Stoichiometry answers the question every chemistry lab eventually asks: if I have this much of one substance, how much of another do I get — or need? This calculator automates the standard gram-to-gram, mole-to-mole, and reverse-solve problems using nothing but the coefficients from a balanced equation and, when needed, molar mass. This article walks through the mole-ratio chain, how to work in either grams or moles, and where the simple math here reaches its limits.
The mole-ratio chain, step by step
Every stoichiometry problem — no matter how it's phrased — reduces to the same three-step chain: convert the known amount to moles, multiply by the mole ratio from the balanced equation, then convert the result back to whatever unit you need. The middle step is the one that actually uses chemistry: molesB = molesA × (coeffB ÷ coeffA). Coefficients come directly from the balanced equation, so getting the equation right is a prerequisite, not an afterthought — a wrong coefficient anywhere silently produces a wrong ratio and a wrong answer downstream.
Working in grams vs. moles
Moles are the currency chemical equations are written in, but lab measurements come off a balance in grams. The Mole Ratio tab skips the gram step entirely and works mole-to-mole, which is the cleanest way to see the ratio in isolation. The Mass-to-Mass tab adds the conversion at both ends: divide the known mass by its molar mass to get moles in, then multiply the computed moles by the other species' molar mass to get grams out. The Both Species tab shows moles and mass for both A and B simultaneously and lets the known amount be in either unit, which is handy when you have grams of the reactant but want to compare moles of both sides.
Solving in reverse: given the product, find the reactant
Stoichiometry is symmetric — the same mole-ratio chain runs backward just as easily as forward. If you know how much product you need and want to know how much reactant to weigh out, switch the Given toggle to Species B. The calculator inverts the ratio (coeffA ÷ coeffB instead of coeffB ÷ coeffA) and runs the identical chain in the other direction. This is the calculation behind questions like "how much starting material do I need to make 50 g of product," which comes up constantly in synthesis planning and lab-scale reaction design.
What this calculator assumes
This tool assumes the equation you enter is already balanced and that the reaction goes to completion exactly as written — it computes the theoretical amount, not what a real reaction jar actually produces. It does not check whether your coefficients balance the atoms on both sides, and it does not account for a limiting reactant: if you're working from two reactants and don't yet know which one runs out first, that's a separate calculation (moles-available ÷ coefficient, compared across both reactants) that determines which reactant's amount should feed into this calculator in the first place.
Why real yields fall short of the stoichiometric number
The mass this calculator reports is the theoretical yield — the maximum possible if every mole of reactant converted perfectly. Real lab yields are almost always lower because of side reactions that consume reactant without forming the target product, reactions that don't run to 100% completion, and material lost during filtration, purification, or transfer between containers. Chemists report a percent yield (actual mass ÷ theoretical mass × 100) to quantify that gap — a useful next calculation once you've measured what a reaction actually produced.