Enter your Input Value, Substrate Concentration [S], and Michaelis Constant Km into the Enzyme Competitive Inhibition Calculator to convert between Ki and IC50 in either direction — then check your [S]/Km Ratio and Fold Difference to see exactly how substrate competition is shifting your inhibitor's apparent potency.
Calculated Value
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Fold Difference (IC50/Ki)
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[S]/Km Ratio
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Ki to IC50 conversion uses the Cheng-Prusoff equation to interconvert between the inhibition constant (Ki) and half-maximal inhibitory concentration (IC50) for competitive enzyme inhibition. This conversion is essential for comparing inhibitor potencies and analyzing enzyme kinetics data.
The Cheng-Prusoff equation is IC50 = Ki × (1 + [S]/Km), where [S] is substrate concentration and Km is the Michaelis constant. It relates the IC50 measured in competition assays to the true inhibition constant Ki.
Use this calculator when analyzing competitive enzyme inhibition data, comparing inhibitor potencies across different assay conditions, or converting between Ki and IC50 values for pharmacological studies.
IC50 is typically higher than Ki because competitive inhibitors must overcome substrate binding. The higher the substrate concentration relative to Km, the more inhibitor is needed to achieve 50% inhibition, making IC50 larger than Ki.
The [S]/Km ratio indicates how the substrate concentration compares to the enzyme's affinity. When [S]/Km is high, more inhibitor is needed for the same effect, and IC50 becomes much larger than Ki.
The conversion is accurate for true competitive inhibition where the inhibitor and substrate bind to the same site. For mixed or noncompetitive inhibition, this equation may not apply accurately.
All concentrations (Ki, IC50, [S], and Km) should be in the same molar units. The calculator uses molarity (M), but you can input values in any consistent molar unit like μM or nM.
No, this calculator is specifically designed for competitive inhibition. For noncompetitive inhibition, IC50 equals Ki regardless of substrate concentration, so no conversion is needed.