Michaelis-Menten Calculator
Enter Vmax, the Michaelis constant Km, and the substrate concentration to get the enzyme reaction velocity — and see why the rate climbs toward Vmax as substrate rises.
Velocity from three inputs
Enter Vmax, Km, and the substrate concentration and the calculator returns the reaction velocity v = (Vmax × [S]) / (Km + [S]) in µmol/min.
Match your units
Vmax sets the unit of the answer (µmol/min here), and Km and [S] must share the same concentration unit (mM) so the ratio stays dimensionless.
What does the Michaelis-Menten calculator do?
The rate of an enzyme reaction
This Michaelis-Menten calculator turns three measurements — the maximum velocity Vmax, the Michaelis constant Km, and the substrate concentration [S] — into the initial reaction velocity of a single-substrate enzyme. The equation v = (Vmax × [S]) / (Km + [S]) is the cornerstone of enzyme kinetics: it describes how the rate rises steeply when substrate is scarce, then bends over and approaches Vmax as the enzyme becomes saturated. It is the number behind enzyme assays, drug-target characterisation, and any experiment that asks how fast a catalyst works at a given substrate level.
Enter Vmax in µmol/min, Km in mM, and the substrate concentration in mM to get the reaction velocity instantly.
The velocity is the maximum velocity scaled by the fraction of enzyme bound to substrate, which is the substrate concentration divided by the sum of Km and the substrate concentration.
v = (Vmax × [S]) / (Km + [S])The ratio [S] / (Km + [S]) is the fraction of active sites occupied by substrate. When that fraction is small the velocity is far below Vmax; when it nears one the velocity approaches Vmax. Because Km and [S] appear as a ratio, only their relative size matters — what counts is how the substrate level compares to Km.
Suppose an enzyme has Vmax = 100 µmol/min, Km = 2 mM, and you supply [S] = 5 mM.
Build the numerator
Vmax × [S] = 100 × 5 = 500 — the maximum velocity scaled by the substrate level.
Build the denominator
Km + [S] = 2 + 5 = 7 — the Michaelis constant plus the substrate level.
Divide
500 / 7 = 71.4286 µmol/min — the reaction velocity at this substrate concentration.
The velocity always sits between zero and Vmax, and where it lands tells you how saturated the enzyme is. The single most useful landmark is that when the substrate concentration equals Km, the velocity is exactly half of Vmax — that is the definition of the Michaelis constant. Below Km the rate is roughly proportional to substrate, so doubling [S] nearly doubles the velocity; above Km the curve flattens and extra substrate buys less and less speed, until at very high [S] the velocity approaches Vmax and adding more substrate changes almost nothing. In the worked example, [S] = 5 mM is well above Km = 2 mM, so the result of 71.4 µmol/min is already past the half-maximal point and heading toward the 100 µmol/min ceiling. A low Km means the enzyme reaches half speed at low substrate, usually read as tight binding, while a high Km means it needs plenty of substrate before it gets going.
The equation is exact within its assumptions, but those assumptions are worth keeping in mind.
Single substrate, steady state, no inhibition
Michaelis-Menten kinetics describe a single substrate under the steady-state assumption, where the enzyme-substrate complex forms and breaks down at a constant level. It does not model inhibitors, allosteric (cooperative) enzymes, or multi-substrate reactions, which need extended equations. It also gives the initial velocity, before product accumulation or substrate depletion shifts the rate, so keep Km and [S] in the same concentration unit and read the answer as a snapshot at the start of the reaction.