Health / Cycle calendar
Period Calculator: Next Period and Cycle Calendar
Estimate your next six periods from your last start and cycle length. Add past starts to see your average and variability, then view the dates on a calendar or download an .ics file.
Period Calculator: Next Period and Cycle Calendar: Shows predicted bleeding ranges after the last observed period, with estimated fertile windows and ovulation days before them. Cycle averages are rounded to a whole day for prediction. When an expected date has passed, it stays visible rather than being treated as an observed period. Estimates are less reliable for irregular cycles and must not be used as contraception. Runs 100% locally in your browser with zero server file uploads.
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- Health & sport tools
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- Free · no sign-up
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Runs entirely in your browser
Estimates only. Not contraception. Predictions are less reliable for irregular cycles. Calendar dates cannot tell you when pregnancy is impossible.
Use past starts to estimate your cycle
Enter up to six consecutive starts before your last period, in any order. Leave unused dates blank. Skipping a period makes the average misleading. The calculated average replaces the cycle length above.
Nothing is stored by default. If you choose to remember, your dates stay in this browser on this device. They are not uploaded. Clearing removes the saved copy and resets these inputs.
Choose the first day of your last period to see six predicted periods.
How the dates are estimated
Each next start adds the average cycle length to the last start. Bleeding lasts for the period length you enter. Ovulation is estimated 14 days before each predicted start, with a six-day fertile window ending on ovulation. Actual timing varies. ACOG: fertility awareness
When to seek medical advice
See a doctor for very irregular or very heavy periods, missed periods, or a change that concerns you. NHS guidance advises seeing a GP after three missed periods in a row. These estimates cannot diagnose a cause. NHS: irregular periods · NHS: heavy periods · NHS: missed periods
Ovulation calculator · Due-date calculator · Printable calendar
Cycle and fertile estimates
Cycles run from one period start to the next. Predicted starts add the rounded average cycle length; bleeding end dates include the entered number of days. Estimated ovulation is 14 days before the next start, with the six-day fertile window ending on ovulation. Timing varies and these predictions are not contraception. ACOG fertility awareness guidance: https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning.
Variability and medical advice
ACOG describes cycle-length variation greater than 7–9 days as irregular, depending on age; this tool flags above seven days as a caution. ACOG abnormal uterine bleeding: https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding. See a doctor for very irregular, very heavy or missed periods. NHS guidance includes seeking advice after three missed periods in a row: irregular periods: https://www.nhs.uk/symptoms/irregular-periods/, heavy periods: https://www.nhs.uk/conditions/heavy-periods/ and missed periods: https://www.nhs.uk/symptoms/missed-or-late-periods/.
How to use it
- Enter the first day of your last period, average cycle length and bleeding length.
- Optionally enter up to six consecutive earlier starts to calculate your average cycle.
- Read the six predicted ranges, estimated fertile days and days until the first expected period, or download the calendar file.
Privacy & limitations
Nothing is stored by default. Remember on this device saves your inputs only in this browser; clear removes them. Dates are not uploaded by this tool. An exported calendar contains sensitive dates and may be uploaded by your calendar provider when you import it.
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Frequently asked questions
How is this different from the ovulation calculator?
This page plans bleeding dates and period length on a month calendar, with an .ics export and a summary of past cycle lengths. The ovulation calculator focuses on fertile timing for trying to conceive.
Can I use these dates as contraception?
No. Calendar predictions cannot tell you when pregnancy is impossible. Ovulation can move, even in regular cycles.
What if the predicted date has already passed?
The date remains visible, with how many days have passed. Later dates are estimates, not evidence that periods occurred. Update the last start when a period happens; consider a pregnancy test if pregnancy is possible, and seek medical advice for missed periods.
What does the variability warning mean?
The tool flags a difference above seven days between recorded cycle lengths. ACOG describes variation greater than 7–9 days as irregular, depending on age. A short history cannot diagnose irregularity. See a doctor for very irregular or heavy periods, missed periods or changes that concern you.
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