Michaelis-Menten Calculator
Find the reaction velocity of an enzyme from Vmax, the Michaelis constant Km and the substrate concentration, plus how close it runs to its maximum rate.
How it works
v = Vmax · [S] ÷ (Km + [S])
The Michaelis-Menten equation describes how an enzyme's reaction rate depends on substrate concentration. At low substrate the rate rises almost linearly, but as substrate climbs the enzyme saturates and the rate approaches its maximum Vmax. The Michaelis constant Km is the substrate concentration at which the reaction runs at exactly half of Vmax, so it indexes how tightly the enzyme grabs its substrate — a low Km means high affinity.
Worked example
With Vmax = 100, Km = 25 and [S] = 50: v = 100 × 50 ÷ (25 + 50) = 5,000 ÷ 75 ≈ 66.7, which is about 66.7% of Vmax. Note that at [S] = Km = 25 the rate would be exactly half of Vmax.
Frequently asked questions
What does Km tell me?
Km is the substrate concentration that produces half the maximum velocity. A small Km means the enzyme reaches half-speed at low substrate — high affinity — while a large Km means it needs a lot of substrate to get going.
Why does velocity level off?
As substrate increases, more and more enzyme active sites are occupied. Once nearly all sites are working, adding more substrate cannot speed things up, so the rate plateaus at Vmax — the enzyme is saturated.
Can the reaction ever reach Vmax exactly?
Not quite. The curve approaches Vmax asymptotically as substrate rises toward infinity, so real reactions get very close but never fully reach it. At [S] = 10 × Km the rate is already about 91% of Vmax.
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Educational estimate, not medical advice — verify formulas, units and assumptions for your specific case.