The theoretical yield is the most product a reaction could possibly make — the ceiling, assuming nothing is wasted. It is fixed by the limiting reactant, the ingredient that runs out first, because once it is gone the reaction stops. The recipe is always the same three steps: turn the limiting reactant's mass into moles, scale by the mole ratio from the balanced equation to get moles of product, then convert back to grams with the product's molar mass. Compare your real result against this ceiling and you have the percent yield.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: stoichiometric mole-ratio yield calculation.
The theoretical-yield equations
Balanced equations relate substances by moles, so you must work in moles, not grams — the coefficients are the mole ratio. The product-to-reactant coefficient ratio scales the limiting reactant's moles into product moles: 1 if they react one-for-one, 2 if two products form per reactant, 0.5 if two reactants make one product. Multiplying by the product's molar mass returns a mass you can weigh, the theoretical maximum for that amount of limiting reactant.
Worked example — a one-step synthesis
Scenario: 25 g of a limiting reactant (molar mass 100 g/mol) reacts 1:1 to give a product of molar mass 120 g/mol.
The reaction could make at most 30 g of product. Double the limiting reactant to 50 g and the yield scales straight to 60 g, since yield is proportional to the limiting reactant. If the balanced equation instead gave two product molecules per reactant (ratio 2), the same 0.25 mol would make 0.5 mol of product — 60 g. And if you actually isolated only 25.5 g, your percent yield would be 25.5 / 30 × 100 = 85%.
Frequently Asked Questions
moles of limiting reactant × mole ratio × product molar mass. 25 g/100 = 0.25 mol → ×1 ×120 = 30 g.
The one that runs out first. Convert each reactant to moles ÷ its coefficient; the smallest is limiting.
Product coefficient ÷ reactant coefficient from the balanced equation. 2H₂+O₂→2H₂O: H₂O:H₂ = 1, H₂O:O₂ = 2.
Theoretical = max from stoichiometry; actual = what you isolate (lower). actual ÷ theoretical × 100 = percent yield.
Balanced equations relate moles, not masses. Mole is the common currency: mass → moles → ratio → moles → mass.