Equilibrium Constant Calculator
The equilibrium constant from equilibrium concentrations — with the Kc to Kp conversion and the free energy it implies.
K is products over reactants, each raized to its coefficient, at equilibrium.
How the equilibrium constant calculator works
K is products over reactants, each raized to its coefficient, at equilibrium. For gases, Kp = Kc(RT)^Δn where Δn is the change in moles of gas.
The magnitude tells you where equilibrium sits: K above about 10³ means the reaction essentially goes to completion, below 10⁻³ it barely proceeds, and in between both are present in meaningful amounts.
Formula: K = [C]^c[D]^d / ([A]^a[B]^b); Kp = Kc(RT)^Δn
Worked examples
| Inputs | Equilibrium constant Kc | Note |
|---|---|---|
| A 1:3 to 2 reaction | 46.2962963 | K = 46.3 — products favored |
| Reactants favored | 0.01 | K = 0.01 |
| Essentially complete | 1,000,000 | K = 10⁶ |
FAQFrequently asked questions
What does the equilibrium constant tell me?
Where equilibrium sits. Above 10³ the reaction is essentially complete; below 10⁻³ it barely proceeds.
What is the difference between Kc and Kp?
Kc uses concentrations, Kp uses partial pressures. They are related by Kp = Kc(RT)^Δn, and they are equal when Δn is zero.
Does a catalyst change K?
No. It speeds both directions equally, so equilibrium arrives sooner at the same place.
What changes K?
Only temperature. Concentration and pressure shift the position of equilibrium but leave the constant alone.
How does K relate to ΔG?
ΔG° = −RT ln K. A K of 10 corresponds to about −5.7 kJ/mol at 25 °C.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- National Institute of Standards and Technology — the US measurement and reference-data authority
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.