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Dose-Response & IC50 (4PL)

Log-dose vs response scatter with an adjustable 4-parameter logistic curve, plus an interpolation-based IC50. The same math as the Excel workbook, live in your browser.

What this is for: Plots a dose-response curve with an adjustable 4-parameter logistic fit, plus an independent interpolation IC50 between the points bracketing 50%.

The formula: response = Bottom + (Top − Bottom) ÷ (1 + 10^((logIC50 − log dose) × Hill slope))

How to use:

  1. Enter the data as one log dose, % response pair per line, with points on both sides of 50% response.
  2. Adjust Top, Bottom, logIC50 and Hill slope until the charted curve tracks your points.
  3. Read logIC50 (interpolated), IC50 in mol/L and IC50 in nM from the results.
  4. Compare those values with the IC50 implied by the curve parameter as a check; if no pair of points brackets 50%, the interpolation has nothing to work from.

Dose-Response & IC50 (4-parameter logistic)

Enter log10 dose (e.g. log10 mol/L) and % response — one log dose, % response pair per line. Adjust the 4PL parameters (Top, Bottom, logIC50, Hill slope) and watch the fitted curve on the chart move — fit by eye until the curve tracks your points. The IC50 below the table is an independent check, computed by linear interpolation between the two points bracketing 50% response.

%
%
logIC50 (interpolated)
IC50 (mol/L, interpolated)
IC50 (nM)
IC50 implied by the curve parameter (mol/L)
Prefer to work offline or keep a copy? The download link is right below.
Download the Dose-Response & IC50 (4PL) workbook (.xlsx)
Notes & limits (from the workbook): The curve is the 4-parameter logistic % response = Bottom + (Top − Bottom)/(1 + 10^((logIC50 − log dose) × Hill slope)) with the parameters you set — the workbook adjusts them by eye against the chart; it does not perform a nonlinear least-squares fit. The reported IC50 is independent of the curve parameters: it is computed exactly as the sheet does, by linear interpolation in log-dose between the two data points bracketing 50% response (helper = x₁ + (y₁−50)/(y₁−y₂)·(x₂−x₁), summed over brackets), then IC50 = 10^logIC50. If no pair of points brackets 50%, the sheet's SUM is 0 and the interpolated IC50 reads 1 mol/L — enter points on both sides of 50% response.