Period Calculator
A free, browser-based calculator. Runs entirely in your browser — no sign up, nothing stored.
100% FreeRuns in your browserBasic
Enter details
Result
Enter a date to see your estimate.
Estimate only — not medical advice. Due dates, ovulation and fertile windows vary between people and cycles. Confirm with a healthcare 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
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.