Variable cycle lengths: should you enter an average or 28 days?
A fixed 28-day assumption may not describe your records. Look at observed intervals and preserve the variation instead of hiding it behind one date.
Period calculator →Calculate the intervals you actually observed
Suppose the last three recorded cycles were 27, 29 and 31 days. Their average is 29 days. Entering 29 creates a scenario based on those observations, not a guarantee about the next cycle. One recorded interval is also too little information to establish a long-term pattern.
Compare more than one scenario
With a most recent start of 2026-06-01, adding 27 days gives June 28, adding 29 gives June 30, and adding 31 gives July 2. This comparison shows how sensitive the result is to the chosen length. It is not a statistical confidence interval and does not promise that the next period will occur inside those dates.
Keep the record, not just the average
Retain actual start dates and relevant changes so the history remains useful. Averaging persistent irregularity does not explain it or make it disappear. The calculator cannot identify a medical cause, rule out pregnancy or determine safe days for unprotected sex. Use the average and date tools to organize observations, and seek appropriate advice when the pattern or symptoms warrant it.
Try the calculators
- Period calculator
Estimate your next period start date from your last period and cycle length.
- Average & median
Find the mean, median and sum of a list.
- Date difference
Count calendar days between two dates.