IC50 Calculator | AAT Bioquest (2024)

The Quest Graph™ IC50 Calculator attempts to model an experimental set using a four parameter logistic regression model, so called because it has four key parameters in an equation of the form:

Y = Min +

Max − Min

1 + (




Hill coefficient

This model typically resolves as a sigmoid function, or "S"-shaped curve. For biological promotion, the Hill coefficient of the equation will be (-) negative, with the slope of the curve rising ("S"). For biological inhibition, the Hill coefficient of the equation will be (+) positive, with the slope of the curve falling ("Ƨ").

With regards to IC50, the sigmoid function itself is a special case of the log-logistic distribution, which is part of a broader family of logistic distributions and functions. In contrast to standard logistic distributions, however, a primary distinction of this calculator is that it does not necessitate prior normalization of data, nor does it enforce these boundaries in the modeling of an experimental set. That is to say, in a standard logistic distribution, the response values (Y) range from 0 to 1 probability values. This is typically what is seen in probit/logit analysis and what is commonly used when modeling population survival rates. In this calculator, response values can be any positive real number, which may result in regression models which do not adhere to the 0 to 1 logistic distribution boundaries. This is especially the case where the controls of an experimental set do not make clear the upper and lower bounds of the data. In such cases, it is not uncommon to generate a regression model which extends far into the negative Y-axis or rises exponentially.

Because negative response values are typically incoherent in biological contexts, it is often desirable to analytical restrict regression models to the positive response domain. This can be achieved computational by fixing the minimum response value to zero. This reduces the four parameter logistic model into a three parameter logistic model with the simplified equation as follows:

Y =

With this form, the regression model has a lower-bounds fixed at zero, eliminating any models which may otherwise extend into the negative response domain.

There are two final points to make about this calculator. The first is that the four parameter logistic curve is a symmetric regression model around the inflection point, that is, the IC50. Roughly translated, this implies that the shape of the sigmoid function on one side of the inflection point will mirror that of the other side. In order to model an experimental set which is asymmetric in nature, a five parameter logistic curve is required. Second, it is inadequate to utilize R2 when discussing the goodness of fit of a four parameter regression model. As has been demonstrated in several papers, R2 can be used when describing linear regression models, but fails to capture the degree of noise and variability in non-linear models.

For additional reading, please see the following:

Have a question or a feature request about this tool? Feel free to reach out to us and let us know! We're always looking for ways to improve!


This online tool may be cited as follows


"QuestGraph™IC50 Calculator."AAT Bioquest, Inc.,5Jul.2024,https://www.aatbio.com/tools/ic50-calculator.


AAT Bioquest, Inc. (2024,July5).QuestGraph™IC50 Calculator. AAT Bioquest.https://www.aatbio.com/tools/ic50-calculator.

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IC50 Calculator | AAT Bioquest (2024)


How do I calculate IC50? ›

The simplest estimate of IC50 is to plot x(concentration of extract),y (inhibition of DPPH (%)and fit the data with a straight line (linear regression). IC50 value is then estimated using the fitted line, i.e., Y = a * X + b, IC50 = (50 - b)/a.

How to calculate IC50 in excel? ›

It is possible to calculate IC50 using Excel. The basic equation for IC50 is IC50 = -log10 (x/C), where x is the concentration of your compound and C is the concentration of your inhibitor.

What is considered a good IC50 value? ›

In most cases, the IC50 of the best candidate compound should be lower to 10 micromolar, acceptable for NIH to screen the NCI60 program. In addition, the drug design should think over the related problem about the absorption, distribution, metabolism and excretion in our body.

How do you calculate IC50 for antioxidant activity? ›

Each of the inhibition % value and known concentration plotted on the graph so obtained linear equation y = mx + c to change the value of y with 50 of linear line equation, it will get the value of x as the value of IC50.

How do you calculate IC50 value for antimicrobial activity? ›

To determine the median inhibitory concentration (IC50) value. value is calculated using the linear relation between the inhibitory probability and concentration logarithm according to the method of Sakuma [2]. The IC50 value is expressed as the mean ± standard deviation of three independent experiments.

Is a higher or lower IC50 better? ›

The IC50 value is correlated with drug potency, i.e., the amount of drug necessary to produce the effect—the lower the IC50 value, the more potent the drug (Meyer et al.

What software is used to calculate IC50? ›

As suggested, prism or sigmaplot. GraphPad Prism is the most easy one. It is easy to carry out linear and nonlinear regression analysis and curve fitting to get best graph and IC50 values directly.

What is IC50 percentage? ›

IC50 refers to the concentration of a compound that inhibits 50% of a specific biological activity or response.

How do you calculate IC50 from KD? ›

IC50 = (([Ki]/KD) × [L]) + Ki (i.e., in the format y = mx + c).

What IC50 is considered active? ›

I'd consider IC50 of <100 nM to be active, 101 nM to 300 nM to be moderately active, and >300 nM to be inactive for in silico pharmacophore modeling.

Does IC50 show efficacy? ›

Half-maximal inhibitory concentration (IC50) is the most widely used and informative measure of a drug's efficacy. It indicates how much drug is needed to inhibit a biological process by half, thus providing a measure of potency of an antagonist drug in pharmacological research.

What is the standard type IC50? ›

The IC50 stands for 50% inhibitory concentration, defined as the concentration of the nanoparticles capable of inhibiting 50% of viral replication compared to the virus control.

How do you manually calculate IC50? ›

The simplest estimate of IC50 is to plot x-y and fit the data with a straight line (linear regression). IC50 value is then estimated using the fitted line, i.e., Y = a * X + b, IC50 = (0.5 - b)/a.

How do you predict IC50? ›

IC50 values can be determined in two ways; using curve fitting software, or by a point to point analysis.

How is IC50 expressed? ›

IC50 values are typically expressed as molar concentration.

How to calculate IC50 in R? ›

In this case, you should have enough and linear response data on the either side of the IC50 concentration of your substrate. If so, you can generate regression curve equation in the form "y = mx +b. Then you can assign 50 for y and calculate x-value, which is your IC50.

How to calculate inhibition IC50? ›

You can calculate the IC50 value from the percent inhibition graph by finding the x axis value where the percent inhibition is 50%. The x axis value is the IC50 value.

What is the formula for IC50 to pIC50? ›

pIC50 = -log (IC50*10 -9 ). ... ... Determination of pIC 50 value IC 50 (µM) value of the donepezil and the 33 anticholinesterase alkaloids used in this study were retrieved from literature. pIC 50 values were calculated as -log(IC 50 ) (Selvaraj et al.

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