What is the equilibrium constant Kc?

Kc (equilibrium constant in terms of concentration) is the ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of its stoichiometric coefficient. For aA + bB ⇌ cC + dD, Kc = [C]^c [D]^d / ([A]^a [B]^b). A large Kc means the reaction favors products; a small Kc means it favors reactants.

What units do I enter for concentration?

Enter concentrations in mol/L (molarity, M). Kc is dimensionless in the thermodynamic sense — each concentration is implicitly divided by the standard concentration of 1 mol/L — so the number you enter is just the numerical molarity value.

Why do pure solids and liquids not appear in Kc?

By convention, the activities of pure solids and liquids are defined as 1 and are omitted from the equilibrium expression. Only dissolved species and gases with variable concentrations appear. This calculator is designed for aqueous or gas-phase species where concentrations vary.

How is Kc different from Kp?

Kc uses molar concentrations (mol/L) while Kp uses partial pressures (atm or Pa) for gas-phase reactions. They are related by Kp = Kc(RT)^Δn, where Δn is the change in moles of gas and R = 0.08206 L·atm/(mol·K). This tool computes Kc only.

What does Kc > 1 or Kc < 1 mean?

When Kc > 1 the equilibrium position favors products (more products than reactants at equilibrium). When Kc < 1 it favors reactants. When Kc ≈ 1 significant amounts of both are present. Extremely large or small values (e.g., 10^10 or 10^-10) indicate the reaction is essentially complete or negligible, respectively.

Can I calculate the reaction quotient Q with this tool?

Yes — if you enter non-equilibrium concentrations, the result is the reaction quotient Q rather than Kc. Compare your Q to the known Kc: if Q Kc it proceeds in reverse; if Q = Kc the system is at equilibrium.

Equilibrium Constant Calculator

Enter product and reactant concentrations with their stoichiometric coefficients to calculate the equilibrium constant Kc.

Reaction Species

Enter equilibrium concentrations (mol/L) and stoichiometric coefficients.

aA + bB ⇋ cC + dD

Products (numerator)

Reactants (denominator)

Enter concentrations and click Calculate

Summary

The Equilibrium Constant Calculator computes Kc for a chemical reaction using the law of mass action: the product of product concentrations raised to their stoichiometric exponents, divided by the product of reactant concentrations raised to their exponents. Add up to four products and four reactants, enter each molar concentration and coefficient, and Kc is calculated instantly in your browser with no data sent to any server.

How it works

Enter the molar concentration (mol/L) and stoichiometric coefficient for each product.
Enter the molar concentration (mol/L) and stoichiometric coefficient for each reactant.
Add more species rows if your reaction has more than two products or reactants.
Click Calculate — the tool applies [Products]^n / [Reactants]^m to compute Kc.
Read the Kc value and its magnitude interpretation (favors products, reactants, or neither).
Click Reset to clear all fields and start a new calculation.

Use cases

Calculate Kc for a homework or exam equilibrium problem.
Verify that a given set of equilibrium concentrations satisfies a known Kc.
Compare the reaction quotient Q to Kc to predict reaction direction.
Explore how changing concentrations affects the equilibrium expression.
Support lab reports that require computing Kc from measured equilibrium data.
Quickly check multi-species reactions with coefficients other than 1.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu