Period Calculator Online
Predict your next several periods, ovulation days and fertile windows from your average cycle. A general estimate, not medical advice — your dates stay on your device.
The Period Calculator runs entirely in your browser. The dates you enter never leave your device and nothing is uploaded to ArrayKit. Results are a general estimate from a standard formula, not medical advice.
Open the Ovulation Calculator
About Period Calculator
The Period Calculator projects your next several expected periods, estimated ovulation days and fertile windows from the first day of your last period and your average cycle and period length. It uses the standard calendar-based formula — each future period lands one cycle length after the previous one, and ovulation is estimated 14 days before the following expected period — the same method used by most period-tracking apps and calculators. Adjust the cycle length and period length to match your own averages for a closer estimate, and copy a plain-text summary of every upcoming cycle. This is a general estimate from a standard formula, not medical advice; real cycles vary in length and timing, so treat the dates as a planning guide rather than a diagnosis. Everything is calculated locally in your browser — your dates never leave your device.
Features
- Projects the next 6 expected periods from your last period's start date
- Estimates ovulation day and fertile window for each upcoming cycle
- Customize average cycle length (21-45 days) and period length (1-10 days)
- Shows a countdown in days to your next expected period
- Copy a plain-text summary of all upcoming cycles, ovulation days and fertile windows
- Clear disclaimer that results are a general estimate, not medical advice
- Updates instantly as you change the date, cycle length or period length
- Runs entirely in your browser — your cycle data is never uploaded
How to use the Period Calculator
- Enter the first day of your last period
- Adjust the average cycle length and period length if they differ from 28/5 days
- Review the projected periods, ovulation days and fertile windows
- Copy the summary to save or share your upcoming cycle dates
Example
Input
last 2026-06-01, 28d
Output
Next period: ~Jun 29
Common errors & troubleshooting
- The predicted next period date looks too far off from what actually happened last cycle. — Adjust the cycle length field to match your true average (count days from the start of one period to the start of the next) rather than leaving the default 28 days.
- Fertile window dates seem too wide or too narrow. — The Period Calculator uses a standard 5-day-before to 1-day-after ovulation window based on typical sperm and egg survival; individual fertility can vary, so treat it as an estimate, not a guarantee.
- Predictions drift further from reality with each future cycle. — This is expected — the calculator assumes a fixed cycle length, but real cycles fluctuate, so accuracy naturally decreases the further out you project. Re-enter your actual last period date each month for the most accurate estimate.
- Cycle length or period length field shows an error and won't calculate. — Enter a whole number of days: cycle length between 21 and 45, period length between 1 and 10. Decimal or negative values are rejected.
Frequently asked questions
- How does the Period Calculator predict my next period?
- It adds your average cycle length to the start date of your last period, then repeats that for each following cycle. This is the same calendar-based method most period-tracking apps use, and it is a general estimate rather than a guarantee.
- How does the Period Calculator estimate ovulation?
- Ovulation is estimated 14 days before the next expected period, based on the typical length of the luteal phase (the second half of the cycle). Your actual ovulation day can shift earlier or later, especially with irregular cycles.
- What is the fertile window shown by the calculator?
- It spans 5 days before the estimated ovulation day through 1 day after, covering the days sperm can typically survive and the roughly 24-hour window an egg remains viable. It is an estimate for planning, not a fertility diagnosis.
- Is the Period Calculator accurate for irregular cycles?
- It works best with fairly regular cycles because it assumes a fixed cycle length. If your cycles vary a lot month to month, treat the projected dates as a rough guide and re-enter your latest period start date often.
- Does the Period Calculator store or upload my cycle data?
- No. All dates and calculations happen locally in your browser. Nothing you enter into the Period Calculator is uploaded to ArrayKit or stored on a server.
- Is the Period Calculator a substitute for medical advice?
- No. It is a general estimate based on a standard calendar formula, not medical advice. For questions about your cycle, fertility or reproductive health, talk to a qualified clinician.
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