Count from the first day, not the last
Your cycle is counted from the first day of one period up to the first day of the next. So the date to start from is the day your last period began, not the day it ended.
If you only remember when your period ended, count back. A period that ended on May 12 after 5 days started on May 8.
A worked example
Say your last period started on May 8 and your cycles are usually 28 days long. Add 28 days to May 8 and you get June 5. That's the day your next period is most likely to start.
If you don't know your cycle length yet, 28 days is a common starting point. The NHS says most women have a period around every 28 days, though it can range from every 21 days to every 35 days.
The period calculator does this math for you and also shows your ovulation day and fertile window.
Why it might come early or late
A predicted date is an estimate. It's common for one cycle to be a little shorter or longer than the one before. Stress, illness, travel, weight changes, hard exercise and some types of hormonal contraception can all shift when your period comes. Periods are also less regular in the first years after they start and as you get closer to menopause.
Tracking a few cycles shows you how much yours usually moves. If you log your periods in a free account, your predictions start using your own cycles after 3 to 4 periods. How our predictions work explains the rest.
When a late period is worth checking
If there's any chance you could be pregnant, a pregnancy test is the quickest way to find out. The NHS suggests seeing a GP if you have missed 3 periods in a row, if your periods have become irregular or if your periods have not started by the time you're 15.
Sources
- Periods, NHS
- Missed or late periods, NHS
- Your menstrual cycle, Office on Women's Health