Period Calculator

About Period Calculator

Understanding your menstrual cycle is one of the most practically useful things you can do for your reproductive health, whether you are trying to conceive, trying to avoid pregnancy, managing symptoms around specific life events, or simply building a clearer picture of your body's monthly rhythm. This free online period calculator and ovulation predictor takes two inputs — your last period start date and your average cycle length (defaulting to 28 days, adjustable from 21 to 45) — and forecasts up to 12 future menstrual cycles, showing for each: the predicted period start date, the period end date (based on your configured period duration), the fertile window (the 5 days leading up to and including ovulation), and the estimated ovulation day. An advanced settings panel lets you customise both your period duration and luteal phase length for greater personal accuracy. The luteal phase defaults to 14 days but can be adjusted for women whose cycles differ. The current cycle phase is shown in real time based on today's date relative to your last period: menstruation, follicular phase, ovulation window, or luteal phase. Estimated Due Date (EDD) is calculated as 280 days from your last period start as a basic pregnancy date reference. A Google Calendar integration lets you export predicted cycle dates as calendar events with one click. All calculations are statistical predictions based on a regular cycle — not a medical device or a contraception method.

How to Use Period Calculator

1. Enter Your Last Period Start Date: Select the first day of your most recent period from the date picker.
2. Set Your Cycle Length and Period Duration: Enter your average cycle length (21–45 days; default 28) and your typical period duration (2–8 days). For more accuracy, open Advanced Settings to adjust the luteal phase length.
3. Review Predictions and Export: See your next 3, 6, or 12 cycles with all dates highlighted. Use the Google Calendar export to sync predictions to your calendar.

Key Features

• Up to 12 Future Cycle Predictions: Forecasts your next 3, 6, or 12 menstrual cycles — showing period start date, period end date, fertile window start/end, and ovulation day for each cycle.
• Fertile Window and Ovulation Day: The fertile window (5 days before ovulation plus ovulation day) and estimated ovulation day are shown for each predicted cycle. Ovulation is calculated as cycle start + cycle length − luteal phase length.
• Adjustable Luteal Phase: The luteal phase (the time between ovulation and your next period) defaults to 14 days but can be adjusted in advanced settings for personalised ovulation prediction.
• Current Cycle Phase Display: Shows your current phase in real time: menstruation, follicular phase, approaching ovulation, or luteal phase — based on today's date relative to your last period.
• Estimated Due Date (EDD): Calculates a basic estimated due date as 280 days (40 weeks) from your last period start date — the standard Naegele's Rule calculation used as a preliminary reference.
• Google Calendar Export: Export your predicted cycle dates (period days and fertile windows) as Google Calendar events with one click, so your cycle is visible alongside your schedule.

Benefits

• Plan Around Your Cycle: Knowing your upcoming period dates, fertile window, and ovulation day lets you plan travel, events, workouts, or medical appointments with your cycle in mind rather than being caught off-guard.
• Fertility Awareness: The fertile window and ovulation predictions provide a clear baseline for those trying to conceive. Combining this with LH surge tests (ovulation predictor kits) gives the most reliable results.
• Understand Your Cycle Phases: Many women notice differences in energy, mood, focus, and sleep quality across the four phases of their cycle. Knowing which phase you are currently in helps contextualise those patterns.
• No App, No Account, No Tracking: Period tracking apps collect detailed personal health data. This calculator runs entirely in your browser with no data transmitted or stored — your menstrual cycle information stays private.

Supported Formats & Capabilities

• Date input: last period start date
• Cycle length: 21–45 days (adjustable)
• Period duration: 2–8 days (adjustable)
• Luteal phase: adjustable in advanced settings

Common Problems & Solutions

My cycles are irregular. Will the predictions be accurate?

This calculator assumes a regular cycle length. For irregular cycles, predictions become less reliable further out. Use the results as a rough guide and recalibrate with your actual period date each month. Using the average of your last 3–6 cycles as your cycle length input improves accuracy.

How is the ovulation day calculated?

Ovulation is estimated as: cycle start date + cycle length − luteal phase length. With a 28-day cycle and the default 14-day luteal phase: day 1 + 28 − 14 = ovulation on day 14. Adjust the luteal phase in Advanced Settings if yours differs.

What is the fertile window?

The fertile window is the 5-day period before ovulation plus ovulation day itself, reflecting the fact that sperm can survive in the reproductive tract for up to 5 days. This is the window when conception is most likely.

Can I use this as a contraceptive method?

No. This tool provides statistical predictions based on calendar arithmetic. It is not a medically approved contraceptive method. Irregular cycles, illness, stress, and other factors affect ovulation timing. For contraception, consult a healthcare provider.

Pro Tips & Best Practices

• Track your actual period start dates over 3–6 months and use the average as your cycle length input — this single adjustment makes predictions significantly more accurate for your body.
• The 5-day fertile window is an average. LH surge tests (ovulation predictor kits, available at any pharmacy) confirm actual ovulation timing and are the most reliable way to identify your peak fertile days.
• Stress, illness, significant weight change, and intense exercise can all shift ovulation timing — sometimes by several days. If your period arrives notably earlier or later than predicted, these are the most common explanations.
• Use the Google Calendar export to create a 3 or 6-month view of your cycle on your personal calendar. Having period dates and fertile windows visible alongside your schedule makes planning much easier.

Privacy & Data Security

All period, ovulation, and fertile window calculations are performed entirely in your browser using calendar arithmetic. Your last period date, cycle length, and any other inputs are processed in local JavaScript memory and are never transmitted to any server. No menstrual health data is collected, stored, or shared.

Frequently Asked Questions

Is a 28-day cycle normal?

A 28-day cycle is the textbook average, but healthy cycles range from 21 to 35 days. The important thing is consistency — your cycle length should not vary by more than 7–9 days from month to month. Significant irregularity is worth discussing with a healthcare provider.

When do I ovulate?

Ovulation is calculated as: period start date + (cycle length − luteal phase). With the default 14-day luteal phase and a 28-day cycle, ovulation falls on day 14. For a 30-day cycle, it falls on day 16. Adjust the luteal phase length in Advanced Options if yours differs from 14 days.

What is an EDD and why does this calculator show it?

EDD stands for Estimated Due Date — calculated as 280 days (40 weeks) from the start of that period using Naegele's Rule. This is provided purely as an informational planning tool for users who are trying to conceive.

How do I add a predicted period to my Google Calendar?

Each predicted cycle card has an 'Add to Calendar' button. Tapping it opens Google Calendar with the event title and dates pre-filled — you just click Save.

How accurate is this period predictor?

For people with consistent cycles, predictions are typically accurate within 1–3 days. For irregular cycles, the margin of error increases. Tracking 3–6 months of actual data gives a much more reliable personal baseline.

What is the fertile window and how long does it last?

The fertile window is the 5 days before ovulation plus ovulation day itself — 6 days total. Sperm can survive up to 5 days in the reproductive tract, and an egg is viable for 12–24 hours after ovulation.

Can I use this calculator if my cycle is longer than 28 days?

Yes. The cycle length is fully adjustable from 21 to 45 days. Enter your actual average cycle length and all predictions — periods, ovulation, fertile window — will be calculated based on your cycle, not the 28-day average.

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