Equilibrium Constant Calculator
This equilibrium constant calculator will help you understand reversible chemical reactions, which are reactions in which both the forward and backward reactions occur simultaneously.
After a certain amount of time, an equilibrium is formed, meaning that the rate of reactants being turned into products is the same as the rate of products being turned back into reactants. At this point, the reaction is considered stable. To determine the state of this equilibrium, the reaction quotient should remain constant. With this tool, you can calculate the value of an equilibrium constant for a reaction while learning how to calculate the equilibrium constant with ease!
Below you can find the reversible reaction and equilibrium constant equations:
a[A] + b[B] ⇌ c[C] + d[D]
K = ([C]^{c} × [D]^{d})/([B]^{b} × [A]^{a}),
where [A] and [B] are the molar concentrations of the reactants, and [C] and [D] are the molar concentrations of the products. To understand those concepts better, take a look at the molarity calculator
Calculating the value of the equilibrium constant for a reaction is helpful when determining the amount of each substance formed at equilibrium as a ratio of each other. The constant doesn't depend on the initial concentrations of the reactants and products, as the same ratio will always be reached after a certain period of time. However, the constant may be influenced by:
 Temperature;
 Solvent; and
 Ionic strength.
If the reaction is still underway, with oscillations between reagents and products, you have to use the reaction quotient calculator instead. However, even if it applies in a different context, it is defined in the same way as the equilibrium constant!
When is equilibrium constant used?
Equilibrium constants are useful if you want to understand biochemical processes such as oxygen transport by hemoglobin or acidbase homeostasis in humans. The changes in acidbase homeostasis are mainly reflected in changes in the arterial and venous blood pH. Doctors will also check the equilibrium constant of transferrin in the blood, as transferrin saturation is a symptom of iron deficiency anemia.
This equation helps explain what will be favored by the equilibrium – the reactants or the products. This can give important information about the nature of the reaction and its mechanism. You will find more on this topic below.
Equilibrium constant equation
The equilibrium constant of a reaction relates to all of the species present in the reaction. However, in this calculator, we assume that there is a maximum of two main reactants and two main products. For the hypothetical reaction:
a[A] + b[B] ⇌ c[C] + d[D]
the equilibrium constant equation has the following formula:
K = ([C]^{c} × [D]^{d})/([B]^{b} × [A]^{a})
The constant K reflect two measurements of quantity:
 K_{c}  represents concentration, molarity, expressed as moles per liter (M=mol/L)
 K_{p}  a function of both reactants and products partial pressure, usually in atmospheres, useful for calculations in the gas phase
If K > 1 – equilibrium favors the products
If K < 1 – equilibrium favors the reactants
If K = 1 – the mixture contains similar amounts of both products and reactants at equilibrium
If you're not sure how to switch from moles to other units and the other way around, take a look at our mole calculator.
Let's calculate the value of the equilibrium constant for a reaction
To give you more insight into how this equation works in practice, we created this example.
You have a mixture of gaseous sulfur dioxide and oxygen, from which you can react to form sulfur trioxide. This is one of the steps in synthesizing sulphuric acid:
2 SO₂ + O₂ ⇌ 2 SO₃
Therefore the equilibrium constant equation for this reaction is:
K = [SO₃]²/([SO₂]² × [O₂])
The reaction mixture is left for a while until an equilibrium is established. The reactants and products have the following concentrations:
 SO₂: 0.03 mol/L
 O₂: 0.035 mol/L
 SO₃: 0.5 mol/L
When you put these numbers into the equation, K is found to be:
K = 0.05²/(0.03² × 0.035)
K = 7.937 × 10³
As K > 1, the equilibrium favors the products.
In our example, the concentrations of reactants and products at equilibrium were provided. We then used this information to calculate the equilibrium constant. But what if you knew the equilibrium constant and the unknown was the initial concentration or coefficient of a component? Well, don't worry! Our calculator works in reverse – so it solves both kinds of problems. Just input all of the data you have, and the results will be computed for you in an instance.
How to calculate the equilibrium constant?
This paragraph mainly focuses on how the equilibrium constant is determined analytically. To calculate the value of the equilibrium constant for a reaction, you need to measure (maybe with our titration calculator) the concentrations of the reactants and/or products. There are both experimental and computational methods for constant evaluation. Among experimental methods, you can find:
 Potentiometry
 Spectrophotometry
 NMR chemical shift
 Calorimetry
Within computational methods, there are:
 Chemical model
 Speciation calculations
 Refinement
 Model selection.
Although you already know how to calculate equilibrium constant, save yourself some time and make use of our calculator!
FAQ
What is an equilibrium constant?
The equilibrium constant, K, determines the ratio of products and reactants of a reaction at equilibrium.
For example, having a reaction a[A] + b[B] ⇌ c[C] + d[D], you should allow the reaction to reach equilibrium and then calculate the ratio of the concentrations of the products to the concentrations of the reactants:
K = ([C]^{c} × [D]^{d})/([B]^{b} × [A]^{a}).
How can I write an equilibrium constant expression?
To write out an equilibrium constant expression, follow these steps:

Calculate the product of the equilibrium concentrations of the products (raised to their coefficients in the balanced chemical equation).

Calculate the product of equilibrium concentrations of reactants (raised to their coefficients in the balanced chemical equation).

Find the ratio of the two products.
For example, the equilibrium constant for the reaction Cl₂ + 2NO₂ ⇌ 2NO₂Cl will be [NO₂Cl]²/([Cl₂] × [NO₂]²).
How can I find the equilibrium constant?
Let's say you have a reaction of synthesis of ammonia: N₂ + 3H₂ ⇌ 2NH₃, where the concentrations are: N₂ = 0.04 mol/L, H₂ = 0.125 mol/L, and NH₃ = 0.003 mol/L.
To find its equilibrium constant:

Write the equilibrium constant equation:
K = ([NH₃]^{2})/([H₂]^{3} × [N₂]^{1}).

Enter the concentration:
K = ([0.003 mol/L]^{2})/([0.125 mol/L]^{3} × [0.04 mol/L]^{1}) = 0.1152 = 1.152 × 10⁻¹.
What changes the equilibrium constant?
The equilibrium constant is changed by temperature, the direction of the writing equation, the stoichiometry of the chemical equation, and concentration units. In contrast, the equilibrium constant does not depend on changes in concentration or pressure or the presence of a catalyst.
What will be the concentration of reactant if the equilibrium constant is 0.03?
1.33 mol/L, assuming that you have a reaction of [A] + [B] ⇌ [C] + [D], and concentration of A is 0.5 mol/L, C is 0.2 mol/L, and D is 0.1 mol/L. To find it:

Write the equilibrium constant equation:
K = ([C] × [D])/([B] × [A]).

Solve for [B]:
[B] = ([C] × [D])/(K × [A]).

Enter data:
[B] = (0.2 × 0.1)/(0.03 × 0.5) = 1.33 mol/L.