The formula
each cycle begins where the previous one ends
Where it comes from
The forecast assumes your cycles are regular and always the same length. In practice a variation of two or three days is normal, and a cycle only counts as irregular when the shortest and the longest differ by more than eight days.
How to work it out by hand
- Take the first day of your last period
- Add the length of your cycle for the next one
- Repeat for as many cycles as you want to forecast
- Work out the fertile window of each one
What is worth knowing
Noting the real dates for three or four months gives a far better estimate than any theoretical average, because every woman has her own pattern. A cycle counts as normal between 21 and 35 days. If yours routinely falls outside that range, or varies by more than eight days month to month, it is worth raising at an appointment.