Reaction Completion Calculator
Reaction completion, or percent conversion, measures how much starting substance has reacted: conversion = (amount reacted ÷ initial amount) x 100. It is not the same as percent yield, which measures how much of the possible product you obtained. A reaction can hit 100% conversion while still giving a mediocre yield if side reactions consumed some of the material.
What Reaction Completion (Extent of Reaction) Measures
Reaction completion, often called extent of reaction or percent conversion, tracks how much of a starting substance has been used up. The calculator above applies one formula: conversion (%) = (amount reacted ÷ initial amount) x 100, with the remainder reported as amount remaining = initial amount minus amount reacted. Enter 1.00 mol initial and 0.72 mol reacted and it returns 72.0% conversion with 0.28 mol left unreacted.
Conversion answers a narrow question: what fraction of the substance you started with is gone? It says nothing about what that consumed material became. A substrate can be fully converted and still produce a disappointing amount of the desired product if a competing pathway diverted some of it elsewhere. Conversion tracks the reactant side of a reaction; percent yield tracks the product side. For the full mechanics of percent yield itself, see the percent yield calculator on the homepage.
Conversion is usually measured against whichever reactant is tracked as the limiting reactant, since that is the substance whose depletion caps how much product the reaction can form. If you are working from a reagent present in surplus, see how leftover material is handled on the excess reactant page instead.
Conversion vs Percent Yield: Two Different Questions
Conversion asks how much starting material disappeared. Percent yield asks how much of the wanted product you actually collected, compared against the maximum the stoichiometry allows. These only match exactly when a reaction is perfectly selective, meaning every mole of substrate that reacts turns into the target product with no side products and no losses during recovery.
In practice they diverge constantly. A reaction can reach 95% conversion of its substrate while a side reaction, an over-reaction, or a competing mechanism siphons off a third of that converted material into byproducts, leaving percent yield far below 95%. The reverse is less common but possible in multi-step processes: a low single-pass conversion paired with efficient recycling of unreacted starting material can still deliver a strong overall product recovery. Because yield and conversion measure different things, a related but distinct figure worth knowing is yield-based efficiency, which expresses actual yield against the theoretical maximum rather than against the starting material consumed. None of these numbers substitute for one another; each is diagnosing a different stage of the process.
Worked Examples of Percent Conversion
Example 1: straightforward lab-scale conversion. A student starts with 1.00 mol of substrate and measures 0.72 mol reacted after the reaction is worked up. Conversion = (0.72 / 1.00) x 100 = 72.0%, with 0.28 mol of starting material recovered unreacted. This is a typical single-pass result for a reaction that has not been pushed to completion.
Example 2: an equilibrium-limited reaction. Starting from 0.500 mol of substrate, only 0.310 mol reacts before the system settles at equilibrium. Conversion = (0.310 / 0.500) x 100 = 62.0%, leaving 0.190 mol unreacted. Running the reaction longer under the same conditions will not raise this number further; 62.0% is a genuine equilibrium ceiling, not a sign that the reaction stopped too early or that something was measured incorrectly.
Example 3: a near-complete industrial-style conversion. With 2.00 mol initial substrate and 1.94 mol reacted, conversion = (1.94 / 2.00) x 100 = 97.0%, leaving just 0.06 mol unreacted. Processes engineered with excess reagent, continuous product removal, or favorable equilibrium conditions can approach this kind of near-complete conversion, in sharp contrast to Example 2's 62.0% plateau.
Why Equilibrium Reactions Plateau Below 100% Conversion
A reversible reaction does not run to completion; it runs until the forward and reverse rates become equal, at which point conversion stops climbing regardless of how long you wait. As product accumulates, the reverse reaction becomes increasingly significant, and net conversion of the starting material levels off at whatever value the equilibrium constant and the starting conditions dictate. Example 2 above, plateauing at 62.0%, illustrates exactly this: extending reaction time will not push that number higher on its own.
Le Chatelier's principle describes how shifting conditions can move that plateau. Removing product as it forms, adding an excess of one reagent, or adjusting temperature or pressure for gas-phase equilibria can pull the equilibrium position toward more product and raise the achievable conversion. None of these changes affect whether the reaction is selective, only how far the underlying equilibrium is willing to shift. Conversion driven by equilibrium position is a separate limitation from conversion lost to a slow reaction or an incomplete reaction time, and treating the two the same way leads to the wrong fix.
