Stoichiometry Calculator

Calculate reactant and product quantities from a balanced chemical equation using mole ratios, molar masses, grams, moles, molecules, atoms, and formula units. This stoichiometry solver balances the reaction automatically and shows each conversion step.

How to Use the Stoichiometry Calculator

Enter a reaction, choose the substance you know, then choose the substance you want to find. The calculator does the balance-and-ratio work in between.

Step 1
Enter the chemical equation
Type H2 + O2 -> H2O, CH4 + O2 -> CO2 + H2O, or another neutral equation. The tool accepts ->, =, or → and balances the equation before using it.
Step 2
Enter the known quantity
Choose the known reactant or product, then enter an amount such as 4 g H₂, 6 mol H₂, 250 mg NaCl, or 6.022e23 molecules O₂. Units can be g, mg, kg, mol, molecules, atoms, or formula units.
Step 3
Choose the target substance
Pick the reactant or product you want to calculate, and choose whether the answer should be mass or moles.
Step 4
Calculate
Review the balanced equation, molar masses, mole ratio, final quantity, step-by-step stoichiometry calculation, and dimensional analysis.

How to Calculate Stoichiometry

Stoichiometry is a mole-ratio calculation. The balanced equation tells you how many moles of each substance react or form; molar mass only appears when the problem gives or asks for grams.

1Balance the chemical equation
Coefficients must conserve every atom. Unbalanced H₂ + O₂ -> H₂O becomes 2H₂ + O₂ → 2H₂O, so the usable coefficients are 2 : 1 : 2.
2Convert the known quantity to moles
If the known amount is in grams, divide by its molar mass. If it is already in moles, keep it as moles.
3Use the stoichiometric mole ratio
Multiply known moles by target coefficient / known coefficient. This ratio is the core of every stoichiometry calculation.
4Find the target moles
The ratio gives moles of the target substance, even if the final answer needs mass.
5Convert the target moles to the desired unit
For grams, multiply by the target molar mass. For mg or kg, convert from grams at the end. For molecules, atoms, or formula units, multiply moles by Avogadro's number.

What this calculator does not do

This calculator handles one known substance → one target substance. It does not decide which reactant runs out first, and it does not calculate percent yield.

Stoichiometry Equation and Formula

The stoichiometry equation changes slightly depending on whether you start and end in moles or mass.

Mole-to-mole formula

target moles = known moles × target coefficient ÷ known coefficient

Mass-to-mass formula

target mass = known mass ÷ known molar mass × target coefficient / known coefficient × target molar mass

For 4.00 g H₂ in 2H₂ + O₂ → 2H₂O, the known molar mass is 2.016 g/mol and the H₂:H₂O mole ratio is 2:2. The target mass is 4.00 ÷ 2.016 × 2/2 × 18.015 = 35.74 g H₂O.

Particle-count stoichiometry uses the same middle ratio: particles ÷ Avogadro's number gives moles before the ratio, and target moles × Avogadro's number gives molecules, atoms, or formula units at the end.

Stoichiometry Conversion Chart

Use this stoichiometry chart as a roadmap: mass must become moles before the balanced-equation ratio can connect one substance to another. The central step is always moles A → moles B.

Stage 1

Grams A

Known mass from the problem

÷ molar mass of A

Stage 2

Moles A

Amount of the known substance

× coefficient B / coefficient A

Stage 3

Moles B

Amount of the target substance

× molar mass of B

Stage 4

Grams B

Target mass answer

final unit

Dimensional-analysis roadmap

grams A × 1 mol A / molar mass A × coefficient B mol B / coefficient A mol A × molar mass B / 1 mol B = grams B

Common Stoichiometry Conversions

The same balanced-equation ratio sits in the middle of every conversion. Only the first and last unit steps change.

Gram-to-Gram Stoichiometry

grams A → moles A → moles B → grams B. This is the classic mass-to-mass pathway, because grams of different substances cannot be compared directly.

Grams-to-Moles Stoichiometry

grams A → moles A → moles B. Use this when the known is mass but the target answer should stay in moles.

Moles-to-Grams Stoichiometry

moles A → moles B → grams B. Start with the mole ratio, then multiply target moles by target molar mass.

Mole-to-Mole Stoichiometry

moles A → moles B. No molar mass is needed unless the problem also asks for a mass.

Particle Stoichiometry: Molecules, Atoms, and Formula Units

The calculator also supports particle counts. They do not change the mole-ratio method; they add Avogadro's number before or after the ratio step.

Molecules-to-moles stoichiometry

For molecular substances such as O₂, H₂O, CO₂, or NH₃, divide molecule count by Avogadro's number to get moles, apply the balanced-equation ratio, then multiply by Avogadro's number if the target also asks for molecules.

6.022 × 10²³ O₂ molecules → 2 × 6.022 × 10²³ H₂O molecules in 2H₂ + O₂ → 2H₂O

Atoms in reaction stoichiometry

Use atoms when the species is a single element written as atoms in the equation, such as Na or Fe. The calculator treats atoms as countable particles and still routes the math through moles.

6.022 × 10²³ Na atoms → 6.022 × 10²³ NaCl formula units in 2Na + Cl₂ → 2NaCl

Formula units for ionic compounds

Ionic compounds such as NaCl and CaCO₃ are counted as formula units, not molecules. The Avogadro conversion is the same; the name of the particle changes to match the chemistry.

