How the Period Calculator works
Enter the first day of your last period, your average cycle length and how many days your period usually lasts. The calculator adds your cycle length again and again to predict the start of your next six periods, shows which days you can expect to bleed, and marks the fertile window — around ovulation, about two weeks before each period — so you can plan ahead or understand your cycle better.
How to work out your cycle length
Your cycle starts on the first day of full bleeding (day 1) and ends the day before your next period starts. Count the days between the first day of one period and the first day of the next. Because cycles vary a little, the best figure is an average of your last three to six cycles. For example, cycles of 27, 29 and 28 days give an average of 28.
What is a normal period?
| Cycle length | About 21–35 days in adults (often longer and more variable in the first years after periods start) |
|---|---|
| Period length | Usually 2–7 days |
| Variation | A difference of a few days between cycles is common |
It is a good idea to talk to a doctor or nurse if your periods suddenly change, become very heavy or painful, come less than 21 days apart, stop for three months or more without pregnancy, or if you bleed between periods or after sex.
Using your predictions
- Planning: know when to expect your period for travel, events or sport.
- Trying to conceive: the fertile window before each period is when pregnancy is most likely; the Ovulation Calculator gives more detail.
- A late period: if your period has not arrived and pregnancy is possible, the Pregnancy Test Calculator shows when a test is most reliable.
Predictions are only as regular as your cycle. Period-tracking over several months — writing down the first day of each period — makes them more accurate and gives you useful information to share with a healthcare professional. This calculator is not a method of contraception.