How to use the period tracker
- Enter the First day of your last period, counting from the first day of full bleeding.
- Set Cycle length (days), from the first day of one period to the first day of the next (use 28 if unsure), and Period length (days), the number of days you bleed.
- Optional: press Add an earlier period and enter the first day of a few past periods. The tool measures the gaps and offers your average cycle length.
- Tick Remember these dates on this device if you want them kept for next time. Untick it to erase them.
How the dates are predicted
Your next period starts on the first day of your last period plus your cycle length. It ends after your period length, so a 5-day period starting October 29 ends November 2. Each later period adds one more cycle. Logged dates give an average cycle, the mean of the gaps between consecutive start dates, counting gaps of 15 to 90 days.
The input limits follow published ranges. The NICHD says the average cycle is 28 days and runs 21 to 35 days for most women and 21 to 45 days in teens, with an average period of about 5 days. The Cleveland Clinic puts typical bleeding at 3 to 7 days.
Worked example
The starting values are a period that began 5 days ago, a 28-day cycle and a 5-day period, so the next one is due in 23 days. For a last period starting October 1, 2026, the six predicted periods are Oct 29 to Nov 2, Nov 26 to Nov 30, Dec 24 to Dec 28, Jan 21 to Jan 25, Feb 18 to Feb 22 and Mar 18 to Mar 22.
Limits and edge cases
Predictions assume every cycle is the same length, but real cycles drift by a few days, and stress, illness, travel, weight change and hormonal birth control all move them. The tool shows a warning when a cycle is over 35 days or a period over 7 days, and it does not predict ovulation, so use the ovulation calculator for fertile days.
The last period must fall within the past year and not in the future. Once a period is more than a cycle overdue, the tool lists the periods still ahead and asks for a fresh start date. If you might be pregnant, use the pregnancy due date calculator.
Frequently asked questions
How do I calculate when my next period is due?
Add your cycle length to the first day of your last period. With a last period starting October 1 and a 28-day cycle, the next one starts October 29. Count the first day of full bleeding as day 1 and measure the cycle from first day to first day.
What is a normal menstrual cycle length?
The NICHD says the average cycle is 28 days and that most women have cycles of 21 to 35 days. In teens, cycles of 21 to 45 days are normal. A typical period lasts about 5 days, and 3 to 7 days is usual.
Why is my period late?
Pregnancy is the first thing to rule out. Stress, large changes in weight or exercise, thyroid problems, polycystic ovary syndrome, hormonal birth control and perimenopause can also shift your timing. Mayo Clinic advises seeing a doctor if you miss three periods in a row and are not pregnant.
Does this period tracker save my data?
Only if you tick Remember these dates on this device. The dates are then kept in this browser on this device and are never sent to us. Untick the box to erase them. Otherwise nothing is stored and the dates disappear when you close the page.
Sources: NICHD, Menstruation fact sheet; Cleveland Clinic, Menstrual Cycle; Mayo Clinic, Amenorrhea.