About this tool
The Equilibrium Constant & Reaction Quotient Calculator is a free, in-browser chemistry tool that evaluates the mass-action expression for a reversible reaction. Enter up to two reactants and two products, each with a molar concentration and a stoichiometric coefficient, and it returns Kc (using equilibrium concentrations) or Q (using current concentrations), along with the product and reactant terms that make up the ratio.
The expression is K (or Q) = Π[product]^coefficient ⁄ Π[reactant]^coefficient — every product concentration is raised to its coefficient and multiplied together for the numerator, every reactant likewise for the denominator, and the numerator is divided by the denominator. Leave a row's concentration blank to omit that species; a coefficient left blank defaults to 1. Concentrations are in mol/L and must be positive so the ratio stays finite.
Everything runs locally in your browser with no uploads. In Q mode you can also enter a known Kc, and the tool compares them to predict the shift by Le Chatelier's principle: Q < Kc means the reaction proceeds forward toward products, Q > Kc means it runs in reverse toward reactants, and Q = Kc means the system is already at equilibrium.