Average of Three Numbers Calculator

Formula

Arithmetic mean = (First+Second+Third) ÷ 3

Worked example

With First: 10, Second: 20, Third: 30, arithmetic mean is 20.

Calculation method

The rule is Arithmetic mean = (First+Second+Third) ÷ 3.

For this average of three numbers example, changing first from 10 to 11 while keeping the other inputs fixed changes arithmetic mean from 20 to 20.333333.

Reading arithmetic mean

Arithmetic mean adds and divides; weighted mean accounts for assigned weights; geometric and harmonic means suit multiplicative changes and rates respectively. Range is largest minus smallest. In the worked values on this page, first 10, second 20, third 30 produce arithmetic mean 20.

Practical use

Use these summaries for small sets of measurements, scores, rates, or repeated results. The specific task here is to find arithmetic mean from first, second, third.

Limits and common mistakes

Choose the summary that fits the question. An average can hide important differences between the original observations. Verify first before interpreting arithmetic mean; a value measured on another basis would answer a different question.

Why the relationship works

Different summary measures preserve different aspects of the inputs. Arithmetic mean balances additive values, geometric mean describes multiplicative factors, harmonic mean can summarize equal-distance rates, RMS emphasizes magnitude, and range uses only the extremes. Choose the measure from the question, not from which result looks most familiar. The average of three numbers rule uses first, second, third in Arithmetic mean = (First+Second+Third) ÷ 3; read each symbol as the field named on this page.

Interpreting another set of values

For average of three numbers, the fields labeled first, second, third are the known quantities. The calculator solves for arithmetic mean using Arithmetic mean = (First+Second+Third) ÷ 3. The output summarizes the entered observations; it does not preserve every detail of them. Keep the original values alongside a mean or spread measure, especially when only three observations are used. When comparing two calculations, keep their units and input definitions identical.

For a second average of three numbers check, keep the other default inputs fixed and change first from 10 to 12. Arithmetic mean changes from 20 to 20.666667. This is a comparison of two particular cases. Repeat it near the values relevant to your task; do not assume that every formula changes at a constant rate.

Checks and limits

For three values, inspect all three original observations as well as the summary. The same mean can describe very different spreads. A range can change when only an extreme changes, while the mean can change when any observation changes. These three-number examples demonstrate formulas but do not establish a population pattern. The reported arithmetic mean is 20 in the worked example. Keep its stated unit or counting convention when checking it against another calculation.