Chemical Equilibrium Constant Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The equilibrium constant Kc of a reaction aA + bB ⇌ cC + dD is calculated with Kc = ([C]^c × [D]^d) ÷ ([A]^a × [B]^b), from the molar concentrations at equilibrium. For ammonia synthesis (N2 + 3H2 ⇌ 2NH3) with [N2]=0.1, [H2]=0.3, and [NH3]=0.4 mol/L, Kc is about 59.3.
Explanation
The equilibrium constant Kc characterizes the final state of a reversible chemical reaction, the point where the forward and reverse reaction rates become equal, which stabilizes the concentrations of all reactants and products. Its numerical value indicates which direction the equilibrium is shifted toward: a Kc far greater than 1 means products dominate heavily at equilibrium (the reaction is nearly complete), while a Kc far below 1 means the starting reactants remain in the majority. Each concentration is raised to the power of its stoichiometric coefficient in the balanced equation — which is why entering these coefficients correctly is just as important as the concentrations themselves: a coefficient error fundamentally changes the result, not just proportionally. Unlike the constant used by our Boyle's law calculator, which describes a gas's physical behavior at fixed chemical composition, Kc describes an equilibrium between different chemical species that transform into each other. Kc's value depends only on temperature (not on starting concentrations or pressure, except for any indirect effect on those concentrations themselves) — one reason it's always given alongside a reference temperature in thermochemical data tables.
Example: ammonia synthesis
Inputs
N2 + 3H2 ⇌ 2NH3. At equilibrium: [N2] = 0.1 mol/L, [H2] = 0.3 mol/L, [NH3] = 0.4 mol/L.
Calculation
Kc = (0.4² × 1⁰) ÷ (0.1¹ × 0.3³) = 0.16 ÷ (0.1 × 0.027) = 0.16 ÷ 0.0027 ≈ 59.26.
Result
This reaction's equilibrium constant, under these conditions, is about 59.3 — the equilibrium clearly favors ammonia formation.
Frequently asked questions
What happens if my reaction has only one product?
Leave D's concentration at its default value (1) and its stoichiometric coefficient at 0: mathematically, any positive number raised to the power 0 equals exactly 1, so this term has no effect on the final result. The calculator then reduces to the single-product formula, Kc = [C]^c ÷ ([A]^a × [B]^b).
Why doesn't Kc depend on the starting concentrations before the reaction?
Kc is a property of the reaction itself at a given temperature, not of the initial amounts of reactants used: whatever the starting concentrations, the system always evolves toward an equilibrium state where the ratio of concentrations raised to their coefficients gives back the same Kc value, as long as the temperature doesn't change. This is precisely what makes Kc useful as a tabulated reference value.
Does the equilibrium constant change if a catalyst is added?
No: a catalyst speeds up how quickly equilibrium is reached, in both directions of the reaction at once, but never changes the position of that equilibrium or the value of Kc itself. Only a change in temperature actually changes a given reaction's equilibrium constant — see our Hess's law calculator to estimate the energy exchanged by a reaction, a factor directly tied to this temperature sensitivity.