanced diet and proper hydration can support cycle regularity. Tools like our calorie calculator and water intake calculator can help you stay on top of your daily nutrition and hydration needs. If your cycle changes a lot from month to month, the predictions from any period calculator will be less accurate. Talk to a doctor if your periods are very irregular, very painful, or suddenly change.

Important Reminder

This calculator gives estimates based on averages. It is not a medical tool and should not be used as a form of birth control. Every person's body is different. For medical advice about your menstrual cycle, fertility, or reproductive health, always speak with a healthcare provider. If you are going through IVF treatments, our IVF due date calculator can provide more tailored pregnancy timeline estimates.

Frequently asked questions

How does the period calculator figure out my next period date?

The calculator adds your average cycle length to the first day of your last period. For example, if your last period started June 1 and your cycle is 28 days, your next period is estimated to start on June 29. It repeats this for each future cycle you choose to project.

What should I enter if I don't know my exact cycle length?

If you are not sure, start with 28 days. That is the most common cycle length. As you track your periods over a few months, you can update this number to match your real pattern.

What do the colors on the calendar mean?

Red marks your predicted period days. Orange shows your fertile window. Dark green is your estimated ovulation day. Dark pink marks the pre-period days right before your next period starts. White cells are standard days with no special phase.

Can I use this calculator to prevent pregnancy?

No. This tool gives estimates only. It is not a birth control method. Cycle length can shift from month to month, so predictions may not be accurate. Talk to your doctor about reliable ways to prevent pregnancy.

What is the luteal phase and should I change it?

The luteal phase is the time between ovulation and the start of your next period. It is usually about 14 days. Most people can leave this at the default setting. Only change it if your doctor has told you your luteal phase is shorter or longer than 14 days.

How accurate is the ovulation date shown here?

The estimated ovulation date is based on your average cycle length and luteal phase. It works best if your cycles are regular. For a more precise reading, you can use ovulation test strips or track basal body temperature along with this tool.

What are pre-period days?

Pre-period days are the 1 to 3 days right before your period is expected to start. You may notice symptoms like bloating, mood changes, or cramps during this time. The calendar marks them in dark pink so you can prepare.

Can I print or save my cycle calendar?

Yes. Click the Print Tracker button to print your calendar. You can also click Save as PDF to create a file you can keep on your phone or computer. This is handy to share with your doctor.

Why does my predicted period date not match the real one?

Your cycle length can change due to stress, sleep, diet, exercise, illness, or hormonal shifts. This calculator assumes every cycle is the same length. If your cycle varies, the predictions will be less exact.

What does the timeline chart show?

The timeline chart is a visual bar chart that shows your projected period days, fertile window, and ovulation day for each cycle on a horizontal timeline. It gives you a quick overview of all your upcoming cycles at once.

Is 21 to 44 days a normal cycle length?

Yes. While 28 days is the most common, doctors consider cycles between 21 and 35 days typical for most adults. The calculator allows up to 44 days to cover people with longer but still regular cycles.

Can I project more than 12 cycles?

No. The calculator lets you project up to 12 cycles, which is roughly one year. Predictions become less reliable the further out they go because your cycle length may change over time.

Does this tool store my personal data?

No. All calculations happen right in your browser. No data is sent to a server or saved online. If you close or refresh the page, your inputs will reset.

What does the bold outline on a calendar day mean?

A bold purple outline marks today's date on the calendar. It helps you see where you currently are in your cycle at a glance.

How do I count my cycle length?

Count from the first day of one period to the day before your next period starts. For example, if one period starts June 1 and the next starts June 30, your cycle length is 29 days.

Your Cycle Forecast