Stoichiometry Calculator
The Stoichiometry Calculator works out the amounts of every reactant and product in a chemical reaction. Type a chemical equation, balance it automatically, then enter any known amount to find the rest in grams, moles, liters at STP, and particles. It is built for chemistry students, teachers, and anyone solving reaction-quantity problems.
Beyond simple conversions, the tool finds the limiting reagent when you supply two or more reactants, shows how much of each excess reactant is left over, and calculates percent yield against the theoretical maximum. A step-by-step solution explains every part of the math.
How to Use the Stoichiometry Calculator
Enter and balance the equation
Type a chemical equation, using + between compounds and =, →, ->, or => as the arrow. Parentheses for polyatomic ions are supported, such as Ca(OH)2 or Ca3(PO4)2. Click Balance or press Enter to balance it with the smallest whole-number coefficients.
Enter known amounts
Every substance appears with its own input. For each amount you know, type the value and pick the unit — grams, moles, liters (STP), or particles. You can fill in one substance or several.
Calculate the results
Click Calculate. The tool converts your input to moles, applies the mole ratios from the balanced equation, and shows the amount of every substance in all four units in the results table.
Check limiting reagent and yield
Enter amounts for two or more reactants and the limiting reagent is found automatically, along with the excess left for each other reactant. Open Percent Yield and enter your actual yield in grams to compare it with the theoretical maximum.
Features
Automatic Equation Balancing
Balances chemical equations to the smallest whole-number coefficients using Gaussian elimination, from simple reactions to multi-compound equations.
Four-Unit Conversion
Reports every substance in grams, moles, liters at STP (22.414 L/mol), and particles using Avogadro's number (6.022 × 10²³).
Limiting Reagent Detection
With two or more reactant amounts, it identifies the limiting reagent and calculates the excess remaining for every other reactant.
Percent Yield
Compares your actual experimental yield to the theoretical maximum, color-coded for normal, over-100%, and below-50% results.
Step-by-Step Solution
Walks through the full calculation in five steps: identify knowns, convert to moles, find the limiting reagent, apply mole ratios, and convert to all units.
Full Results Table
A single table lists grams, moles, liters (STP), and particles for every reactant and product, with very large or small values in scientific notation.
12 Example Reactions
Load preset reactions — combustion, neutralization, synthesis, decomposition, single and double replacement, photosynthesis, and thermite — in one click.
Molar Mass Display
Each compound shows its molar mass in g/mol from standard atomic masses, so you can check every gram-to-mole conversion.
Polyatomic Group Support
Parses formulas with parenthesized groups like Ca(OH)2 and Ca3(PO4)2, and accepts =, →, ->, and => as the reaction arrow.
Fast Workflow
Press Enter to balance instantly, and use Clear or Reset to start a new equation or clear amounts without retyping.
Frequently Asked Questions
How do you calculate stoichiometry step by step?
Start from the balanced equation, then identify your known amount and convert it to moles. Use the mole ratio between coefficients to find the moles of the substance you want, and convert that back to grams, liters, or particles. This calculator runs all five steps automatically and shows each one in the Step-by-Step Solution panel.
How do you convert moles to grams?
Multiply the number of moles by the molar mass of the substance in g/mol. To go the other way, divide grams by the molar mass. The calculator does both conversions and displays each compound's molar mass so you can verify the result.
How do you find the limiting reactant?
The limiting reactant is the one that runs out first, capping how much product can form. Enter amounts for two or more reactants and the calculator divides each by its coefficient: the smallest ratio is the limiting reagent. It then shows how much of every other reactant is left in excess.
How is theoretical yield and percent yield calculated?
Theoretical yield is the maximum product predicted by the balanced equation from the limiting reactant. Percent yield is the actual amount you obtained divided by the theoretical yield, times 100. Open the Percent Yield panel, enter your actual yield in grams, and the tool reports the percentage; a yield above 100% usually points to impurities or measurement error.
What is the mole ratio in a balanced equation?
The mole ratio is the proportion between the coefficients of two substances in a balanced equation. For example, in 2 H&sub2; + O&sub2; → 2 H&sub2;O the ratio of hydrogen to water is 2:2, or 1:1. The calculator uses these ratios to convert the amount of one substance into the amount of any other.
What does liters (STP) mean?
STP stands for Standard Temperature and Pressure. At STP, one mole of an ideal gas occupies 22.414 liters, so the liter values in the results table assume STP conditions. This is most meaningful for gaseous substances in the reaction.
Why does it say "Could not balance this equation"?
This appears when the equation is chemically impossible to balance — for example an element shows up on one side only — when a formula has a typo, or when the equation is too complex for the solver. Check that every element is present on both sides and that your formulas are spelled correctly.
How accurate are the results, and is my data private?
Calculations use standard atomic masses and show up to four significant figures, with scientific notation for very large or small numbers — accurate for schoolwork and everyday lab estimates. Everything runs in your browser, so no equations or amounts are ever sent to a server or saved between sessions.
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