Menstrual Cycle Calculator
Estimate your next period, ovulation date and fertile window.
Assumptions used in this calculation
- Regular cycles: Assumes a consistent cycle length, actual ovulation timing varies and this isn't a substitute for medical tracking methods.
About this calculator
Tracking a cycle by hand means juggling three different date calculations at once, when your next period is due, when you likely ovulated, and your fertile window, each derived differently from the same starting date. This calculator consolidates all three: enter the first day of your last period, your average cycle length, and average period length, and it computes your next expected period date, estimated ovulation date (working backward from the next period using the standard 14-day luteal phase, rather than forward from the last period), and a fertile window around that ovulation estimate. Counting back from the next period rather than forward from the last one is deliberate, the luteal phase (ovulation to next period) is far more consistent in length than the earlier follicular phase, so it produces a more reliable ovulation estimate. As with any calendar-based method, this assumes a consistent cycle length and isn't a substitute for medical tracking methods like basal body temperature charting if precise timing matters to you.
Worked example
Last period started 1 Jan, 28-day average cycle
Result: Next period ≈ 29 Jan, ovulation ≈ 15 Jan
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