Equilibrium Constant Calculator
The equilibrium constant calculator computes Kc from equilibrium concentrations for up to two reactants and two products in the general reaction aA + bB = cC + dD. The equilibrium constant K is a fundamental quantity in chemistry that predicts the composition of a reaction mixture once equilibrium is established. It is used in analytical chemistry to calculate species concentrations, in industrial chemistry to optimize reaction conditions, and in biochemistry to understand metabolic pathways and enzyme function. Enter the equilibrium concentrations and stoichiometric coefficients to compute Kc and assess whether the system favors products or reactants.
Equilibrium constant formula
aA + bB = cC + dD
Kc = ([C]^c * [D]^d) / ([A]^a * [B]^b)
Kp = Kc * (R*T)^(delta_n)
delta_n = (c + d) - (a + b) for gases
Interpreting the value of K
- K greater than 10^4: products heavily favored, reaction goes essentially to completion.
- K between 0.01 and 100: significant amounts of both reactants and products at equilibrium.
- K less than 10^-4: reactants heavily favored, very little product forms.
- K = 1: equal amounts of reactants and products at standard concentrations.
Equilibrium constant: frequently asked questions
What is the equilibrium constant?
The equilibrium constant (K) expresses the ratio of product to reactant concentrations at equilibrium. For aA + bB = cC + dD: Kc = ([C]^c * [D]^d) / ([A]^a * [B]^b). Pure solids and liquids are excluded. A large K means products are heavily favored at equilibrium; a small K means reactants are favored.
What is the difference between Kc and Kp?
Kc uses molar concentrations (mol/L). Kp uses partial pressures (atm or bar) and applies to gas-phase reactions. They are related by: Kp = Kc * (RT)^(delta n), where delta n is the change in moles of gas (moles gas products minus moles gas reactants) and R = 0.08206 L*atm/(mol*K).
What is the reaction quotient Q?
Q is calculated the same way as K but using current (not equilibrium) concentrations. If Q less than K: reaction proceeds forward (more products form). If Q greater than K: reaction proceeds backward. If Q = K: system is at equilibrium. Comparing Q and K tells you the direction of shift.
What is Ksp?
Ksp is the solubility product constant for sparingly soluble ionic compounds. For CaCO3(s) = Ca^2+(aq) + CO3^2-(aq): Ksp = [Ca^2+][CO3^2-]. The solid is omitted. Ksp determines the maximum concentration of ions in solution. Adding a common ion decreases solubility; this is the common ion effect.
How does temperature affect K?
Temperature changes K via the van't Hoff equation: d(ln K)/dT = delta H_rxn / (R*T^2). For exothermic reactions (delta H negative), K decreases with temperature (Le Chatelier's principle: heating shifts equilibrium toward reactants). For endothermic reactions, K increases with temperature.
Official sources
- NIST: NIST Chemistry WebBook.
- IUPAC: IUPAC Recommendations on Equilibrium.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.