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Enter grams, moles, or particle counts for the compound you know — the calculator converts automatically.
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Results list moles and grams for every species in the equation, not just the one you asked about.
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The tool solves the smallest whole-number coefficients using matrix reduction over the atom-count system.
Choose one compound and give its mass, moles, or particle count — plus any other reactant amounts you have.
See mole ratios, converted mass for every species, and which reactant limits the reaction, all in one table.
Stoichiometry is the part of chemistry that turns a balanced equation into a set of usable numbers. Every reaction is really a recipe: a fixed ratio of reactants combines to form a fixed ratio of products, and that ratio never changes regardless of how much material you start with. A stoichiometry calculator online exists to save you from doing that ratio arithmetic by hand every time a problem changes scale, whether you're working through a textbook exercise or scaling up a reaction in a lab.
The core idea rests on the mole. Atoms and molecules are far too small to count individually, so chemists use the mole as a bridge between the microscopic world of particles and the macroscopic world of a lab balance. One mole of any substance always contains the same number of particles, roughly 6.022 × 10²³, known as Avogadro's number. Once an amount is expressed in moles, the coefficients of a balanced equation tell you exactly how many moles of any other substance are involved, which is the entire stoichiometry equation in a nutshell.
Stoichiometry answers practical questions: how much product will a reaction actually yield, how much reactant should be measured out, and which ingredient will run out first if the ratio supplied doesn't match the ratio required. That last question introduces the idea of a limiting reagent, the reactant that is used up before the others and therefore caps the amount of product that can form. A reaction stoichiometry calculator checks every reactant amount against the balanced ratio and reports which one is limiting, along with how much of each excess reactant remains unused.
The general stoichiometry formula follows four steps. First, write and balance the chemical equation so the number of atoms of each element matches on both sides. Second, convert the given amount of a known substance into moles, using molar mass if you started with grams. Third, apply the mole ratio taken directly from the balanced coefficients to move from moles of the known substance to moles of the substance you want to find. Fourth, convert that mole value into whatever unit the question requires, typically grams, using the molar mass of the target compound again.
| Conversion | Formula | Example |
|---|---|---|
| Grams to moles | n = mass ÷ molar mass | 18 g H2O ÷ 18 g/mol = 1 mol |
| Moles to grams | mass = n × molar mass | 2 mol O2 × 32 g/mol = 64 g |
| Mole ratio | mol B = mol A × (coeff B ÷ coeff A) | 2H2 + O2 → 2H2O |
| Moles to particles | particles = n × 6.022×10²³ | 1 mol → 6.022×10²³ molecules |
| Percent yield | actual ÷ theoretical × 100 | 18 g ÷ 20 g × 100 = 90% |
Consider the combustion of methane: CH4 + 2O2 → CO2 + 2H2O. If a problem gives 16 grams of methane, its molar mass of about 16 g/mol converts that to 1 mole. The balanced coefficients show a 1:2 ratio between methane and oxygen, so 1 mole of methane requires 2 moles of oxygen, which is 64 grams. The same ratio shows that 1 mole of methane produces 1 mole of carbon dioxide (44 grams) and 2 moles of water (36 grams). A stoichiometry solver performs exactly this sequence for any equation you type in, including reactions with several reactants where a limiting reagent decision matters.
People searching for a stoichiometry calculator online usually want one of three things solved quickly: converting a known mass into the mass of another substance in the same reaction, identifying which reactant limits a yield when multiple starting amounts are given, or checking that a hand-balanced equation is actually correct before using it further. This calculator is built around exactly those three tasks, with a balancer, a mole-ratio solver, and a limiting-reagent comparison built into a single workflow so you don't need three separate tools.
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