Online Period Calculator
Period Calculator predicts your upcoming periods and fertile days from your last period, period length, and cycle length — shown on a calendar and a six-cycle table.
Result
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| Important dates for the next 6 cycles | ||
|---|---|---|
| Period | Ovulation Window | |
| -- | ||
What Is This Tool?
The Period Calculator predicts your upcoming menstrual periods and fertile days. You enter the first day of your last period, your typical period length, and your average cycle length, and it projects your cycle forward. The results appear on a three-month calendar that marks period days in one color and ovulation days in another, with controls to scroll through other months, plus a table listing the period and ovulation windows for the next six cycles. The result can be downloaded as a PDF.
How to Use This Tool?
- Enter the first day of your last period.
- Choose your period length and average cycle length.
- Click Calculate to see the calendar and upcoming dates.
- Use Prev/Next to view other months, and download a PDF if you'd like.
Key Features
- Predicts your upcoming period dates from your last period and cycle length.
- Highlights period days and ovulation days on a three-month calendar.
- Lets you scroll forward and back through the months.
- Lists period and ovulation windows for the next six cycles.
- Accounts for your period length and offers a PDF download.
Examples
- A period starting Oct 4 with a 28-day cycle marks Oct 4–8 as period days.
- The fertile window for that cycle falls on Oct 16–20.
- The next period is predicted to begin around Nov 1.
- The six-cycle table runs from October through the following March.
Common Use Cases
- Predicting when your next period will arrive.
- Planning around your cycle for events or travel.
- Identifying fertile days for or against conception.
- Tracking cycle regularity over several months.
- Keeping a simple visual record of your cycle.
Tips & Best Practices
- Use the first day of bleeding as your last-period date.
- Enter your typical period length and average cycle length.
- Track a few cycles to find a reliable average.
- Re-calculate if your cycle length changes.
- Don't rely on it as a contraceptive method.
Limitations
- Predictions assume a regular cycle; irregular cycles reduce their accuracy.
- It must not be used as a form of birth control.
- It isn't medical advice and can't confirm ovulation.
- Nothing is saved between sessions — only the current result can be exported as a PDF.
Frequently Asked Questions
- How does it predict my period?
- It projects your cycle length forward from your last period to estimate upcoming period and fertile days.
- Can I use it to avoid pregnancy?
- No — it's an estimate for tracking and should never be relied on as contraception.
- What do the colors mean?
- One color marks predicted period days and the other marks the fertile or ovulation window.
- What if my cycle is irregular?
- Predictions become less accurate; track several cycles or consult a healthcare provider.
Key Terminology
- Menstrual cycle
- The time from the first day of one period to the first day of the next.
- Period length
- The number of days bleeding typically lasts.
- Cycle length
- The number of days from the start of one period to the next.
- Ovulation window
- The fertile days around ovulation when conception is most likely.
- Fertile window
- The span of days in a cycle when pregnancy is possible.