How the projection is built
Your cycle length is the number of days from the first day of one period to the first day of the next. Starting from the last period you enter, the calculator adds that length repeatedly to map out when each future period should begin.
For each projected cycle it also estimates an ovulation day, placed about 14 days before the following period starts. Because the luteal phase tends to be the steady part of the cycle, counting back from the next period gives a better ovulation estimate than counting forward from the last one.
Why later dates drift
Real cycles rarely run to exactly the same length every month. A small difference is harmless for the next period, but the same few days are added again and again as the projection extends, so the error compounds.
That means the first one or two predicted periods are usually close, while dates many months out should be treated as rough guides. Re-running the calculator after each period, using your actual most recent start date, keeps the projection on track.
Tracking tips and what affects your cycle
Keeping a simple log of period start dates makes your personal average far more accurate than any default. A number of everyday factors can also shift timing:
- Stress, travel, and major changes in sleep or routine.
- Significant weight change, intense exercise, or illness.
- Hormonal contraception and certain medications.
- Approaching perimenopause or recovering after pregnancy or breastfeeding.
Medical disclaimer
These projections are estimates for general planning and education only and are not medical advice. Persistently irregular, very heavy, very painful, or absent periods can have medical causes and should be discussed with a doctor or qualified healthcare provider.
Formula
periodStart_k = lastPeriod + cycleLength·k; ovulation_k ≈ periodStart_(k+1) − 14Frequently asked questions
- How accurate is this projection?
- It assumes a constant cycle length. Most cycles vary by a few days, so later projections drift more than the next one or two.