Donaldson R F Stoichiometry Calculator
Which reactant runs out first, and how much product a balanced reaction can make.
Enter a balanced equation's formulas and coefficients (this tool doesn't balance equations — use the Chemical Equation Balancer first if you need to).
How this is worked out
For a balanced equation like 2H₂ + O₂ → 2H₂O, each reactant's mass is converted to moles (mass ÷ molar mass), then divided by its own coefficient to find how far the reaction could proceed on that reactant alone — whichever gives the smaller number is the limiting reagent: it runs out first and caps how much product can form. That capped amount, converted back through the product's own coefficient and molar mass, is the theoretical yield. If you also enter how much product you actually collected, the tool works out the percent yield (actual ÷ theoretical × 100%) — always below 100% in a real experiment, because of side reactions, incomplete reactions or losses during handling.
Frequently asked questions
Why does the equation need to already be balanced?
Because the whole calculation depends on the coefficients representing the true mole ratio the reaction actually uses — an unbalanced equation would give a limiting reagent and yield that don't correspond to real chemistry.
What if I have excess of one reactant?
That's exactly what "limiting reagent" identifies — whichever reactant isn't limiting is the one in excess, and some of it will be left over once the reaction finishes.
Related tools: Chemical Equation Balancer, Molar Mass Calculator, Empirical & Molecular Formula Calculator.
Source / Last verified: standard general chemistry (limiting reagent and percent yield, from mole ratios in a balanced equation) — read/checked 1 October 2026. The default example (4 g H₂ + 4 g O₂ → H₂O) correctly identifies O₂ as limiting and gives a theoretical yield of 4.50 g water.