Kp Calculator

Q: Does this calculator balance my chemical equation?

No. You must balance the equation yourself and type the correct coefficients. The calculator uses your coefficients as exponents, so a wrong coefficient gives a wrong K p .

Q: How many species can I add?

As many as you need. Click Add Species for each new chemical. There is no limit, so big reactions with five or six gases work fine.

Q: Why did the Include box untick itself when I picked solid?

Solids, liquids, and aqueous species do not belong in a K p expression, so the tool unticks them for you. Tick the box again if your class wants that species shown.

Q: Why do I get an error about partial pressure?

Every included species needs a pressure greater than zero. A blank, a zero, or a negative number stops the math, since you cannot take the log of zero and a zero denominator breaks the fraction.

Q: Can I use decimal or fraction coefficients?

Yes. You can type values like 0.5 or 1.5. The calculator raises the pressure to that power. Just keep them the same as your balanced equation.

Q: What does the bar chart show?

It shows how much each gas adds to log 10 (K p ). Product bars are positive and reactant bars are negative. All the bars added together equal log 10 (K p ).

Q: Why does the tool show log and ln of Kp?

Logs make huge or tiny numbers easier to compare. ln(K p ) is also used in the free energy equation ΔG° = −RT ln K, so you can carry it straight into thermodynamics problems.

Q: Do I have to enter a temperature?

No. Temperature is only used for the K p ⇄ K c conversion. K p itself comes straight from your partial pressures.

Q: Can I calculate Q, the reaction quotient, with this?

Yes. Type in pressures that are not at equilibrium and the same math gives you Q. Compare Q to K p to see which way the reaction will shift.

Q: How do I get Kp for the reverse reaction?

Swap the roles: change every reactant to product and every product to reactant. The new answer is 1 divided by the old K p .

Kp Calculator

Updated Aug 31, 2026 By Jehan Wadia

Introduction

The K_p Calculator finds the equilibrium constant for gas reactions using partial pressures. Type in each chemical, its coefficient, its phase, and its partial pressure. The tool builds the K_p expression, plugs in your numbers, and shows the answer with clear steps.

K_p tells you which side of a reaction wins at equilibrium. If K_p is bigger than 1, products win. If it is smaller than 1, reactants win. The calculator says this for you in plain words.

Solids and liquids are left out of the K_p expression on their own, since pure solids and liquids do not have partial pressures. You can still turn them back on if your teacher wants that. This makes heterogeneous equilibrium problems, like CaCO₃(s) ⇌ CaO(s) + CO₂(g), quick and safe to solve.

You can work in atm, kPa, bar, mmHg, or Pa. Change the unit and every pressure you typed changes with it. The tool also finds Δn (the change in moles of gas), shows log and ln values, and converts between K_p and K_c with the formula K_p = K_c(RT)^Δn when you add a temperature. If you need to work out those concentrations first, the Molarity Calculator and the Mole Calculator pair well with this tool.

Six ready-made reactions, like the Haber process and PCl₅ decomposition, load with one click so you can check your work fast. A bar chart shows how much each gas adds to log₁₀(K_p), and you can copy the full step-by-step solution for your notes or homework.

How to use our Kp Calculator

Enter each chemical in your reaction, its coefficient, its phase, and its partial pressure at equilibrium. The calculator gives you the K_p expression, the K_p value, Δn, unit conversions, and a step-by-step solution.

Quick-Pick Reactions: Click a preset like the Haber Process or PCl₅ decomposition to fill the table with a ready-made example. This is the fastest way to start.

Species Name: Type the formula of each chemical, such as N₂, H₂, or NH₃. This name shows up in the K_p expression. To check the mass behind a formula, use the Molecular Weight Calculator.

Role: Pick Reactant or Product. Products go on top of the fraction, and reactants go on the bottom.

Coefficient (ν): Enter the number in front of the chemical in the balanced equation. It must be greater than 0, and it becomes the exponent on that pressure term. Our Stoichiometry Calculator can help you balance the equation first.

Phase: Choose gas, aqueous, liquid, or solid. Only gases count in K_p, so solids and liquids are dropped for you.

Partial Pressure: Type the equilibrium pressure of each gas. It must be a positive number, and it uses the pressure unit you picked. If you only know moles, volume, and temperature, the Ideal Gas Law Calculator will give you the pressure.

