Menstrual Cycle Predictor

Predict the next 6 expected period start dates from a recent period and cycle length. Not medical advice.

Inputs

Result

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General calculation reads

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How Not to Be Wrong
Jordan Ellenberg - math thinking
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The Joy of X
Steven Strogatz - accessible math
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Scientific calculator (Casio fx-991EX)
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How to use this calculator

  • Enter last period start.
  • Enter average cycle length.
  • See next 6 expected dates.

About this calculator

Predicts upcoming period start dates assuming a constant cycle length. Average cycle is 28 days; normal range is 21-35. For irregular cycles use the average over 3-6 months. Not medical advice.

Frequently asked

Average cycle length?+
28 days is the textbook average. Real-world: 21-35 days is normal. Outside that range, consult a provider.
Why is the prediction wrong sometimes?+
Cycles vary by ±2-3 days even in regular cyclers. Stress, illness, travel, age, and hormonal changes shift them.
Is this medical advice?+
No. It's simple date arithmetic. For clinical questions about cycle health, see a provider.
What about perimenopause?+
Cycles become more variable in the years before menopause. This calculator assumes a stable cycle and may not be useful.
How does this differ from ovulation prediction?+
Period prediction is based on adding cycle length. Ovulation prediction subtracts 14 days from the next period.

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