Period Calculator
Predict your next period, cycle length and fertile days with this free period calculator that works entirely in your browser.
Enter details
Result
Enter a date to see your estimate.
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the first day of your last period, your cycle length and period length.
- Read your next period dates and upcoming cycles.
How it works
Each cycle starts cycle-length days after the last. The tool projects the next several start dates and the approximate end of your next period.
Worked example
28-day cycle from 1 Jan. Next periods: 29 Jan, 26 Feb, 26 Mar…
Tips
- Estimate only — not medical advice. Dates and fertile windows vary between people and cycles; confirm with a healthcare professional.
Frequently asked questions
What if my cycle is irregular?
The projection assumes a regular cycle, so irregular cycles will be less accurate.
Privacy?
Runs entirely in your browser. Your inputs are not saved or transmitted.
Related tools
- Ovulation Calculator
- Pregnancy Due Date Calculator
- Conception Date Calculator
- Care Percentage Calculator
- Child Support Estimator (Australia)
- Custody Nights Percentage Calculator
Explore more in Family, Parenting & Custody.
Cite this tool
Zerdly. (2026). Period Calculator. Retrieved from https://www.zerdly.com/tools/period-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.