Equilibrium Constant Calculator

At chemical equilibrium, forward and reverse reaction rates are equal, and the equilibrium constant Keq captures the resulting balance as a ratio: product concentrations raised to their stoichiometric coefficients, divided by reactant concentrations raised to theirs — Keq = [P1]^a × [P2]^b / ([R1]^c × [R2]^d). This calculator supports up to two products and two reactants; a second species with its coefficient left at 0 is simply excluded from the ratio (raised to the zero power effectively drops out), so single-product or single-reactant equilibria work the same way as more complex ones. The coefficients must match the balanced equation exactly, since they're exponents — getting a coefficient wrong doesn't shift the answer by a small amount, it changes the order of magnitude. From Keq, the calculator derives ΔG° = -RT·ln(Keq) at the specified temperature (298.15 K, or 25°C, by default), connecting the equilibrium position to Gibbs free energy: a large Keq (products favored) corresponds to a negative ΔG°, meaning the forward reaction is thermodynamically favorable as written.

It also classifies the equilibrium position into a plain-language label — strongly product-favored, product-favored, reactant-favored, or strongly reactant-favored — based on how far Keq sits from 1. One easy mixup: the concentrations entered here should be equilibrium concentrations, not initial concentrations before any reaction occurred; feeding in starting concentrations instead of the values actually present once the system has settled will produce a meaningless Keq. Also remember Keq itself is dimensionless in the strict thermodynamic definition but is commonly reported this way (Kc, from raw molarities) for practical lab use, and its true value still depends on temperature even though this simplified model treats concentrations and Keq as directly computed from each other.