ConductScienceConductScience

IC50 Calculator

Fit 4PL or 5PL dose-response curves and compute IC50 with confidence intervals. Publication-ready plots, Hill slope, and pIC50. Data never leaves your browser.

Dose-Response CurvesIC50 with CIsHill Slope

Try it out

Load example IC50 data to see the full workflow

Two columns: Concentration and Response (% viability, % inhibition, or raw signal). Replicates at the same concentration are averaged automatically.

  • Determine IC50/EC50 from dose-response experiments
  • Fit 4PL or 5PL logistic models to inhibition or activation data
  • Compare potency across compounds using pIC50 values
  • Generate Hill slope and confidence interval estimates
  • Export publication-ready dose-response curves

Don't use for

  • Time-course kinetic data (use kinetic models instead)
  • Single-dose screening data (need multiple concentrations)
  • Allosteric modulators with bell-shaped dose-response

How Dose-Response Curve Fitting Works

Dose-response analysis quantifies the relationship between drug concentration and biological effect. The standard approach is to fit a sigmoidal (S-shaped) model to the data using nonlinear regression.

The 4-Parameter Logistic (4PL) model is the most widely used: Y = Bottom + (Top − Bottom) / [1 + (x / IC50)^HillSlope]. The four parameters are: Top (maximum response), Bottom (minimum response), IC50 (half-maximal inhibitory concentration), and Hill slope (curve steepness).

This tool uses the Levenberg-Marquardt algorithm, the same method used by GraphPad Prism and R's drc package. It iteratively adjusts all four parameters simultaneously to minimize the sum of squared residuals between the model and your data.

The IC50 is not simply read off the graph at Y=50%. It is mathematically derived from the fitted curve parameters, which accounts for noise in individual data points and produces a more accurate estimate than visual interpolation.

Designing an IC50 Experiment

A well-designed dose-response experiment uses 8–12 concentrations spanning at least 3 log units (e.g., 0.001 µM to 100 µM), with 2–3 replicates per concentration. Half-log dilution series (3.16-fold dilutions) are standard.

Critical requirements: include concentrations that fully define both the top plateau (minimal drug effect) and bottom plateau (maximal drug effect). If the curve plateaus are missing, the fitted IC50 will be unreliable.

Normalize your data: express response as percent of untreated control (Top = 100%) or percent of maximum inhibition (Bottom = 0%). Vehicle-only controls should be included at every plate.

For screening campaigns with many compounds, a 10-point half-log dilution series from 100 µM to 0.003 µM is standard. For lead optimization, use a 12-point series centered around the expected IC50, with tighter spacing (quarter-log) near the inflection.

Frequently Asked Questions

Next Steps

Cell Culture Supplies

Cell culture consumables and reagents for cytotoxicity studies

Browse →

Chemicals & Reagents

MTT, XTT, and resazurin assay kits for viability assays

Browse →

Need Help?

Get a quote on equipment or talk to our team.

Request quote →