Predict your next period dates, estimated ovulation day, and fertile window in seconds. Enter the first day of your last period, your average cycle length, and how long your period usually lasts. You’ll get a clear timeline for planning, tracking, and staying prepared.
How to Use the Period Calculator
- Pick the start date of your last period (the first day your flow began).
- Please enter your average cycle length. For example, it could be 28 days.
- Please enter how long your period usually lasts (for example, 5 days).
- Click Calculate to view your cycle timeline.
- If your results don’t match your recent pattern, update your averages and calculate again.
How This Calculator Works
This tool estimates dates using the values you enter. It does not detect ovulation. It predicts timing based on typical cycle patterns.
Next period estimate
Your next period is predicted by adding your cycle length to the start date of your last period.
Period window estimate
Your period length helps estimate how many days your flow may last in the upcoming cycle.
Ovulation estimate
Ovulation is estimated using your cycle length as a guide. For many people, it happens around the middle of the cycle, but it can move earlier or later.
Fertile window estimate
The fertile window is estimated around ovulation. A helpful range exists, not a guarantee.
For a more focused fertility view, you can also use our Ovulation Calculator.
Quick Example
Here’s a simple example to show how the timeline is estimated:
- Last period start: January 3
- Cycle length: 28 days
- Period length: 5 days
The calculator will estimate your next period start about 28 days after January 3. It will show your expected period days, estimated ovulation day, and fertile window.
What Your Results Mean
Next period
Your predicted next cycle start date. Useful for organizing tasks, trips, and schedules.
Period window
The expected range of days your period may occur, based on your usual period length.
Fertile window
A range of days when pregnancy is more likely for people trying to conceive. Timing can still vary.
Estimated ovulation day
Your most likely ovulation day based on your average cycle length. Ovulation can shift month to month.
Cycle day today
This shows where you are in your cycle today. It starts from Day 1, which is the first day of your last period.
Menstrual Cycle Basics
What is a menstrual cycle?
A menstrual cycle starts on the first day of one period and ends on the first day of the next.
Why cycle length matters
Cycle length is the main number used for prediction. Even a small shift—like 2–3 days—can move your next period estimate.
Why cycles can change
Cycle timing can change for several reasons:
- Stress or poor sleep
- Travel or routine changes
- Diet or exercise changes
- Hormonal changes
- Illness or medication
If your cycle changes often, predictions will be less exact.
Irregular Cycles and Better Accuracy
If your cycle varies month to month
If your cycle length swings a lot, predictions become a rough estimate.
Tips to get more accurate results
- Track your last 3–6 cycles and use the average.
- Update your cycle length after significant routine changes.
- Use notes for late periods to spot patterns over time.
If you often miss your periods, have heavy bleeding, or feel severe pain, talk to a healthcare professional.
よくある質問
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生理計算ツールは、次の生理がいつ始まるかを予測するのに役立ちます。
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簡単に推定するには、最後の生理の初日から始めます。
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周期が28 日の場合、排卵は14 日目頃に起こる可能性があります。