How this calculator works
This single-substrate steady-state model describes initial rates when substrate is in excess and product accumulation is negligible. Km has the same concentration units as substrate; Vmax and rate share your selected rate basis. Km is the substrate concentration at half maximum rate. Inhibition, cooperativity and substrate depletion can invalidate the simple curve. Inferring Km from one rate requires Vmax to be known independently.
Formula
v = Vmax [S]/(Km + [S]); [S] = v Km/(Vmax - v); Km = [S](Vmax - v)/v.
Worked example
At [S] = Km = 2 mmol/L and Vmax = 10 micromol/min, v = 5 micromol/min. With [S] = 4 mmol/L, Vmax = 12 and v = 8, Km = 2 mmol/L.
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Frequently asked questions
Does Km always equal binding affinity?
No. Km combines kinetic constants and equals a dissociation constant only under additional assumptions.
Can the rate equal Vmax exactly?
The model approaches Vmax as substrate increases; a finite positive Km never reaches it exactly.
What if Km is unknown?
Select Michaelis constant and provide substrate, initial rate and an independently determined Vmax.