Accurate Period Calculator

Stop relying on generic 28-day assumptions. Enter your real past period start dates for a weighted statistical forecast. If you only have your most recent start date, use our standard period calculator.

Enter Past Period Start Dates

Add 3 to 13 consecutive start dates (most recent first).

Calculated on your device. Nothing is saved.
Key Takeaway: Linear recency weighting ensures that your most recent cycle patterns influence predictions more than older historical data.

How Statistical History Modeling Works

Rather than taking a simple arithmetic mean, our algorithm:

  1. Sorts and deduplicates your entered dates chronologically.
  2. Computes exact integer intervals between consecutive periods.
  3. Applies a linear weight ($1 \dots k$) to prioritize recent cycles.
  4. Calculates standard deviation ($\sigma$) to define a 1-SD ("most likely") and 2-SD ("possible") arrival window.

When to See a Doctor

Online calculators provide general estimates based on biological averages. Consult a healthcare provider (such as an OB/GYN or general practitioner) if you notice:

  • Your periods suddenly become irregular after being consistent
  • Your cycle is consistently shorter than 21 days or longer than 35 days
  • Your period bleeding lasts longer than 7 days or is unusually heavy (soaking through a pad/tampon every hour)
  • You experience severe, debilitating pelvic pain or cramping that disrupts daily life
  • You experience bleeding between periods or after intercourse
  • You are under 35 and have been trying to conceive for 12 months (or 6 months if 35+)
  • Your period has stopped completely for 90 days or more (and you are not pregnant)
Next Logical Step

Explore Cycle Length Trends

View detailed length distributions and bar charts of your cycle history.

Open Cycle Length Calculator

Frequently Asked Questions

Evidence-based answers to common questions about your cycle and fertility.

Why is a multi-date period calculator more accurate?

Standard calculators rely on a single assumed cycle length (like 28 days). By calculating the mathematical intervals and standard deviation between your actual past start dates with recency weighting, this tool captures your true personal biological baseline.

How many past period dates should I enter?

A minimum of 3 consecutive period start dates is required to calculate cycle intervals. Entering 4 to 6 dates provides optimal statistical confidence without outdated history skewing results.

Are my entered dates saved anywhere?

No. All dates are processed in volatile browser memory. When you refresh or navigate away, the form clears completely. No data is sent over the network.

Explore other free, private tools for tracking your menstrual cycle and fertility.

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Medical & Scientific Sources

  1. American College of Obstetricians and Gynecologists (ACOG)(2020). "Using the Menstrual Cycle as a Vital Sign"
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