1 mol NaCl = 6.022 × 10²³ formula units of NaCl

Stoichiometry Examples

Try a gram-to-gram, mole-to-mole, or moles-to-grams problem. Each example shows the key steps used in the calculator.

Example 1 — Gram to Gram
2H₂ + O₂ → 2H₂O
4.00 g H₂ → 35.74 g H₂O
1. 1Balance the equation: 2H₂ + O₂ → 2H₂O.
2. 2Convert the known mass: 4.00 g H₂ ÷ 2.016 g/mol = 1.984 mol H₂.
3. 3Apply the mole ratio: 1.984 mol H₂ × 2 mol H₂O / 2 mol H₂ = 1.984 mol H₂O.
4. 4Convert to grams: 1.984 mol × 18.015 g/mol = 35.74 g H₂O.
Try This Example →
Example 2 — Mole to Mole
N₂ + 3H₂ → 2NH₃
6 mol H₂ → 4 mol NH₃
1. 1Balance the equation: N₂ + 3H₂ → 2NH₃.
2. 2The known amount is already in moles: 6 mol H₂.
3. 3Apply the mole ratio: 6 mol H₂ × 2 mol NH₃ / 3 mol H₂ = 4 mol NH₃.
4. 4No molar mass is needed because the target unit is moles.
Try This Example →
Example 3 — Moles to Grams
2Na + Cl₂ → 2NaCl
2 mol Na → 116.88 g NaCl
1. 1Balance the equation: 2Na + Cl₂ → 2NaCl.
2. 2Use the mole ratio: 2 mol Na : 2 mol NaCl, simplified to 1:1.
3. 3Convert target moles: 2 mol Na × 2 mol NaCl / 2 mol Na = 2 mol NaCl.
4. 4Convert to grams: 2 mol × 58.44 g/mol = 116.88 g NaCl.
Try This Example →

Common Stoichiometry Mistakes

Most wrong answers come from putting the right numbers in the wrong order. Check these before trusting a final mass, mole, or particle count.

Using an unbalanced equation

Mole ratios come only from balanced coefficients. H₂ + O₂ → H₂O suggests a false 1:1 oxygen-to-water ratio; 2H₂ + O₂ → 2H₂O gives the correct 1:2 ratio for O₂ to H₂O.

Comparing grams directly

Four grams of H₂ does not become four grams of H₂O. Convert grams to moles first, use the ratio, then convert the target moles back to grams.

Using the wrong molar mass

The known substance and target substance usually have different molar masses. In H₂ → H₂O, use 2.016 g/mol before the ratio and 18.015 g/mol after the ratio.

Solving a limiting-reactant problem here

If a problem gives amounts for two or more reactants, stoichiometry alone is not enough. Compare reactants in the limiting reactant calculator, then calculate theoretical yield.

Related Chemistry Calculators

Stoichiometry connects the equation balancer and molar-mass tools. Move to limiting reactants only when you have more than one known reactant amount.

Frequently Asked Questions

What is a stoichiometry calculator?

A stoichiometry calculator finds reactant or product quantities from a balanced chemical equation. Enter the reaction, choose a known substance and amount, then choose the target substance. The calculator balances the equation, converts mass to moles when needed, applies the mole ratio, and converts the target moles to grams, milligrams, kilograms, or moles.

How do you calculate stoichiometry?

Balance the chemical equation first. Convert the known quantity to moles, multiply by the target coefficient divided by the known coefficient, then convert target moles to the requested unit. For mass-to-mass problems, the chain is grams A → moles A → moles B → grams B.

What is the formula for stoichiometry?

For mole-to-mole stoichiometry: target moles = known moles × target coefficient / known coefficient. For mass-to-mass stoichiometry: target mass = known mass / known molar mass × target coefficient / known coefficient × target molar mass.

What is a mole ratio?

A mole ratio is the ratio of coefficients in a balanced chemical equation. In N₂ + 3H₂ → 2NH₃, the ratio between H₂ and NH₃ is 3 mol H₂ : 2 mol NH₃. That ratio converts moles of one substance into moles of another substance in the same reaction.

How do you convert grams to grams in stoichiometry?

Convert grams of the known substance to moles using its molar mass, apply the balanced-equation mole ratio, then convert moles of the target substance to grams using the target molar mass. Do not compare grams of two different substances directly.

Does the chemical equation need to be balanced first?

Yes. Stoichiometric ratios come from the coefficients of a balanced equation. This calculator balances the equation automatically before using the mole ratio, so H₂ + O₂ -> H₂O is treated as 2H₂ + O₂ → 2H₂O.

Is a stoichiometry calculator the same as a grams-to-moles calculator?

No. A grams-to-moles calculator converts one substance using only molar mass. A stoichiometry calculator uses a reaction: it balances the equation, converts the known amount to moles, applies a mole ratio, and finds a different reactant or product amount.

Can I use molecules, atoms, or formula units?

Yes. Choose molecules, atoms, or formula units as the known or target unit. The calculator converts particle counts through Avogadro's number, 6.022 × 10²³ particles per mole, then applies the balanced-equation mole ratio.

Is stoichiometry the same as a limiting reactant calculation?

No. This page uses one known substance to find one target amount. A limiting reactant calculation starts with multiple reactant amounts, compares which one runs out first, then finds theoretical yield. Use the limiting reactant calculator for that problem type.