Michaelis-Menten Enzyme Kinetics Calculator
Reaction rate, fraction of Vmax and catalytic efficiency from Vmax, Km and substrate concentration.
Results
What this tool does
Enzymes do not speed up indefinitely as you feed them more substrate: every active site is eventually busy, and the rate flattens out. Leonor Michaelis and Maud Menten put numbers on that saturation curve in 1913, and the two constants they introduced — the ceiling rate Vmax and the half-saturation point Km — are still how every enzyme in every textbook is characterised.
Formula
v = Vmax [S] / (Km + [S])
Variables
| Symbol | Meaning | Unit |
|---|---|---|
vm | Maximum rate Vmax | µmol/min |
km | Michaelis constant Km | mM |
ss | Substrate concentration | mM |
VV | Reaction rate | µmol/min |
PC | Share of Vmax | % |
S9 | Substrate for 90% of Vmax | mM |
CE | Catalytic efficiency (Vmax/Km) | — |
Worked example
- Maximum rate Vmax100 µmol/min
- Michaelis constant Km5 mM
- Substrate concentration2 mM
- Reaction rate28.5714 µmol/min
- Share of Vmax28.57 %
- Substrate for 90% of Vmax45.000 mM
- Catalytic efficiency (Vmax/Km)20.0000
Limitations
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
- Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.
Frequently asked questions
What does Km actually tell me?
It is the substrate concentration at which the enzyme runs at exactly half its maximum rate, so a low Km means the enzyme grabs its substrate tightly and saturates early. It is a property of the enzyme and substrate pair, not of how much enzyme you have — adding more enzyme raises Vmax and leaves Km untouched. Reaching 90% of Vmax always takes nine times Km.