A typical menstrual cycle is 21–35 days. Ovulation occurs ~14 days before the next period.
Fertile window: 5 days before ovulation + ovulation day = 6 days total.
Only ~13% of women have a 28-day cycle — individual variation is normal.
This tool is most useful for tracking trends rather than obsessing over a single data point. Run the calculation monthly with consistent measurement conditions to see whether your metric is moving in the right direction.
The most common mistake is treating the result as a precise measurement rather than an evidence-based estimate. All body metric calculators have margins of error — use the result as a tracking baseline, not a clinical diagnosis.
A personal trainer uses the calculator with a new client to set a measurable starting point. Re-running the calculation at 4-week intervals provides an objective progress metric that supports motivation and programme adjustments.
Add your average cycle length to the first day of your last period.
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