Indicator Range
The span of pH over which an indicator's colour visibly changes, roughly one unit either side of its pKa.
Formula / equation
pH = pK_In ± 1
Unit
Dimensionless
In depth
Outside this range one form is present in such excess that its colour masks the other completely. A suitable indicator must have its range lying entirely within the vertical section of the titration curve, otherwise the colour change is gradual and the end point ill-defined.
Examples in the real world
Methyl orange changes between pH 3.1 and 4.4; phenolphthalein between 8.3 and 10.0.