IC50 Calculator (Dose–Response Curve Fit)
Fit a four-parameter logistic dose–response curve to your concentration–viability data and get the IC50, Hill slope, plateaus and R². Accepts % viability or raw MTT/XTT absorbance with control and blank.
The IC50 Calculator fits y = Bottom + (Top − Bottom)/(1 + 10^((logIC50 − log x)·Hill)) to concentration–response points by least squares (Nelder–Mead, same model as GraphPad Prism's variable-slope inhibitor fit) and reports the IC50, log IC50, Hill slope, top and bottom plateaus, R² and per-point residuals. Raw MTT absorbances are converted to % viability with your untreated-control and blank wells.
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How to use it
- Paste concentrations and responses (or absorbances with control and blank).
- Read IC50, Hill slope and R²; check residuals.
- Tick the 0–100 % constraint for normalised data to get the absolute IC50.
- Copy the results for your figure legend.
What you enter
- ≥ 4 concentrations spanning the response
- % viability or absorbance per concentration
- Control and blank OD for absorbance mode
What you get
- IC50 and log IC50
- Hill slope
- Top and bottom plateaus
- R² and residuals
Formula
y = Bottom + (Top − Bottom) / (1 + 10^((logIC50 − log₁₀x) · Hill)) | % viability = (OD − blank) / (OD_control − blank) × 100
The four-parameter logistic (4PL) describes a sigmoidal response on a log-concentration axis. The IC50 is the concentration at the midpoint between the fitted plateaus (relative IC50); constraining the plateaus to 0 and 100 % gives the absolute IC50 at exactly 50 % of control.
Example: 9-point cytotoxicity curve
Input: 0.01–100 µM; viability 98 96 91 78 55 30 12 6 4 %
Result: IC50 = 1.18 µM, Hill slope −0.98, plateaus 98.6 % / 2.2 %, R² = 0.9998
The compound halves viability at about 1.2 µM with a standard one-site slope; identical to the SciPy/Prism fit.
Questions
How do I calculate IC50 from an MTT assay?
Convert each absorbance to % viability with (OD − blank)/(control − blank) × 100, plot against log concentration, and fit a four-parameter logistic curve; the IC50 is its midpoint. This tool does all three steps.
Why does my IC50 differ from a straight-line interpolation?
Interpolation between the two points around 50 % ignores the curve's shape and plateaus; the logistic fit uses all points. The calculator shows both so you can compare.
What is a Hill slope?
The steepness of the transition. A magnitude near 1 is typical for one-site binding; steeper or shallower slopes suggest cooperativity, heterogeneity or assay artefacts. Negative values simply mean response falls with concentration.
IC50 vs EC50?
IC50 is the concentration giving half-maximal inhibition; EC50 half-maximal effect (activation). Same model; use the EC50 calculator for increasing responses.
Limitations
- Needs both plateaus defined by data; an IC50 outside the tested range is an extrapolation and is flagged.
- Average replicates or paste each replicate with its concentration; the fit weights all points equally.
- Reports the relative IC50 unless plateaus are constrained.
References
- Sebaugh JL (2011). Guidelines for accurate EC50/IC50 estimation. Pharm Stat 10:128-134.
- Mosmann T (1983). Rapid colorimetric assay for cellular growth and survival. J Immunol Methods 65:55-63.
Related reading
- MTT Assay Protocol: Principles, Calculations & Cell Viability
- How to Calculate Cell Viability: Methods and Formulas
- Cytotoxicity Assay: Overview of LDH, MTT, and ATP-Based Methods
- Cell Viability Assays: Choosing the Right Method
More Cell Biology
- EC50 Calculator (Dose–Response Curve Fit)
- MTT Assay Calculator: Cell Viability % and IC50
- Water Potential Calculator
- Microscope Magnification Calculator
All free tools · By Dr. Zubair Khalid, DVM, MS, PhD.