Using an unbalanced equation
Coefficients become exponents, so an unbalanced reaction gives the wrong expression even when the arithmetic is clean.
Equilibrium Constant Calculator for Kc, Kp and Q
Inputs
Paste balanced reaction
Use <=>, ⇌, or =. Phases like (g), (aq), (l), and (s) are detected automatically.
Used only when comparing Q with a known K.
Enter equilibrium concentrations for dissolved species and gases. Pure solids and pure liquids are omitted from Kc and Qc expressions.
Balanced reaction table
For Kc, enter molar concentrations for included species. Phases (s) and (l) are omitted automatically.
| Species | Side | Coeff. | Phase | Value | Unit | Initial | Change | Equilibrium | Row |
|---|
For included Kc species, equilibrium concentration is calculated from the ICE columns before evaluating the expression.
Positive x assumes forward progress: reactants decrease by coefficient × x and products increase by coefficient × x. You can also type each change manually.
Results
Primary result
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Expression used
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Included and omitted species
Interpretation
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Calculation steps
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Formula summary
| Quantity | Formula | Use |
|---|---|---|
| Equilibrium constant in concentration form | Kc = [products]^coeff / [reactants]^coeff | Use equilibrium molar concentrations for included species. |
| Equilibrium constant in pressure form | Kp = (Pproducts)^coeff / (Preactants)^coeff | Use gas partial pressures only. |
| Reaction quotient | Q = same expression as K, using current values | Compare with K to predict net direction. |
| Kc to Kp conversion | Kp = Kc(RT)^Δn | For gas-phase stoichiometry, with Δn = gas products − gas reactants. |
This page uses the standard textbook approximation that activities can be represented by molarity for solutes or partial pressure for gases. Pure solids and pure liquids are treated as constant and omitted from the equilibrium expression.
Worked examples
| Case | Balanced reaction | Inputs | Result |
|---|---|---|---|
| Ammonia synthesis at equilibrium | N2(g) + 3H2(g) ⇌ 2NH3(g) | [N2] = 0.200 M, [H2] = 0.300 M, [NH3] = 0.500 M | Kc ≈ 46.3 |
| Reaction direction check | N2(g) + 3H2(g) ⇌ 2NH3(g) | Current values: [N2] = 0.400 M, [H2] = 0.800 M, [NH3] = 0.200 M, known Kc = 46.3 | Qc ≈ 0.195, so the reaction tends to move toward products. |
| Heterogeneous equilibrium | CaCO3(s) ⇌ CaO(s) + CO2(g) | CO2 partial pressure = 0.250 atm | Kp = 0.250; both solids are omitted. |
The calculator assumes your reaction is already balanced. Stoichiometric coefficients become exponents in the equilibrium expression.
How this calculator handles phases and direction
For Kc or Qc, the tool includes aqueous species and gases, while pure solids and pure liquids are omitted. You can enter concentrations in M, mM, or uM; values are converted to mol/L before calculation. For Kp or Qp, only gases appear in the expression, and pressure inputs in atm, bar, kPa, or torr are converted to atm.
When you choose a quotient comparison, the page evaluates the same expression as K but uses the current values you entered. If Q < K, the system tends to move toward products; if Q > K, it tends to move toward reactants; and if Q = K, the system is already at equilibrium within numerical tolerance.
Reading a plain K result: K >> 1 means products are favored, K ≈ 1 means both sides may be present in meaningful amounts, and K << 1 means reactants are favored. For a fixed balanced reaction, the equilibrium constant changes mainly with temperature.
Balanced equation required
Coefficients become exponents
Pure solids/liquids omitted
Q < K: forward shift
Q > K: reverse shift
Common equilibrium constant mistakes
Including pure solids or liquids
Pure solids and pure liquids usually have constant activity and are absorbed into K. Do not include them unless your convention explicitly says to.
Mixing Kc and Kp inputs
Concentration quotients use concentration-style values. Pressure quotients use gas partial pressures. The unit selectors convert within each mode, but they do not turn Kc into Kp without the conversion mode.
Using Q values as equilibrium values
Q can be calculated from any current mixture. K must be calculated from equilibrium values at the stated temperature.
Forgetting temperature dependence
Changing the starting amounts changes Q and the position the system moves from. Changing temperature is what changes K for a given reaction.
FAQ
When should I use Kc instead of Kp?
Use Kc when your data are concentrations for gases or aqueous species. Use Kp when the included species are gases and your data are partial pressures.
How do I write an equilibrium expression?
Put included products in the numerator and included reactants in the denominator. Raise each species term to its coefficient from the balanced equation.
Does an equilibrium constant have units?
Thermodynamic equilibrium constants are dimensionless because they use activities. Introductory chemistry often computes concentration or pressure quotients and reports the numerical K without units by convention.
Why do catalysts not change K?
A catalyst speeds the forward and reverse paths but does not change the equilibrium composition at a fixed temperature, so it changes the time to equilibrium rather than the value of K.
What changes the equilibrium constant?
Temperature is the main condition that changes K for a given reaction. Changing concentration, pressure, volume, or adding a catalyst can change Q or the rate, but not K at the same temperature.
What do K > 1 and K < 1 mean?
A large K means products are favored at equilibrium. A small K means reactants are favored. A K near 1 means neither side overwhelmingly dominates.
Does the equation need to be balanced?
Yes. The coefficients in the balanced equation become exponents in the equilibrium expression.
Why is water sometimes omitted?
Water is omitted when it is a pure liquid or the solvent because its activity is effectively constant. Include water when it is treated as a gas or as a non-solvent reacting species under your course convention.
Does this solve ICE tables or equilibrium concentrations from a starting mixture?
The optional ICE helper calculates equilibrium concentration from an initial row plus a change row, including known x progress. It does not solve nonlinear equilibrium-composition problems from K alone.
Is this suitable for graded homework or lab method verification?
It is useful for checking expressions and arithmetic, but it does not replace your course conventions, activity-based thermodynamics, or any validated laboratory method.
Reviewed and sources
Last reviewed: June 9, 2026 by Starlight Tools editorial review. This page is designed as an educational calculator for introductory chemistry conventions, not as a substitute for a course rubric or laboratory method.
Assumptions and limits
This page assumes the reaction is already balanced and uses the common chemistry approximation of activities by molarity or partial pressure. It does not infer stoichiometry, handle activity coefficients, ionic-strength corrections, fugacity, solvent standard states, or solve full equilibrium-composition problems. For assessed, research, or regulated work, verify the exact convention your source uses.