Percent Yield Calculator
Calculate the percent yield of a chemical reaction from actual and theoretical yields.
Understand why reactions never give 100% yield.
Percent yield compares how much product you actually obtained to the maximum theoretically possible.
Formula:
% yield = (actual yield / theoretical yield) × 100
Theoretical yield is the maximum amount of product calculated from stoichiometry, assuming the limiting reagent is completely consumed and no product is lost.
Actual yield is what you physically collect after the reaction and purification.
Why is percent yield rarely 100%? Real reactions rarely produce 100% yield due to:
- Incomplete reaction: Equilibrium limits products formed
- Side reactions: Other products form alongside the desired product
- Product loss: During transfer, filtration, crystallization, or distillation
- Impure reagents: Contaminated starting materials
- Measurement errors: Losses during weighing or handling
Typical yield ranges in different fields:
| Context | Typical % Yield |
|---|---|
| Industrial chemical synthesis | 85–95% (optimized processes) |
| Multi-step pharmaceutical synthesis | Often 40–60% per step |
| Organic chemistry lab (first attempt) | 40–80% |
| Biological systems (ATP synthesis) | ~40% efficiency |
| Combustion engine | 15–40% mechanical efficiency |
Atom economy (related concept):
Even with 100% yield, a reaction may waste atoms if large byproducts are formed.
Atom economy = MW of desired product / sum of MW of all reactants × 100
For green chemistry, high atom economy is just as important as high percent yield.
Theoretical yield comes first
Percent yield is meaningless until the theoretical yield is right, and that number comes from the limiting reagent rather than from whichever reactant you happened to weigh out. Work out the moles of each reactant, divide each by its coefficient in the balanced equation, and the smallest result identifies the limiting reagent. Everything else is in excess and will be left over.
Getting this wrong is the usual reason a percent yield comes out absurd. Calculate against a reagent that was in excess and the theoretical figure is too large, so a perfectly good reaction reports a disappointing 40%.
What a yield above 100% really means
It is never a reaction that overperformed. In practice it is one of three things: the product is still wet with solvent and you weighed the solvent too, an impurity has been carried through the workup, or the theoretical yield was calculated with the wrong molar mass. Drying the sample to constant weight settles the first case, and it is the commonest by a wide margin.
Reported yields in the literature are usually of purified, characterised product, which is why a paper’s 85% and an undergraduate’s 85% are not always the same claim.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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