Period Calculator
Predict your next period date, current cycle phase, and fertile window from your last period date and average cycle length.
First day of your last period
Average cycle length
days
Period length
days
Next period expected
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Days until next period
—
Current cycle day
—
Current phase
—
Estimated ovulation
—
Fertile window
—
Following 2 cycles
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How the Period Calculator Works
This calculator predicts your next period date, current cycle phase, and fertile window from the first day of your last period and your average cycle length. It projects forward using your typical cycle length (day 1 of one period to day 1 of the next), and estimates ovulation around 14 days before your next expected period — the standard assumption used by most period trackers.
How to Use It
- Enter the first day of your most recent period.
- Set your average cycle length (most people range 21–35 days; 28 is the textbook average).
- Set how many days your period usually lasts.
- See your next expected period, current cycle day and phase, and fertile window instantly.
Example
If your last period started on June 1st with a 28-day cycle, your next period is expected around June 29th, with ovulation and the fertile window falling roughly between June 10th and June 15th.
Common Use Cases
- Planning ahead for your next period so you're not caught off guard.
- Getting a quick estimate of your fertile window when trying to conceive or avoid pregnancy.
- Understanding which phase of your cycle (menstrual, follicular, ovulation, luteal) you're currently in.
FAQs
How accurate is this period calculator?
It's an estimate based on your average cycle length, not a measurement of your actual hormones. Cycle length naturally varies month to month due to stress, illness, travel, and other factors, so treat the predicted dates as a helpful range rather than an exact schedule. This tool is not a substitute for medical advice or a reliable method of contraception.
What are the four phases of the menstrual cycle?
The menstrual phase is your period itself. The follicular phase runs from the end of your period until ovulation. Ovulation is the release of an egg, roughly midway through the cycle. The luteal phase follows ovulation until your next period begins.
How is this different from the Ovulation Calculator?
This tool is built around predicting your next period date and current cycle phase, with ovulation shown as a secondary estimate. If fertility planning is your main goal, the dedicated Ovulation Calculator leads with the fertile window and ovulation date instead.