Calculate an equilibrium constant K from equilibrium concentrations and coefficients, then get log K, the standard free energy change and how far it proceeds.
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Results
Calculated
Equilibrium constant K
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Kc from the values entered
log10 K
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Each unit is a factor of ten
ΔG° at T
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kJ/mol, −RT ln K
Direction favored
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Which side dominates at equilibrium
Ready
Enter equilibrium concentrations and coefficients (0 to skip a species), then press Calculate.
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What this calculator finds
The equilibrium constant K compares products to reactants once a reversible reaction has settled. This calculator builds K from equilibrium concentrations and stoichiometric coefficients for up to two products and two reactants, and then converts it to the standard Gibbs free-energy change at your chosen temperature.
Use it for general chemistry problems, for checking an ICE-table result, or for judging whether a reaction goes to completion. It reports K, its base-10 logarithm, ΔG°, and a plain-language reading of how far the reaction proceeds.
The equations
K = ([C]c [D]d) / ([A]a [B]b) for a A + b B ⇄ c C + d D, using equilibrium concentrations in mol/L.
ΔG° = −R T ln K, with R = 8.314 J/(mol·K) and the result shown in kJ/mol.
K > 1 (log K > 0) means products are favored; K < 1 means reactants are favored.
Worked example
Ammonia synthesis N2 + 3 H2 ⇄ 2 NH3 at 298.15 K with equilibrium concentrations [NH3] = 0.20 M, [N2] = 0.50 M and [H2] = 1.50 M, the default inputs.
K = (0.20)2 / (0.50 × 1.503) = 0.0400 / (0.50 × 3.375) = 0.0400 / 1.6875 = 0.02370. Then log K = −1.625 and ΔG° = −8.314 × 298.15 × ln(0.02370) / 1000 = +9.28 kJ/mol. K lies between 10−3 and 103, so the calculator reports significant amounts of both sides, with reactants slightly favored.
Common mistakes and how to read the result
Using initial instead of equilibrium values. Only equilibrium concentrations give K.
Forgetting the exponents. Each concentration is raised to its coefficient; entering 3 for H2 matters.
Including solids and liquids. Pure solids and liquids have activity 1, so set their coefficient to 0.
Mixing up reversed reactions. Reversing the equation inverts K and flips the sign of ΔG°.
Frequently Asked Questions
Is this Kc or Kp?
It is whatever quantity you enter. Concentrations in mol/L give Kc; partial pressures in atm or bar give Kp. Pure solids and pure liquids are left out, so enter a coefficient of 0 for them.
What if I have more than two products or reactants?
Multiply the extra species together yourself: for a species with concentration c and coefficient n the contribution is c to the power n. You can combine identical-coefficient species by multiplying their concentrations and using the shared coefficient.
Does the value have units?
Strictly, K is dimensionless because activities are used. Textbook Kc values are quoted without units, and this calculator does the same.
What do the values in the boxes have to be?
They must be concentrations at equilibrium. If they are starting concentrations, the same expression gives the reaction quotient Q, which tells you which way the reaction will shift rather than the value of K.
Practical Guide for Equilibrium Constant Calculator
Equilibrium Constant Calculator is most useful when the inputs reflect the situation you are actually planning around, not a best-case estimate. Treat the result as a decision aid: it gives you a structured way to compare assumptions, spot outliers, and decide what to verify next. For Chemistry work, the most important review lens is units, concentration, limiting assumptions, temperature, precision, and significant figures.
Start with a baseline run using values you can defend. Then change one assumption at a time and watch which output moves the most. If one input dominates the result, spend your verification time there first. If several inputs have similar influence, use a conservative scenario and an optimistic scenario to create a practical range instead of relying on a single exact number.
Before acting on the result, verify inputs against lab notes, reagent labels, and the expected reaction or solution model. This is especially important when the calculator supports a purchase, project plan, performance target, or operational decision. The calculator can make the math consistent, but the quality of the conclusion still depends on current data, clear units, and assumptions that match your real constraints.
When the output looks surprising, slow down and inspect each input in order. A small change in one high-leverage field can move the final number more than several low-leverage fields combined. For Equilibrium Constant Calculator, that means you should first confirm the value with the greatest scale, then confirm the value with the greatest uncertainty, then rerun the calculator with conservative and optimistic assumptions. This sequence turns the calculator from a single answer into a practical decision range.
Review Checklist
Confirm every input uses the unit and time period requested by the calculator.
Run a low, expected, and high scenario so the answer has a useful range.
Check whether rounding or a missing decimal place changes the decision.
Update the calculation for every new mixture, batch, reaction, or homework data set.
How to Validate the Result
Use Equilibrium Constant Calculator as a repeatable checkpoint rather than a one-time answer. The safest workflow is to record the original inputs, save the output, and write down which assumption you are testing. Then rerun the calculator with one changed value. If the result changes sharply, that input deserves more attention before you act on the number.
For this topic, the main validation lens is units, concentration, limiting assumptions, temperature, precision, and significant figures. That means a result can be mathematically correct and still be misleading if the inputs come from the wrong time period, use inconsistent units, or mix expected values with best-case values. Keep baseline, conservative, and optimistic runs separate so the final decision is easier to explain later.
When you share the result with someone else, include the assumptions and the date of the calculation. Many calculator outputs become stale after prices, schedules, measurements, or constraints change. A short note about the source of each input makes the calculation auditable and prevents later confusion about why the answer moved.
Label the source for each input before comparing scenarios.
Use the same rounding method across every run.
Flag any input that is estimated rather than measured.
Recalculate for every new mixture, batch, reaction, or homework data set.