Equilibrium Constant Calculator (Kc and Kp)
By Santaji GadepatilinUpdated November 6, 2026
The equilibrium constant Kc is the ratio of product to reactant concentrations at equilibrium, each raised to its coefficient in the balanced equation; for gases it is often given as Kp in partial pressures instead. This equilibrium constant calculator works out Kc for aA + bB ⇌ cC + dD and converts it to Kp with Kp = Kc(RT)^Δn.
How to use the equilibrium constant calculator
Enter the equilibrium concentrations and coefficients for reaction aA + bB ⇌ cC + dD. For a reaction with fewer species, set the unused coefficient to 0. Enter the temperature to convert Kc to Kp — the conversion assumes every species counted in Δn is a gas; set the coefficient of any solid or liquid to 0.
Equilibrium constant formulas
Kc = [C]c[D]d / ([A]a[B]b)
Kp = Kc(RT)Δn · Δn = (c + d) − (a + b), R = 0.0820574 L·atm/(mol·K)
Worked example: N₂ + 3H₂ ⇌ 2NH₃
Illustrative equilibrium mixture at 500 °C: [N₂] = 0.50 M, [H₂] = 1.50 M, [NH₃] = 0.10 M:
- Kc = 0.10² ÷ (0.50 × 1.50³) = 0.005926
- Δn = 2 − 4 = -2
- Kp = 0.005926 × (0.0820574 × 773.15)−2 = 1.472e-06
(RT)Δn conversion factors
| Temperature | RT (L·atm/mol) | Δn = −2 | Δn = −1 | Δn = +1 | Δn = +2 |
|---|---|---|---|---|---|
| 298.15 K | 24.465 | 0.001671 | 0.04087 | 24.47 | 598.6 |
| 500 K | 41.029 | 0.0005941 | 0.02437 | 41.03 | 1683 |
| 773.15 K | 63.443 | 0.0002484 | 0.01576 | 63.44 | 4025 |
| 1000 K | 82.057 | 0.0001485 | 0.01219 | 82.06 | 6733 |
Notes
- With pressures in bar use R = 0.0831446 L·bar/(mol·K).
- The reaction quotient Q uses the same expression with non-equilibrium values: if Q < K the reaction moves forward, if Q > K it moves back.
- K is linked to the standard Gibbs energy by ΔG° = −RT ln K — see the Gibbs free energy calculator.
Frequently asked questions
How do you calculate Kc?
For aA + bB ⇌ cC + dD, Kc = [C]^c[D]^d ÷ ([A]^a[B]^b) using equilibrium molar concentrations. Pure solids and liquids are left out.
What is the relationship between Kp and Kc?
Kp = Kc(RT)^Δn, where R = 0.08206 L·atm/(mol·K), T is in kelvin and Δn is moles of gaseous products minus moles of gaseous reactants. When Δn = 0, Kp = Kc.
What does a large K mean?
K much greater than 1 means products dominate at equilibrium; K much less than 1 means reactants dominate. K says nothing about how fast equilibrium is reached.
Does K change with concentration or pressure?
No — only temperature changes K. Changing concentrations or pressure shifts the position of equilibrium, but the ratio returns to the same K.