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Home›Period & Fertility›Cycle Length Calculator
Period & Fertility · Evidence-aware tool

Cycle Length Calculator

Calculate individual cycle lengths, average, median and variability from at least two period-start dates.

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    Cycle Length: average & Variability from Start Dates

    Calculate individual cycle lengths, average, median and variability from at least two period-start dates.

    Inputs this tool uses

    • Earlier period start
    • Next period start
    • Optional third start

    What you get

    Average cycle length from observed start dates.

    Average28.0 days
    Observed intervals28

    How to use the Cycle Length Calculator

    1. Enter earlier period start, next period start, optional third start.
    2. Select any method, activity level or option shown on this page, then choose Calculate.
    3. Read the main result together with the supporting values, method, assumptions and limitations below before using the estimate.

    How this calculation works

    Cycle tools use date arithmetic from observed period starts and user-entered cycle assumptions. Estimates are never presented as contraception.

    Method transparency: Results are calculated from the values you enter. Health estimates are educational reference values, not diagnoses or treatment advice.

    Example calculation

    The sample values loaded into the calculator produce 28.0 days. This example exists to show how the output is structured; replace the sample values with your own inputs for your calculation.

    Assumptions and limitations

    • Cycle and ovulation dates are estimates and can vary from cycle to cycle.
    • This tool is not contraception and does not diagnose fertility or pregnancy.

    Sources and review status

    Source set last verified 2026-09-25. No clinician review is claimed unless a real named reviewer is displayed.

    Trying to Get Pregnant? Here’s When to Have SexAmerican College of Obstetricians and Gynecologists · current · verified 2026-09-25Ovulation timing and broad fertile-window contextYour menstrual cycleU.S. Office on Women’s Health · current · verified 2026-09-25Menstrual-cycle and ovulation timing contextPregnancy testsU.S. Office on Women’s Health · current · verified 2026-09-25Home pregnancy-test timing and uncertainty

    Connected tools

    Continue with the closest related calculation without re-entering sensitive values automatically.

    ◌Period Calculator

    Estimate the next period date from recent cycle lengths or the last start date, showing uncertainty rather than a single guaranteed date.

    Open tool →    ◌Period Tracker

    Store observed period dates locally, derive cycle statistics and forecasts on-device, and keep estimates visually separate from observations.

    Open tool →    ◌Cycle Day Calculator

    Calculate cycle day from the last period start date and a target/current date using date-only arithmetic.

    Open tool →
    Medical boundary: AG Health Tools provides educational wellness estimates and private tracking utilities. It does not diagnose conditions, recommend treatment, calculate medication doses, or replace a clinician.
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