Include in K_p: This box is ticked for gases and unticked for other phases. Tick or untick it if you want to override the default.

Add Species / Clear All / Remove: Use Add Species for another row, the red X to delete one row, and Clear All to start over.

Pressure unit: Choose atm, kPa, bar, mmHg, or Pa. Changing it converts every pressure you already typed. For other unit swaps, try the Unit Converter Calculator.

Known K_c value: Optional. Enter a K_c to convert it into K_p using K_p = K_c(RT)^Δn.

Temperature: Enter the temperature of the reaction. It is needed for any K_p ⇄ K_c conversion.

Temperature unit: Pick K, °C, or °F. The number converts on its own when you switch units. You can also check values with the Celsius to Fahrenheit Calculator or the Temperature Calculator.

Calculate K_p: Click it to see the full answer. Click Reset to go back to the default Haber Process setup.

What Is K_p?

K_p is the equilibrium constant written with partial pressures. It is used for reactions where gases react and reach equilibrium. Equilibrium means the forward and reverse reactions happen at the same speed, so the amounts of each gas stop changing.

The K_p Formula

For a gas reaction:

aA + bB ⇌ cC + dD

K_p = [P(C)^c × P(D)^d] ÷ [P(A)^a × P(B)^b]

Products go on top. Reactants go on the bottom. Each partial pressure is raised to the power of its coefficient from the balanced equation. If you want to see how those powers behave on their own, the Exponent Calculator is handy.

Which Species Count

Only gases go into a K_p expression. Pure solids (s), pure liquids (l), and aqueous species (aq) are left out. Their amounts do not change the pressure of the system. So for CaCO₃(s) ⇌ CaO(s) + CO₂(g), the answer is simply K_p = P(CO₂).

What the Number Tells You

K_p > 1 — mostly products at equilibrium.
K_p < 1 — mostly reactants at equilibrium.
K_p ≈ 1 — a fair mix of both.

K_p only changes when the temperature changes. Adding a catalyst or changing pressure does not change its value.

Δn and Units

Δn is the change in moles of gas:

Δn = (moles of gas products) − (moles of gas reactants)

If Δn = 0, K_p has no units, and the number is the same in atm, bar, or kPa. If Δn is not zero, the units are the pressure unit raised to Δn, like atm² or atm⁻¹. That is why the same reaction can give different K_p numbers in different pressure units. Very small or very large results are easier to read with the Scientific Notation Calculator.

K_p and K_c

K_c uses molar concentrations instead of pressures. The two are linked by:

K_p = K_c × (RT)^Δn

Here R = 0.082057 L·atm·mol⁻¹·K⁻¹ and T is the temperature in kelvin. Because R is tied to atm, K_p must be in atm for this equation to work. When Δn = 0, K_p and K_c are equal. To get concentrations from a stock solution, the Dilution Calculator saves a step.

Common Pressure Units

1 atm = 101.325 kPa
1 atm = 1.01325 bar
1 atm = 760 mmHg (torr)
1 atm = 101,325 Pa

Tips for Getting It Right

Balance the equation first. Wrong coefficients mean wrong exponents.
Use equilibrium pressures, not starting pressures.
Change temperature to kelvin before any K_c work.
Keep every pressure in the same unit.
Skip solids and liquids, even if they show up in the balanced equation.
Round your final answer sensibly with the Sig Fig Calculator, and compare it to a textbook value using the Percent Error Calculator.

Related Chemistry Tools

pH Calculator — for acid and base equilibrium problems.
Buffer Calculator — Henderson–Hasselbalch buffer work.
% Yield Calculator — compare actual and theoretical yield.
PPM Calculator — trace concentration units.
Serial Dilution Calculator — step-by-step dilution series.
Log Calculator — for the log and ln forms of K_p.

