Kp Calculator

Last updated: June 2026

Use this Kp calculator to organise equilibrium calculations involving gaseous reactants and products. This page helps A-Level and Singapore H2 Chemistry students calculate Kp from partial pressures and understand how Kp is related to Kc.

Key takeaway: Kp is the equilibrium constant written in terms of partial pressures. Gaseous products appear in the numerator, gaseous reactants appear in the denominator, and each partial pressure is raised to the power of its coefficient in the balanced equation.
How to use this calculator
  1. Enter each gaseous reactant and product, together with its coefficient and equilibrium partial pressure.
  2. Leave all three boxes in an optional row blank when that additional gas is absent.
  3. Use the same pressure unit for every partial pressure in one calculation.
  4. For the converter, choose the conversion direction and enter the equilibrium constant, temperature and Δn.
  5. Calculate Δn using gaseous coefficients only: gaseous products minus gaseous reactants.

What is Kp?

Kp is an equilibrium constant expressed using partial pressures instead of concentrations. It is used when the equilibrium system involves gases.

Like Kc, Kp tells us about the position of equilibrium at a fixed temperature. A large Kp value means the equilibrium mixture contains more gaseous products relative to gaseous reactants. A small Kp value means the equilibrium mixture contains more gaseous reactants relative to gaseous products.

Kp expression

For a general gaseous equilibrium:

aA(g) + bB(g) ⇌ cC(g) + dD(g)

The Kp expression is:

Kp = PCcPDd ÷ (PAaPBb)

PA, PB, PC and PD are the equilibrium partial pressures. The powers come from the coefficients in the balanced equation.

Partial pressure and mole fraction

The partial pressure of a gas is the pressure that gas contributes to the total pressure of a mixture. When a question gives mole fraction and total pressure, use:

Pgas = Xgas × Ptotal

The values substituted into a Kp expression must be equilibrium partial pressures. Initial pressures cannot be used unless the equilibrium changes have first been calculated.

How to calculate Kp

First write the balanced equation. Place gaseous product partial pressures in the numerator and gaseous reactant partial pressures in the denominator. Use the balanced coefficients as powers, then substitute the equilibrium values using one consistent pressure unit.

Only gaseous species are included in a Kp expression. Pure solids and pure liquids are not included.

Worked examples

Example 1: Writing a Kp expression

N2(g) + 3H2(g) ⇌ 2NH3(g)
Kp = PNH32 ÷ (PN2PH23)

The partial pressure of NH3 is squared because its coefficient is 2. The partial pressure of H2 is cubed because its coefficient is 3.

Example 2: Calculating Kp from partial pressures

H2(g) + I2(g) ⇌ 2HI(g)

Suppose PH2 = 0.200 atm, PI2 = 0.100 atm and PHI = 0.800 atm.

Kp = (0.800)2 ÷ (0.200 × 0.100) = 32.0

Kp and Kc Converter

Kp and Kc are related by:

Kp = Kc(RT)Δn

Δn is the total coefficient of gaseous products minus the total coefficient of gaseous reactants. Solids, liquids and aqueous species are not counted.

For the Haber equilibrium:

Δn = 2 − (1 + 3) = −2

The value of R must match the pressure unit used for Kp. This calculator uses 0.082057 dm3 atm mol−1 K−1 for atm, 0.08314 dm3 bar mol−1 K−1 for bar and 8.314 dm3 kPa mol−1 K−1 for kPa.

Common exam traps

Trap 1: Do not use total pressure directly in the Kp expression. Use the equilibrium partial pressure of each gas.
Trap 2: Do not forget that balanced coefficients become powers in the Kp expression.
Trap 3: Do not include pure solids or pure liquids in Kp.
Trap 4: When calculating Δn, count gaseous coefficients only.
Trap 5: Use a value of R that matches the pressure unit selected for Kp.
Trap 6: Kp changes only when temperature changes.

Frequently Asked Questions

What does a Kp calculator calculate?

It calculates the equilibrium constant for a gaseous equilibrium using equilibrium partial pressures. It also shows the equation, Kp expression, substitution and step-by-step working.

What is the difference between Kp and Kc?

Kc uses equilibrium concentrations, while Kp uses equilibrium partial pressures.

What is Δn?

Δn is the total coefficient of gaseous products minus the total coefficient of gaseous reactants.

Do I include solids and liquids in Kp?

No. Pure solids and pure liquids are not included in the Kp expression.

Does changing pressure change Kp?

No. Changing pressure may shift the equilibrium position, but Kp itself changes only when temperature changes.

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