Period Calculator
This period calculator predicts your next period and the six cycles after it — enter your last period, average cycle length and how long your period usually lasts to see the start and end date of each one.
Your dates
Your result
Your 40-week timeline
Based on Naegele's rule (ACOG methodology). Estimates only — always confirm with your provider.
How period prediction works
Your cycle length is counted from the first day of one period to the first day of the next — not from the day bleeding stops. This calculator projects that rhythm forward six cycles, so you can plan travel, events and appointments. Real cycles drift a day or two either way, and the further out the forecast, the wider the real-world wiggle room. A normal adult cycle runs anywhere from 21 to 35 days (ACOG), so a result that isn't exactly 28 days is completely expected.
Worked example. Say your last period began on January 1, 2026, your average cycle is 28 days, and you usually bleed for 5 days. Your next period is projected for January 29, 2026 (Jan 1 + 28 days), running through about February 2. Adding 28 days again and again gives the six-cycle forecast below:
| Cycle | Period starts | Period ends (~5 days) |
|---|---|---|
| Next (1) | Jan 29, 2026 | Feb 2, 2026 |
| 2 | Feb 26, 2026 | Mar 2, 2026 |
| 3 | Mar 26, 2026 | Mar 30, 2026 |
| 4 | Apr 23, 2026 | Apr 27, 2026 |
| 5 | May 21, 2026 | May 25, 2026 |
| 6 | Jun 18, 2026 | Jun 22, 2026 |
Enter your own dates above and the live calculator rebuilds this table for your cycle. These are calendar estimates for planning, not a medical assessment — if your periods stop, become very irregular, or change pattern suddenly, confirm what's normal for you with your provider.