How to Calculate IC50 from a Dose-Response Curve
The IC50 is the concentration of a drug that reduces a response (for example cell viability) by half. You get it by fitting a curve to your dose-response data, not by reading it off a bar chart.
1. Plot concentration on a log scale
Doses usually span several orders of magnitude (0.001 to 100 µM). On a log x-axis the response forms an S-shaped (sigmoidal) curve, which is what the fit expects.
2. Fit a four-parameter logistic (4PL) model
Response = Bottom + (Top − Bottom) / (1 + (Concentration / IC50)Hill slope)
- Top and Bottom: the plateaus of the curve.
- IC50: the concentration halfway between Top and Bottom.
- Hill slope: how steep the curve is.
Normalise to your controls (untreated = 100%, fully killed = 0%) before fitting, and use replicates.
3. Report it properly
- Give the IC50 with a 95% confidence interval, for example 0.35 µM (0.28–0.44).
- Show the mean ± SD of each concentration as points with error bars, and the fitted curve as a line.
- Compare drugs on the same axes so differences are visible.
Common mistakes
- Too few concentrations on the steep part of the curve.
- Fitting on a linear x-axis.
- Reporting IC50 values outside the tested range.
The Dose-Response Curve tool fits the 4PL model from your CSV and prints the IC50 in the legend.
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