Methodology
How the estimates are calculated
Every number the tracker shows comes from the rules on this page. There's no hidden model.
Definitions
- Cycle day 1 is the first day of period bleeding (not spotting).
- Cycle length is the number of days from one period’s day 1 to the next period’s day 1.
- All maths uses calendar dates in your local time zone, so clock changes can't shift a day.
The calculation
Everything is counted from the date you enter, your cycle length (21–45 days) and your period length (1–10 days). Nothing is averaged or guessed.
| Next period | last period start + cycle length |
|---|---|
| Predicted period days | next period, lasting your period length |
| Estimated ovulation | next period − 14 days |
| Estimated fertile window | ovulation − 5 days to ovulation + 1 day |
The three-month calendar
The calendar repeats the same pattern every cycle length from the date you entered: period days, then the fertile window and ovulation before each next period. Days after the first cycle are predictions of predictions, so treat them as rough.
Which cycle day is today
Today's cycle day is simply the number of days since the date you entered, plus one. The day-by-day notes come from a typical cycle: they describe what usually happens on that day, not a measurement of your body. If the expected period date has passed, the calculator says so and asks for the new date rather than guessing.
Why these rules
Counting back 14 days from the next period is the standard calendar convention, because the second half of the cycle tends to be more consistent than the first. The NHS says ovulation usually happens 10 to 16 days before the next period and can't be pinpointed exactly. The 5 days before ovulation reflect how long sperm can survive. Every date here is an estimate, and a few days either side is normal.
Known limitations
- A calendar cannot detect ovulation, pregnancy or any medical condition.
- Estimates assume your future cycles resemble the ones you entered.
- Recent hormonal contraception, pregnancy, breastfeeding and perimenopause make calendar estimates much less reliable.
- No day is ever labelled “safe”. Don’t use these estimates as contraception.
The calculation code is small and covered by automated tests. If a date looks wrong, pleasetell us. You don't need to send any health details.