STEMHub.tools
Equilibrium · Thermodynamics

Equilibrium Constant Kc & Kp Calculator

Build a concentration equilibrium expression, calculate Kc, or convert between Kc and Kp using temperature and the change in gaseous moles.

Formula shown Runs locally Reviewed Aug 11, 2026
Follow us on Google
Equation guide

Ideal concentration and partial-pressure equilibrium expressions at one temperature

Concentration form

Kc = Π[products]ν/Π[reactants]ν

Pressure form

Kp = Kc(RT)Δng

Gas mole change

Δng = Σνg,p − Σνg,rp: products; r: reactants
Chemical equilibrium

Build Kc or connect Kc and Kp

Reactants — denominator
Products — numerator

Enter equilibrium molar concentrations. Coefficients become exponents. Do not include pure solids or pure liquids. Inputs and displayed results use at most three decimal places.

Kc

Build the equilibrium expression from active dissolved or gaseous species.

Using the model

Symbols, assumptions and limitations

Interpretation notes

Concentration values must describe one equilibrium state. Pure solids and pure liquids are omitted from equilibrium expressions. The numerical Kc–Kp conversion uses R = 0.082057 L·atm·mol⁻¹·K⁻¹ and the conventional textbook standard-state treatment.

Worked examples

See the method in practice

01

Hydrogen iodide formation

For H₂(g) + I₂(g) ⇌ 2HI(g), Kc = [HI]²/([H₂][I₂]) and Δng = 0, so Kp and Kc have the same numerical value.

02

Ammonia synthesis

For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), Δng = 2 − 4 = −2; temperature therefore affects the numerical Kp/Kc conversion.

Questions

Frequently asked

What concentrations belong in Kc?

Use equilibrium molar concentrations, not initial concentrations, unless the reaction has not changed them.

Why are solids and liquids omitted?

Their activities are effectively constant in the usual ideal treatment and are absorbed into the equilibrium constant.

Does a large K mean a fast reaction?

No. K describes equilibrium composition and thermodynamic favorability, not how quickly equilibrium is reached.

Sources & review

Equations and examples are checked against the references below. Results are educational and should be independently verified for safety-critical work.

OpenStax Chemistry — Equilibrium Constants

Written by the STEM Hub editorial team · Reviewed August 11, 2026 · Review methodology