Stoichiometry calculator: limiting reagent and yield
Stoichiometry calculator with automatic balancing, molar masses, limiting reagent, excess reactants, theoretical yield and actual percent yield.
Enter an equation, then reactant masses such as H2=4; O2=40. Choose the target product and optionally enter the actual product mass in grams.
Results
Calculation details
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How does this tool work?
Stoichiometry turns the coefficients of a balanced chemical equation into quantitative relationships between reactants and products. This tool reuses Bethemesh's equation balancer and molar-mass parser to move from grams to moles, identify the limiting reagent and calculate theoretical yield.
The entered equation is balanced first using the smallest positive whole-number coefficients. These coefficients define the molar ratios of the reaction.
Each reactant mass is converted to moles from its molar mass. Comparing n/ν across reactants gives the maximum reaction extent and therefore the limiting reagent or tied limiting reagents.
The reaction extent determines theoretical product moles and mass. If an actual product mass is supplied, percent yield is actual mass divided by theoretical mass times 100.
How to use this tool
Balance the reaction
Enter the chemical equation using reactant and product formulas. The calculator first applies element conservation and returns the smallest compatible whole-number coefficients.
Convert every reactant to moles
Enter the available mass of each reactant. Molar mass is calculated from the formula, then grams are converted to amount of substance before any stoichiometric comparison is made.
Find the limiting reagent and theoretical product
The engine compares n/ν for all reactants. The smallest ratio fixes the maximum reaction extent, the amount of target product that can form, and the reactants left in excess.
Compare with the actual yield
If you know the isolated product mass, enter it to calculate percent yield. Interpret the gap with possible losses, incomplete conversion, side reactions, impurities or measurement error in mind.
Good to know
The limiting reagent sets the production ceiling
The reactant with the smallest mass is not necessarily limiting. Amounts must first be converted to moles and divided by their stoichiometric coefficients. The smallest n/ν ratio tells you how many complete reaction extents the available mixture can support.
Theoretical yield and percent yield answer different questions
Theoretical mass follows only from the balanced equation and limiting reagent. Percent yield compares the actually isolated mass with that ideal maximum, so it describes experimental performance rather than whether the chemical equation itself is correct.
A balanced equation still has to be the right chemistry
Mathematical balancing does not prove that the entered reaction is chemically realistic. The calculator does not predict products, equilibrium, kinetics or competing pathways; it quantifies the stoichiometry of the equation you provide.
Stoichiometric principle
The calculation combines element conservation, molar masses and reaction extent. For reactant i with amount nᵢ and stoichiometric coefficient νᵢ, nᵢ/νᵢ measures how many reaction extents that reactant can supply.
Reaction extent ratio
ξ = nᵢ / νᵢ
The smallest reactant ratio sets the maximum reaction extent.
This is a stoichiometric model. It assumes the entered equation is correct and does not model kinetics, equilibrium, side reactions or real-world reagent purity.
Use cases
Chemistry homework and exam preparation
Check a complete solution without skipping the reasoning chain: balancing, molar masses, grams-to-moles conversion, limiting reagent and theoretical product mass. This makes it easier to locate the exact step where a hand calculation went wrong.
Planning a laboratory reaction
Compare the quantities actually available before starting an experiment and predict which reactant should be consumed first. The theoretical product mass also gives a useful scale for choosing glassware and evaluating the final isolated mass.
Evaluating experimental yield
Use the measured product mass to calculate percent yield and separate the ideal stoichiometric limit from experimental performance. Low yield can reflect incomplete reaction or handling losses; values over 100% usually call for checks of purity, drying and weighing.
Checking excess reagent
Estimate not only the limiting reagent but also how many moles and grams of each excess reactant should remain once the limiting reagent is consumed. This is useful when an excess is deliberately used to drive a reaction or simplify work-up.
Examples
Forming water
The equation is balanced automatically to 2 H2 + O2 → 2 H2O.
Input
H2 + O2 -> H2O; H2=4; O2=40; product H2OResult
H2 is limiting; theoretical water mass is calculatedExperimental yield
A value above 100% can indicate impurities, moisture or inconsistent measurements.
Input
Add an actual product massResult
Percent yieldTips and best practices
- Use formulas supported by the chemical parser, such as Fe2O3, H2SO4 or Ca(OH)2.
- Separate masses with semicolons and use the exact reactant formulas from the equation.
- The calculation assumes the written reaction stoichiometry and does not model side reactions.
- If two reactants have nearly identical n/ν ratios, both may be stoichiometrically limiting within the precision of your data; avoid deciding from heavily rounded values.
- Keep extra significant digits during intermediate conversions and round mainly at the end so that molar-mass and yield calculations do not accumulate avoidable rounding error.
Sources and references
- OpenStax, 4.3 Reaction Stoichiometry (opens in a new tab), Chemistry 2ePage in English
- IUPAC, stoichiometry (opens in a new tab), Compendium of Chemical Terminology (Gold Book)Page in English
- CIAAW, Abridged Standard Atomic Weights 2024 (opens in a new tab), Commission on Isotopic Abundances and Atomic WeightsPage in English
Frequently asked questions
Does the calculator balance the equation?
Yes. Balancing happens before the stoichiometric calculation.
How do I enter several reactants?
Use entries such as H2=4; O2=40, with masses in grams.
How is the limiting reagent found?
For each reactant, available moles are divided by its stoichiometric coefficient and the smallest ratio limits the reaction.
Can percent yield exceed 100%?
The input can produce such a value, but experimentally it usually warrants checking purity, moisture and measurements.
Why is the reactant with the smallest mass not always limiting?
Because masses must be converted to moles and then compared with the reaction coefficients. Different substances can have very different molar masses and stoichiometric requirements.
Does the calculation account for reagent purity?
Not automatically. If a reagent is impure, enter the effective mass of the reactive substance or correct the available amount before using the calculator.
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