週期計算器

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

1. Pick the start date of your last period (the first day your flow began).
2. Please enter your average cycle length. For example, it could be 28 days.
3. Please enter how long your period usually lasts (for example, 5 days).
4. Click Calculate to view your cycle timeline.
5. 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.