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Two Outcome Expected Value Calculator

Compare two mutually exclusive numeric outcomes using their probability-weighted average and spread. Values can be signed and should be in the same units. The expected value is a long-run model average, and may be a value that never occurs in one trial.

Enter every field in the stated units. Decimal and scientific notation are accepted for non-integer fields; blanks, commas, fractions and non-finite entries are rejected. Zero is accepted only where a field permits it.

Expected value10
Variance2700
Standard deviation51.96152423

How to use this calculator

  1. Identify the quantities required by the model and use the field units.
  2. Replace the example values, then select Calculate or press Enter.
  3. Read the labeled outputs and check the interpretation below.

Formula and interpretation

Mean = pa+(1−p)b; variance = p(1−p)(a−b)²; standard deviation = √variance.

Calculations run in your browser. The previous results are hidden when an input changes or validation fails. This prevents an old answer from being mistaken for the new calculation. Results are displayed to approximately ten significant digits using floating-point arithmetic; that precision does not imply equal precision in measured inputs.

Worked example

Probability of outcome A = 0.25; Value of outcome A = 100; Value of outcome B = -20. Expected value = 10; Variance = 2700; Standard deviation = 51.96152423.

Substitute the example inputs into the stated formula in the same order as the labeled fields. These are the default values shown above, so the explanation and the working interface describe the same case.

Limitations

The two outcomes must exhaust all possibilities. The calculator does not infer probabilities, fees or hidden outcomes. A favorable mean does not eliminate variability or risk.

This is an educational mathematical model. Check the assumptions and units for your situation. Geometry results do not establish material strength, and probability outputs do not predict an individual outcome.

Frequently asked questions

Will one trial return the expected value?

Not generally. Each trial returns one of the two outcomes, while expectation summarizes the probability model.

Why does the calculator reject my entry?

Each field shows its supported range. Integer fields reject fractions; mathematical conditions such as a zero divisor or invalid shape produce a separate explanation. Correct the indicated field or condition and calculate again.

How many digits should I use?

Use enough digits to retain your input accuracy. Round the final answer to the precision justified by the data; tiny residuals around theoretical zero can result from floating-point arithmetic.

Review notes and reference

Reviewed October 3, 2026. The calculator script was tested against the default example and separate boundary cases. Checks covered blank, text, comma-formatted, non-finite and out-of-range entries, plus recovery after errors. Background reference for the mathematical method. Explanations and worked examples are original.

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