How your next period is calculated
Your next period is estimated as the first day of your last period plus your usual cycle length. The range around it comes from how much you say your cycles vary, and grows with the square root of the number of cycles ahead. That’s the standard way independent differences add up, and it’s why the sixth month shows a wider window than the first.
Periods usually come every 21 to 35 days and last between 2 and 7 days. This calculator also accepts lengths outside that range, as those are real experiences too, and it will tell you when a pattern is worth mentioning to a doctor.
For predictions based on your own history rather than a single average, use the period tracker.
Why the window gets wider
Small differences between cycles add up. With cycles that vary by about 3 days, the window around each predicted date grows like this:
| Period | Window around the expected date |
|---|---|
| Next period | ±3 days |
| 2nd period ahead | ±4 days |
| 3rd period ahead | ±5 days |
| 4th period ahead | ±6 days |
| 6th period ahead | ±7 days |
So it’s reasonable to plan around next month’s window, but a date six months from now is only a rough guide.
How to find your cycle length
- Find the first day of proper bleeding for two periods in a row.
- Count the days from the first of those dates up to the day before the second. That total is your cycle length.
- Repeat for a few months if you can. If your numbers differ, use a typical one and choose a wider variation setting.
The average is about 28 days, and anywhere from 21 to 35 days is common. Periods usually last between 2 and 7 days, with the heaviest bleeding in the first 2 days.
Understanding your results
- Next period is the range of days your period is most likely to start, with the single most likely day underneath.
- Could last until adds your period length and your variation to that estimate, which helps when you’re planning travel or an event.
- Likely fertile days and estimated ovulation are calculated from the same dates. The ovulation calculator lets you adjust them further.
When a period calculator is less reliable
A calculator assumes that your next cycle will resemble your usual one, so it’s less useful if your cycles have recently changed. Examples include the first few years after your first period, the years around perimenopause, recent pregnancy or breastfeeding, and starting or stopping hormonal contraception.
If your period doesn’t arrive when expected, the late period checker can help you work out what to do next. Any lasting change in your periods is worth raising with a GP.