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    Sample Size CalculatorBeta

    Design studies with evidence, not guesswork.

    Turn your research question, study design, and assumptions into a transparent sample-size plan you can explain.

    Plan my studySee how it works

    Guided method selection, visible assumptions, and one calculation engine shared across StatWisely products.

    Study signal
    Illustrative model
    Required sample
    64per group
    128total participants
    020406080100050100150200250Sample size per groupPower (%)80%n/group = 64
    Standardized effect
    Target power

    Engine-derived example for a two-sided, two-sample t test with equal allocation and α = 0.05. Use the calculator for a design-specific result.

    Example design
    Two independent means
    Significance
    0.05
    Allocation
    1 : 1
    Relationship
    Smaller expected effects require larger studies.

    A defensible sample starts with the question.

    StatWisely separates the decisions that define your design, shows what each assumption changes, and carries the result into the correct calculation workspace.

    1. Describe the research question

      Start with what you want to estimate, compare, or predict—not with the name of a statistical test.

    2. Make the assumptions visible

      Set the design, outcome type, effect, precision, power, allocation, and expected attrition with guidance at each decision.

    3. Continue with a reasoned result

      See the required sample, recruitment adjustment, sensitivity curve, assumptions, and the method that produced them.

    One calculation truth, wherever you work.

    The web calculator and StatWisely desktop application are designed around the same reusable Python calculation package, keeping formulas and assumptions aligned.

    Explore the methods guide
    Study definitionDesign + assumptions
    Shared scientific corePython calculation package
    Planning outputSample + sensitivity + explanation

    Choose the question. We will guide the method.

    Go directly to a workspace when you know what you need, or start with the guided calculator when you do not.

    Estimate with precisionPlan confidence intervals for means, proportions, correlations, epidemiological measures, regression, and survival estimates.Power a comparisonCalculate sample size for tests of means, proportions, association, regression, survival, and non-inferiority.Check achieved powerEvaluate the power available from an existing or constrained sample size.

    Make the study design explainable before recruitment begins.

    Start with guided questions, then inspect every assumption behind the resulting sample-size plan.

    Start guided planning