What percent yield is and when to use it
Percent yield compares how much product a chemical reaction actually produced (the actual yield, measured on a lab balance) against the maximum amount stoichiometry predicts it could have produced (the theoretical yield, calculated from the limiting reagent). No real reaction reaches 100% — side reactions, incomplete reactions, purification losses, and measurement error all reduce the actual amount of pure product you can recover — so percent yield is the standard way chemists report how efficient a synthesis was.
Use this calculator after any synthesis or lab procedure where you have both a measured mass of product and a calculated theoretical maximum. It is a routine step in general and organic chemistry lab reports, and it is also used industrially to evaluate whether a manufacturing process is efficient enough to be cost-effective at scale.
The formula
Percent Yield = (Actual Yield ÷ Theoretical Yield) × 100%
- Actual yield — the mass (or moles) of product you actually isolated and measured, typically in grams after drying and purification.
- Theoretical yield — the maximum mass (or moles) of product possible, calculated from the balanced equation and the limiting reagent, assuming the reaction goes to completion with no losses.
Worked example
Suppose a stoichiometry calculation predicts a reaction should theoretically produce 12.0 g of product (the theoretical yield), but after running the reaction and purifying the product, you weigh out 9.6 g (the actual yield).
Percent Yield = (9.6 ÷ 12.0) × 100 = 80.00%, which matches what this calculator displays when you enter 9.6 for Actual Yield and 12.0 for Theoretical Yield.
Common mistakes and how to interpret the result
- Mixing up actual and theoretical yield. Entering theoretical yield in the "Actual" field (or vice versa) can produce a nonsensical result over 100% or a very low percentage — double-check which number came from your balance and which came from your calculation.
- Forgetting to identify the limiting reagent first. Theoretical yield must be calculated from the limiting reagent, not just any reactant — using the wrong reactant's stoichiometry gives an incorrect theoretical yield and therefore a wrong percent yield.
- Treating yields over 100% as errors to ignore. A result above 100% usually signals leftover solvent, unreacted starting material, or moisture still present in the "dry" product — it is a sign to re-purify or re-dry the sample, not a real yield.
- Not using consistent units. Both actual and theoretical yield must be in the same unit (both grams or both moles) — mixing units invalidates the ratio.