Percent Yield Calculator
Calculate the percent yield of a reaction from the actual and theoretical amounts of product — the standard measure of reaction efficiency — in grams or moles.
Cómo usar esta calculadora
- 1Calculate or look up the theoretical yield from the reaction's stoichiometry.
- 2Weigh or measure the actual yield you obtained.
- 3Enter both in the same unit — grams or moles.
- 4Read the percent yield and how much product was lost.
Cómo funciona
Percent yield
percent yield = (actual yield ÷ theoretical yield) × 100 theoretical yield = maximum from stoichiometry actual yield = amount really obtained typically below 100%; above 100% means impurity or error
Percent yield expresses how efficient a chemical reaction was by comparing what you actually got against the most you could possibly have gotten. The theoretical yield is calculated from stoichiometry — using the balanced chemical equation and the limiting reactant to predict the maximum amount of product if every reactant molecule converted perfectly with no losses. The actual yield is what you measure after running, isolating, and purifying the product. Dividing the actual by the theoretical and multiplying by 100 gives the percent yield. Because real reactions rarely go to completion, and because product is inevitably lost during transfers, filtration, and purification, the actual yield is almost always less than the theoretical, so percent yield is usually below 100%. A value above 100% is a red flag that the measured product is contaminated with impurities, unreacted starting material, or residual solvent.
Ejemplo resuelto
If a reaction's stoichiometry predicts a theoretical yield of 10 grams of product but you actually isolate 8.5 grams, the percent yield is (8.5 ÷ 10) × 100 = 85% — a good yield. The 1.5 grams of difference is product that either never formed or was lost during workup and purification.
Percent Yield Calculator: la guía completa
Theoretical versus actual yield
Every percent-yield calculation rests on two numbers with very different origins. The theoretical yield is a prediction, calculated on paper from the balanced chemical equation. It uses the mole ratios in the equation and identifies the limiting reactant — the one that runs out first and therefore caps how much product can form — to work out the maximum possible amount of product under ideal conditions. It assumes the reaction goes perfectly and completely, with every available reactant converting to product and nothing lost.
The actual yield, by contrast, is a measurement of reality: the amount of purified product you actually end up holding after the reaction is run and the product is separated from everything else. Reality never matches the ideal, so the actual yield falls short of the theoretical. The gap between the two is the story of everything that went less than perfectly, and percent yield quantifies it in a single, comparable figure. This is why chemists calculate the theoretical yield before starting and weigh the product at the end — the comparison is how they judge whether a reaction worked as hoped.
Why yields fall short of 100%
It is rare for a reaction to deliver its full theoretical yield, and the reasons are worth understanding because they point to where a process can be improved. Many reactions simply do not go to completion — they reach an equilibrium with reactants and products coexisting, so not all the starting material converts. Side reactions divert some reactant into unwanted by-products instead of the target product. And even the product that does form is never fully recovered: some is lost stuck to glassware, some in the filtrate during filtration, some during recrystallisation or other purification steps that trade quantity for purity.
Each of these losses chips away at the yield, and a low percent yield prompts a chemist to ask which is to blame. If the reaction is incomplete, changing temperature, time, or concentrations might help. If side reactions dominate, a different catalyst or milder conditions might steer the reaction. If losses in workup are the culprit, gentler handling and more careful transfers recover more. Percent yield is thus not just a grade but a diagnostic, and improving it — especially in industrial processes where a few percentage points translate to large sums — is a central preoccupation of practical chemistry.
When yield exceeds 100%
Occasionally a percent-yield calculation comes out above 100%, which is physically impossible — you cannot make more product than the reactants allow. Rather than a triumph, this is always a signal of error, and recognising it is an important lab skill. The most common cause is impurity: the measured product is contaminated with something that adds to its mass, most often residual solvent that was not fully dried off, or unreacted starting material or by-products that were not separated during purification.
The fix is to purify the product more thoroughly and dry it completely before weighing, then recalculate. A yield stubbornly above 100% after careful drying suggests a weighing or calculation mistake, such as an error in the theoretical yield or a miscalibrated balance. Because it flags a problem so clearly, an impossible yield is actually useful feedback — it tells the chemist that the product is not yet clean or the arithmetic is off, before any false conclusions are drawn. A believable percent yield always sits between zero and one hundred, and anything outside that range is an invitation to check the work.
Preguntas frecuentes
How do I calculate percent yield?
Divide the actual yield (what you obtained) by the theoretical yield (the maximum from stoichiometry) and multiply by 100. If you got 8.5 g from a reaction predicted to give 10 g, the percent yield is (8.5 ÷ 10) × 100 = 85%. Use the same unit for both.
What is theoretical yield?
The maximum amount of product a reaction could produce, calculated from the balanced equation and the limiting reactant, assuming the reaction goes perfectly with no losses. It's a prediction on paper, and the actual yield you measure is almost always lower.
Why is my percent yield less than 100%?
Because real reactions rarely go to completion, side reactions divert some reactant into by-products, and product is lost during transfers, filtration, and purification. Each factor reduces the actual yield below the theoretical maximum, so yields below 100% are normal and expected.
Can percent yield be over 100%?
Not really — it's physically impossible to make more product than the reactants allow, so a value above 100% signals an error. Usually the product is impure, containing residual solvent or unreacted material that inflates its mass. Purify and dry it fully, then recalculate.