Michaelis-Menten Km Calculator

The Michaelis constant Km is the substrate concentration at which an enzyme-catalysed reaction proceeds at half its maximum velocity (Vmax). It is an inverse measure of an enzyme's affinity for its substrate: a low Km means the enzyme reaches half-maximal speed at a low substrate concentration, indicating high affinity. Km has the same units as substrate concentration, commonly micromolar or millimolar.

The Michaelis-Menten equation describes the rate of many enzyme reactions: v = (Vmax times [S]) divided by (Km + [S]), where v is the reaction velocity, Vmax is the maximum velocity, [S] is the substrate concentration, and Km is the Michaelis constant. The equation produces a hyperbolic curve that rises steeply at low substrate and levels off toward Vmax at high substrate.

Rearranging the Michaelis-Menten equation gives Km = [S] times (Vmax minus v) divided by v. This lets you find Km from a single measured velocity, the substrate concentration at which it was measured, and a known Vmax. The result is valid only when the measured velocity is below Vmax, since the reaction can never exceed its maximum rate.

Vmax is the theoretical maximum rate reached only at saturating substrate, so any real measured velocity must be less than Vmax. If you enter a velocity equal to or greater than Vmax, the formula would give a Km of zero or a negative value, which has no physical meaning. The calculator flags these cases rather than returning a misleading number.