Conversion and Yield Side by Side
| Conversion | Percent yield | |
|---|---|---|
| What it measures | How much starting material has reacted | How much of the desired product was actually obtained |
| Formula basis | Amount reacted / initial amount | Actual product / theoretical product |
| Meaning of 100% | All of the starting material has been consumed | The maximum possible amount of product was recovered |
| Affected by side reactions? | No, side reactions still consume starting material | Yes, side reactions divert material away from product |
| Affected by recovery losses? | No | Yes, losses during isolation and purification lower it |
The table above is why a substrate can show 100% conversion and a weak percent yield in the same experiment: conversion only cares that the starting material is gone, not where it ended up.
What Remains When a Reaction Isn't Complete
Whatever fraction of the starting substance has not reacted stays in the reaction mixture as unconverted material, calculated simply as initial amount minus amount reacted. In Example 1 above, that is 0.28 mol of substrate still present at the end of the reaction, available for recovery, recycling, or further reaction under different conditions.
Whether that leftover material is a problem depends on context. In a stoichiometric reaction between two measured reactants, unreacted substrate sitting alongside an excess reagent is expected and often intentional; see the excess reactant page for how that leftover portion is tracked separately from the limiting reagent. In an equilibrium reaction, unreacted starting material is a structural feature of the system rather than a fault, since the reverse reaction actively regenerates some of it as fast as it forms. For a wider walkthrough of how conversion, yield, and reactant tracking fit together across a full problem set, the chemistry guide works through related calculations step by step.
Frequently asked questions
What is reaction completion or extent of reaction?
Reaction completion, or extent of reaction, is the fraction of a starting substance that has actually reacted, expressed as percent conversion: (amount reacted ÷ initial amount) x 100. It describes how far a reaction has progressed toward using up its substrate, independent of what that consumed material turned into. A value of 100% means none of the original starting substance remains unreacted.
How is conversion different from percent yield?
Conversion tracks how much starting material disappeared; percent yield tracks how much of the desired product you actually collected against the theoretical maximum. Conversion is a reactant-side measurement, yield is a product-side measurement, and they only agree exactly when the reaction is perfectly selective with no side reactions or recovery losses pulling the two apart.
Can conversion be 100 percent while yield is low?
Yes. If every mole of starting material reacts, conversion is 100%, but if a side reaction or competing pathway converts a portion of that material into an unwanted byproduct instead of the target compound, percent yield of the desired product can still be low. Conversion measures depletion of substrate, not the identity of what it became.
Why does an equilibrium reaction not reach 100 percent conversion?
An equilibrium reaction plateaus below 100% conversion because it never truly stops; forward and reverse reactions continue at equal rates once equilibrium is reached. As product builds up, the reverse reaction becomes significant enough to regenerate some starting material as fast as it forms, capping net conversion at whatever value the equilibrium position allows, such as 62.0% in a typical equilibrium-limited example.
How does Le Chatelier's principle affect conversion?
Le Chatelier's principle predicts how shifting conditions moves an equilibrium's conversion ceiling. Removing product as it forms, adding excess reagent, or changing temperature or pressure for gas-phase reactions can push the equilibrium toward more product, raising achievable conversion. It does not make the reaction more selective; it only changes how far the existing equilibrium is willing to shift toward product.
What does it mean if conversion is very low?
Low conversion means most of the starting material never reacted, which could point to insufficient reaction time, low temperature, a sluggish catalyst, an unfavorable equilibrium position, or a reaction that simply has not been driven forward. Unlike low percent yield, low conversion is specifically about the substrate not being consumed, not about what happened to the material that did react.
How do you calculate percent conversion?
Divide the amount of substance that has reacted by the initial amount of that substance, then multiply by 100: conversion (%) = (amount reacted ÷ initial amount) x 100. For example, 0.72 mol reacted out of 1.00 mol initial gives 72.0% conversion, with the remaining 0.28 mol left as unreacted starting material.
What happens to the substance that has not reacted?
Unreacted starting material simply remains in the reaction mixture, equal to the initial amount minus the amount reacted. Depending on the setup, it may be recovered and recycled, left alongside an intentional excess of one reagent, or continually regenerated by the reverse reaction in an equilibrium system, rather than representing lost or wasted material.
Does conversion apply to reversible reactions only, or all reactions?
Percent conversion applies to any reaction, reversible or not. For reactions that run to completion, conversion can approach 100% as the limiting substrate is fully consumed. For reversible reactions that settle at equilibrium, conversion plateaus below 100% because the reverse reaction becomes significant before all of the starting material is used up.
How can you increase conversion in an equilibrium reaction?
Conversion in an equilibrium reaction can be increased by shifting the equilibrium toward product, following Le Chatelier's principle: continuously removing product as it forms, adding an excess of one reactant, or adjusting temperature or pressure for gas-phase systems. These changes move the equilibrium position; they do not speed up the reaction or improve its selectivity.