Interpolate concentration from absorbance using a four-parameter logistic curve.
The four-parameter logistic curve is sigmoidal, so precision collapses near both asymptotes where a large concentration change produces almost no absorbance change. The reliable region is roughly the central 15 to 85 per cent of the response range. Readings outside the central region should be diluted and repeated rather than reported, because the interpolation error there can exceed the measurement itself.
ELISA Standard Curve
4PL: A = bottom + (top − bottom) ÷ (1 + (C ÷ C₅₀)^slope), solved for C
4PL: A = bottom + (top − bottom) ÷ (1 + (C ÷ C₅₀)^slope), solved for C The four-parameter logistic curve is sigmoidal, so precision collapses near both asymptotes where a large concentration change produces almost no absorbance change. The reliable region is roughly the central 15 to 85 per cent of the response range.
Readings outside the central region should be diluted and repeated rather than reported, because the interpolation error there can exceed the measurement itself.
This calculator takes 5 inputs: Sample absorbance, Bottom asymptote, Top asymptote, Inflection point concentration, Hill slope. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.