A survey of 21 free online buffer calculators and apps found only 2 that correctly calculated pH under conditions where [H[superscript +]] and [OH[superscript -]] cannot be ignored in the ratio term of the Henderson-Hasselbalch equation. Traditional guidelines for when use of formal concentrations is acceptable were found to be flawed. For acidic (pK[subscript a] [less than or equal to] 7) buffers, the use of formal concentrations is valid when F[subscript A]-/K[subscript a] > 20. For alkaline (pKa [greater than or equal to] 7) buffers, the approximation is valid when F[subscript BH]+/K[subscript b] > 20. Robust equations for predicting pH of acidic and alkaline buffers are presented, which could be used with programmable calculators and should be used for online buffer calculators and apps.