Formulas used

Equilibrium constant Kp from partial pressures

K_p = \frac{\prod_i (P_i)^{\nu_i}\ \text{(products)}}{\prod_j (P_j)^{\nu_j}\ \text{(reactants)}}

Change in moles of gas

\Delta n = \sum \nu_{\text{gas products}} - \sum \nu_{\text{gas reactants}}

Kp converted to an atm basis (f = unit-to-atm factor)

K_p(\mathrm{atm}) = K_p(\mathrm{unit}) \times f^{\Delta n}

Kp expressed in bar and in pascals

K_p(\mathrm{bar}) = K_p(\mathrm{atm}) \times (1.01325)^{\Delta n}, \qquad K_p(\mathrm{Pa}) = K_p(\mathrm{atm}) \times (101325)^{\Delta n}

Kc from Kp

K_c = \frac{K_p(\mathrm{atm})}{(RT)^{\Delta n}}, \qquad R = 0.082057\ \mathrm{L\,atm\,mol^{-1}K^{-1}}

Kp from Kc

K_p = K_c\,(RT)^{\Delta n}

Logarithmic forms of Kp

\log_{10} K_p = \frac{\ln K_p}{\ln 10}, \qquad \log_{10} K_p = \sum_{\text{prod}} \nu_i \log_{10} P_i - \sum_{\text{react}} \nu_j \log_{10} P_j

Temperature conversions to kelvin

T_K = T_C + 273.15, \qquad T_K = (T_F - 32)\times\tfrac{5}{9} + 273.15

Frequently asked questions

Does this calculator balance my chemical equation?

No. You must balance the equation yourself and type the correct coefficients. The calculator uses your coefficients as exponents, so a wrong coefficient gives a wrong K_p.

How many species can I add?

As many as you need. Click Add Species for each new chemical. There is no limit, so big reactions with five or six gases work fine.

Why did the Include box untick itself when I picked solid?

Solids, liquids, and aqueous species do not belong in a K_p expression, so the tool unticks them for you. Tick the box again if your class wants that species shown.

Why do I get an error about partial pressure?

Every included species needs a pressure greater than zero. A blank, a zero, or a negative number stops the math, since you cannot take the log of zero and a zero denominator breaks the fraction.

Can I use decimal or fraction coefficients?

Yes. You can type values like 0.5 or 1.5. The calculator raises the pressure to that power. Just keep them the same as your balanced equation.

What does the bar chart show?

It shows how much each gas adds to log₁₀(K_p). Product bars are positive and reactant bars are negative. All the bars added together equal log₁₀(K_p).

Why does the tool show log and ln of Kp?

Logs make huge or tiny numbers easier to compare. ln(K_p) is also used in the free energy equation ΔG° = −RT ln K, so you can carry it straight into thermodynamics problems.

Do I have to enter a temperature?

No. Temperature is only used for the K_p ⇄ K_c conversion. K_p itself comes straight from your partial pressures.

Can I calculate Q, the reaction quotient, with this?

Yes. Type in pressures that are not at equilibrium and the same math gives you Q. Compare Q to K_p to see which way the reaction will shift.

How do I get Kp for the reverse reaction?

Swap the roles: change every reactant to product and every product to reactant. The new answer is 1 divided by the old K_p.

I only know the total pressure and mole fractions. What do I enter?

Find each partial pressure first. Multiply the mole fraction of a gas by the total pressure, then type that value into the row for that gas.

Why is my Kp such a huge or tiny number?

That is normal. Equilibrium constants often run from 10⁻³⁰ to 10³⁰. A tiny K_p means almost no product forms. A huge K_p means the reaction goes nearly all the way.

Why does the Kc answer use R = 0.082057?

That value of R works with pressure in atm, volume in liters, and temperature in kelvin. Because R is tied to atm, the tool always uses the atm version of K_p for the conversion, even if you typed kPa or mmHg.

My back-calculated Kc does not match my textbook. Why?

Check three things: the temperature in kelvin, the sign of Δn, and whether your pressures are true equilibrium values. A small rounding gap is also normal.

What if all my reactants are solids?

Then the bottom of the fraction is 1, and K_p is just the product pressures multiplied together. The tool tells you this under the expression.

Can I save or print the work?

Yes. Click Copy Steps to copy the full plain-text solution, then paste it into your notes, a document, or your homework.

Does changing pressure or adding a catalyst change Kp?

No. Only temperature changes the value of K_p. Pressure changes shift the amounts of gas, but the constant stays the same at that temperature.

Why did all my pressures change when I switched units?

The tool converts them for you so every row stays in one unit. Nothing is lost — 1 atm becomes 101.325 kPa, and the K_p is recalculated on the new basis.

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