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Ki calculator

If you’ve conducted a competition experiment using Monolith or Dianthus and obtained an EC50 value, this calculator helps you determine the dissociation constant (Ki) of the interaction.

In a competitive binding assay, the interaction between an unlabeled target and an unlabeled ligand is measured indirectly by tracking the displacement of a labeled competitor from the target. Typically, this occurs when both molecules compete for the same binding site, though competition can also arise from ligand binding to an allosteric site that disrupts the interaction.

Because the binding affinity is determined indirectly, it is expressed as Ki rather than Kd. The Ki is calculated using:

The known Kd of the labeled competitor-target interaction
The EC50 of competitor displacement
The total concentrations of competitor and unlabeled target in the assay

Use this calculator to quickly and accurately determine Ki from your experimental data.

I. Results from Direct Binding Experiment

1. Enter the Kd between fluorescent competitor and unlabeled target.

II. Results from Competitive Binding Experiment

2. Enter the concentration of fluorescent competitor in the assay.

3. Enter the concentration of unlabeled target in the assay.

4. Enter the measured EC50 value for unlabeled ligand displacing the fluorescent competitor.

Your results

Dissociation constant (Ki)

Understand Competitive Binding Assays: Learn more

FAQs

What does the Ki mean?

Ki (Inhibition Constant) represents the dissociation constant (Kd) between an unlabeled ligand and an unlabeled target. It reflects how tightly the inhibitor binds to the target—lower Ki values indicate stronger binding.

Why am I getting ‘NaN’ or ‘Invalid Input’ as a result?

If the result shows “NaN” (Not a Number), it usually means that the input values do not make sense mathematically or physically. Here are two common reasons why this might happen:

Infeasible Stoichiometry: If your input values suggest an impossible situation—such as an EC50 that is too low given the competitor and target concentrations—then the calculation cannot be completed.
Incorrect Input Values: Ensure that all inputs are positive numbers. If any value is zero, negative, or non-numeric, the formula will fail.