September 2026
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Period / menstrual cycle calculator
Predict your next period, ovulation date, and likely fertile window.
Estimate, not a diagnosis. This tool gives an educational estimate based on the numbers you enter. It cannot account for your individual health history — consult a healthcare professional for anything clinical.
How this works
We use the first day of your last period and your average cycle length to estimate your next period by adding the cycle length. Ovulation is estimated as 14 days before the next period (a typical luteal phase), and the fertile window is the five days before ovulation plus ovulation day itself.
Outputs
- Predicted next period date
- Estimated ovulation date
- Fertile window
Limitations
This is an estimate suitable for planning. Irregular cycles, hormonal contraception, pregnancy, and medical conditions can make predictions unreliable. For clinical decisions, consult your healthcare provider.
Related reading
- Understanding your menstrual phase
- How to prevent and ease period cramps
- What to eat during your period
Frequently asked questions
How does a period calculator predict my next period?
It adds your average cycle length to the first day of your last period. Ovulation is then estimated as 14 days before that predicted date, since the luteal phase (after ovulation) is typically more consistent than the follicular phase.
Why might my actual period differ from the prediction?
Irregular cycles, hormonal contraception, pregnancy, stress, and various medical conditions can all shift timing. This tool gives a planning estimate, not a guarantee — the further your cycle varies from your average, the less precise the prediction.
Does this calculator work if my cycles are irregular?
It will still produce an estimate based on your average, but the less regular your cycles are, the wider the real margin of error. The cycle length calculator can help you see your actual variability.