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Stoichiometry Calculator —
mole ratios, mass & yield.

Type any chemical equation, pick a given species, and convert it into the corresponding amount of any other species — in moles, grams, particles, solution volume, or gas volume — with the full mole-ratio working shown step by step.

How it worksAuto-balances

Inputs

Equation & amounts

Chemical equatione.g. CH4 + O2 = CO2 + H2O

Balanced: CH₄ + 2O₂ → CO₂ + 2H₂O

Quick start

Given

Compound

Find

Compound

Theoretical product

43.891 g

of CO₂ from 16 g of CH₄.

n(CH4)

0.9973 mol

n(CO2)

0.9973 mol

Working

Step by step

1
Convert mass to moles: n = m / M = 16 g ÷ 16.043 g/mol = 0.9973 mol.
2
Apply the mole ratio from the balanced equation (1 CO2 : 1 CH4): n(CO2) = 0.9973 mol × (1 / 1) = 0.9973 mol.
3
Convert moles to mass: m = n × M = 0.9973 mol × 44.009 g/mol = 43.891 g.

Compounds in this equation

Coefficients & molar masses

Compound	Role	Coefficient	Molar mass (g/mol)
CH₄	Reactant	1	16.043
O₂	Reactant	2	31.998
CO₂	Product	1	44.009
H₂O	Product	2	18.015

Field guide

How stoichiometry works.

Every balanced chemical equation is a recipe written in moles. The coefficients in front of each formula tell you the relative number of moles consumed or produced — not grams, not litres, not particles. Once you accept that and convert your actual measured quantity to moles, the rest is just a single multiplication.

n_target = n_given × ( coefficient_target ÷ coefficient_given )

That ratio of coefficients is the mole ratio, and it’s the only piece of the calculation that comes from the equation itself. Everything else is unit conversion.

The three-step workflow.

Convert the given amount to moles. Use the molar mass for mass; Avogadro’s number for particles; molarity × volume for a solution; or the ideal-gas law for a gas.
Apply the mole ratio from the balanced equation to find the moles of the target species.
Convert moles of the target back to whatever unit you actually need — grams, litres of solution, litres of gas, etc.

The calculator above does exactly this and shows each step, so you can copy the working into homework or a lab report. The equation is balanced for you (with the same balancer used by our Chemical Equation Balancer), and molar masses are pulled from IUPAC standard atomic weights (via the Molecular Weight Calculator).

Worked example — combustion of methane

Take the default reaction, the combustion of methane:

CH₄ + 2 O₂ → CO₂ + 2 H₂O

How much CO₂ comes from burning 16 g of methane? Molar mass of CH₄ is about 16.04 g/mol, so 16 g is roughly 1 mole. The CO₂ : CH₄ coefficient ratio is 1 : 1, so you produce 1 mole of CO₂ — about 44 g.

Working with gases and solutions.

Gases at STP

At standard temperature and pressure (0 °C, 1 atm), one mole of any ideal gas occupies about 22.414 L. So 11.207 L of H₂ at STP is half a mole; 44.828 L is two moles. Modern IUPAC actually defines STP as 0 °C and 100 kPa (giving 22.711 L/mol), but the older 1 atm convention is still the one taught in most general-chemistry textbooks and is what this calculator uses for the “gas at STP” option.

Gases at any P, T

For non-STP conditions, use the ideal-gas law n = PV ⁄ RT, with R = 0.082057 L·atm/(mol·K), pressure in atm, and temperature in kelvins (K = °C + 273.15). The calculator accepts pressure in atm or kPa (it converts kPa internally using 1 atm = 101.325 kPa).

Solutions

For aqueous reagents, the path to moles goes through molarity: n = M × V, where M is the molarity in mol/L and V is the volume in litres. 0.500 L of 2.00 M HCl contains 1.00 mol of HCl — which, in HCl + NaOH → NaCl + H₂O, neutralises exactly 1.00 mol of NaOH and produces 1.00 mol (≈ 58.44 g) of NaCl.

Common pitfalls.

Don’t skip balancing. Coefficient ratios on an unbalanced equation are meaningless. The calculator balances automatically; if you copy from a textbook, double-check.
Watch your units. Mass in grams divides by molar mass in g/mol — not mg or kg. The calculator handles the conversion, but the principle matters.
STP and the gas constant must agree. 22.414 L/mol comes from 1 atm and 0 °C; if you switch to kPa, switch the constant too (or use the ideal-gas law with consistent units).
One-at-a-time vs. limiting reactant. Plain stoichiometry assumes the given reactant is fully consumed. If you have measured amounts of two reactants, run the calc for each and the smaller predicted product is the real one — the other reagent is in excess.

Related calculators

Balance an equation with the Chemical Equation Balancer, look up a molar mass with the Molecular Weight Calculator, prepare solutions with the Molarity Calculator, or work with gases via the Ideal Gas Law Calculator.

Disclaimer: Molar masses are computed from IUPAC’s 2021 standard atomic weights of the elements. The gas constant R = 0.082057 L·atm·mol⁻¹·K⁻¹ and Avogadro’s number Nₐ = 6.02214076 × 10²³ mol⁻¹ are the CODATA / NIST recommended values. This calculator is provided for educational reference only and is not a substitute for professional chemistry, engineering, or safety review. Always verify critical results against primary sources.

FAQ

Frequently asked

What is stoichiometry, in one sentence?+

Stoichiometry is the math of how much of one substance reacts with, or produces, how much of another in a balanced chemical equation — using the coefficients of the equation as a ratio of moles.

Why do I have to convert everything to moles first?+

Coefficients in a balanced equation are mole ratios, not mass ratios. A reaction that consumes one mole of one substance per two moles of another does NOT consume them in a 1:2 mass ratio, because the two species have different molar masses. Converting to moles puts everything on the common 'how many particles' currency that the coefficients actually describe.

What is the mole ratio formula?+

n_target = n_given × (coefficient_target ÷ coefficient_given). If a balanced equation reads 2 H₂ + O₂ → 2 H₂O, then for every 1 mole of O₂ you consume 2 moles of H₂ and produce 2 moles of H₂O — the ratios are 1:2 and 1:2.

How do I do stoichiometry with a gas or solution?+

Start by converting the amount to moles. For a gas at STP (0 °C, 1 atm) use n = V ÷ 22.414 L/mol. For a gas at other conditions use the ideal-gas law n = PV ÷ RT (with R = 0.082057 L·atm/mol·K and T in kelvins). For a solution use n = molarity × volume in litres. Once you have moles, apply the mole ratio, then convert moles back to the target unit with the inverse of the same formulas.

What's the difference between stoichiometry and a limiting-reactant problem?+

Plain stoichiometry assumes the given reactant is fully consumed and asks how much product (or co-reactant) that implies. A limiting-reactant problem starts from two or more given reactants and asks which one runs out first — you do a stoichiometry calculation for each reactant separately, and the smallest predicted product amount tells you the limiting reactant. This tool handles one-at-a-time stoichiometry; for limiting reactants, run it once per starting material and pick the smaller result.

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