Cell viability from replicate wells
Compare cell viability across a dose–response curve.
8 concentrations · 3 replicate wells
IC50 Calculator
FigranPaste concentrations and responses to fit a dose–response curve and calculate the IC50.
- 4PL fit with 95% CI
- IC50, EC50 and pIC50
- Free SVG and 300 DPI PNG
Dose–response curve examples
Explore replicate wells, increasing responses and independent experiments.
Ryegrass root growth
Fit a sigmoidal dose response curve for ryegrass root inhibition; compare 4PL and 5PL.
Source: R drc package (ryegrass)
Compare two herbicides
Compare dry matter responses to two herbicides, fitted separately.
Source: R drc package (S.alba)
Cell-based reporter assay
Fit a vinclozolin androgen receptor reporter assay.
Source: R drc package (vinclozolin)
EC50 from an increasing curve
Fit the first run of a DNase ELISA assay.
Source: R datasets (DNase)
Repeat reporter experiments
Fit six vinclozolin experiments independently.
Source: R drc package (vinclozolin)
How to calculate IC50
Move from a concentration series to a fitted curve and an IC50 parameter table.
1Enter your data
Paste or edit concentrations and responses.
2Set units
Choose concentration units and normalize if needed.
3Choose a model
Use 4PL or select 5PL and set justified constraints.
4Review & export
Check diagnostics, confidence intervals and download the results.
The four-parameter logistic (4PL) equation
The IC50 calculator fits a four parameter logistic curve with two plateaus, a midpoint and a Hill slope.
Y = Bottom + (Top − Bottom) / (1 + 10(log₁₀IC50 − X) · Hill slope)
Here X is log₁₀ concentration. Hill slope controls transition direction and steepness. A five parameter logistic model adds an asymmetry factor. See Ritz et al. (2015) for dose–response fitting with R.
- Top: upper plateau
- Bottom: lower plateau
- IC50: midpoint concentration
- Hill slope: steepness and direction
Relative and absolute IC50
Relative IC50 is the concentration halfway between the fitted plateaus. Absolute IC50 is the concentration where the fitted response reaches 50%.
When to use 5PL or fix a plateau
Use 5PL when the response is asymmetric and the data support an extra parameter. Fix a plateau or slope only when it is justified by the assay. Gottschalk and Dunn (2005) compare 4PL and 5PL models.
IC50, EC50 and pIC50
Read the midpoint in the context of the direction and concentration units.
| Value | Meaning |
|---|---|
| IC50 | Midpoint of a decreasing response |
| EC50 | Midpoint of an increasing response |
| pIC50 | −log₁₀(IC50 in mol/L) |
For example, an IC50 value of 1 µM corresponds to pIC50 6.
IC50 calculator FAQ
How do I calculate IC50 from a dose–response curve?
Enter concentrations and responses into the IC50 calculator and fit a 4PL curve. The relative IC50 is the concentration at the midpoint between the fitted plateaus.
What is the difference between IC50 and EC50?
IC50 describes an inhibitory response; EC50 describes an increasing response. This calculator names the midpoint according to the fitted curve direction.
How do I convert IC50 to pIC50?
Take the negative base-10 logarithm of IC50 in mol/L. For example, 1 µM is 10⁻⁶ M, so pIC50 is 6.
Should I use 4PL, 5PL or a three-parameter fit?
Start with 4PL; consider 5PL for a clearly asymmetric response. You can fix a plateau or Hill slope; “3PL” has more than one convention.
Why is my IC50 extrapolated or not determined?
The tested concentrations may not define both plateaus. Measure more concentrations around the transition and plateaus, or fix a plateau when justified.
What is relative versus absolute IC50?
Relative IC50 is the fitted plateau midpoint; absolute IC50 is the concentration at 50% response. The latter is available for percentage responses and may not be reached.
Should I include the zero-concentration control?
Yes, when it belongs to the experiment. Zero is included at the model limit and shown separately on the log axis.
Can I calculate IC50 in Excel or GraphPad Prism, and will the result match?
To fit a four parameter logistic curve in Excel, enter the 4PL equation, provide starting parameters and use Solver to minimize squared residuals. GraphPad Prism also fits 4PL; confidence intervals may differ by method. R² alone does not establish fit quality. This IC50 calculator compares its fits with R drc and keeps your data in the browser.