Irregular Cycle Calculator
Quick answer
If your cycle length varies from month to month, narrow your next-period prediction to a range instead of a single date — using your shortest and longest observed cycles. When the range is reasonably tight, it also estimates a most-likely date.
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Your details
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How the prediction works
Earliest next period = last period + shortest cycle length
Latest next period = last period + longest cycle length
Most likely date = last period + average cycle (auto: (shortest + longest) ÷ 2, or your override)
Variability = longest − shortest (days)
A fertile-window estimate is only shown when variability is ≤ 10 days — beyond that, timing is too unpredictable to estimate reliably.
Worked example
- 1
Your last period started September 1; your shortest observed cycle is 26 days and longest 33.
- 2
Earliest next period ≈ September 27; latest ≈ October 4.
- 3
Average cycle ≈ (26 + 33) ÷ 2 = 29.5 → most likely date ≈ September 30.
- 4
Variability = 7 days — tight enough for a rough fertile-window estimate.
Why cycles vary
| Your result | Category | What it means |
|---|---|---|
| Up to ~7 days of variation | Normal variability | Cycle length shifts a little between months for most people — a small shortest-to-longest window is typical. |
| Wider variation | Track more cycles | A longer history gives a truer shortest/longest window. More data beats a single month of measurement. |
| Very irregular or absent cycles | See a provider | Irregularity can relate to stress, PCOS, thyroid conditions or perimenopause. A doctor can help identify the cause. |
A few days of month-to-month variation is normal. Larger swings have many possible causes worth exploring with a provider.
Frequently asked questions
Sources & further reading
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Open the logThis calculator provides general estimates for informational purposes only and is not medical advice. For personal guidance, consult a qualified healthcare provider.