Enzyme Kinetics Calculator
Reaction velocity from substrate concentration by the Michaelis–Menten equation — how far below Vmax the enzyme is running, and the substrate needed to reach any fraction of it.
Enzyme velocity rises with substrate concentration but saturates: v = Vmax × [S] / (Km + [S]).
How the enzyme kinetics calculator works
Enzyme velocity rises with substrate concentration but saturates: v = Vmax × [S] / (Km + [S]). At [S] equal to Km the enzyme runs at half speed; at ten times Km it is at 91%; it never quite reaches Vmax.
Km is the enzyme's affinity in disguise — a low Km means it is half-saturated at low substrate. The two parameters are what a Lineweaver–Burk or a direct fit extracts from the lab data.
Formula: v = Vmax [S] / (Km + [S])
Worked examples
| Inputs | Velocity (µmol/min) | Note |
|---|---|---|
| Substrate below Km | 46.1538 | 38% of Vmax |
| At Km | 60 | exactly half |
| Well above Km | 109.0909 | 91% — nearly saturated |
FAQFrequently asked questions
What is the Michaelis–Menten equation?
The rate law for an enzyme with a single substrate: velocity rises with substrate but saturates at Vmax because every active site is busy.
What does Km mean?
The substrate concentration at which the enzyme runs at half Vmax. A low Km means high affinity — the enzyme is saturated at low substrate.
Why does the velocity never reach Vmax?
Because there is always some fraction of free enzyme. Ten times Km gives 91%; a hundred times gives 99%.
How are Vmax and Km measured?
From velocity at several substrate concentrations, by non-linear fit or a Lineweaver–Burk plot. Modern practice fits directly; the double-reciprocal plot distorts the errors.
What is catalytic efficiency?
kcat over Km — how well the enzyme works at low substrate. Vmax over Km here is the same idea without the enzyme concentration.
Where these figures come from
- NCBI Bookshelf — Molecular Biology of the Cell — growth kinetics and molecular conventions
- Wellcome Sanger Institute — a leading UK genomics institute
Last checked: September 2026. Formulas are the standard textbook forms; assumptions are stated on each page because they are where these models break.