How this instrument works
Percent yield is the ratio of actual yield — the mass of product you actually isolated and weighed — to theoretical yield — the maximum mass stoichiometry predicts from your limiting reagent — multiplied by 100. It answers a practical question every synthetic chemist asks after a reaction finishes: how much of the possible product did I actually get to keep? A percent yield of 100% would mean every bit of limiting reagent converted cleanly into isolated product with nothing lost.
In practice, actual yield is almost always lower than theoretical yield. Reactions rarely go to 100% completion, side reactions consume some of the limiting reagent into unwanted products, and purification steps like filtration, recrystallization or column chromatography physically lose some material even when the chemistry itself worked perfectly. A percent yield in the 70-90% range is often considered good for a straightforward synthesis; more complex, multi-step routes routinely yield less.
Percent yield can occasionally read above 100%, and that is a warning sign rather than a triumph. It usually means the isolated product still contains residual solvent, unreacted starting material, or another impurity adding extra mass to the balance reading — a genuinely pure product can never physically exceed the maximum the stoichiometry allows.
This instrument pairs directly with this site's theoretical yield calculator: that one computes the theoretical yield figure from moles of limiting reagent and molar mass, and this one takes that theoretical figure together with your actual measured mass to report the efficiency percentage.
- Enter the mass of product you actually isolated and weighed into Actual yield (g).
- Enter the maximum possible mass predicted by stoichiometry into Theoretical yield (g) — use this site's theoretical yield calculator first if you haven't computed that figure yet.
- Read Percent yield (%) directly beneath both fields — it updates the instant either mass changes.
- Make sure both masses are in the same unit (grams) and refer to the same product; mixing units or comparing different substances invalidates the ratio.
- Theoretical yield (g) must be greater than zero — the instrument has nothing to divide against otherwise.
Worked example — 74.817 g actual against an 88.02 g maximum
Enter 74.817 into Actual yield (g) and 88.02 into Theoretical yield (g) — the 88.02 g figure is exactly what this site's theoretical yield calculator returns for 2 mol of limiting reagent forming carbon dioxide (molar mass 44.01 g/mol) at a 1:1 stoichiometric ratio. Percent yield reads 85.0%.
That means 74.817 g of the 88.02 g stoichiometry allowed for was actually recovered from the reaction — a solid, typical result for a straightforward synthesis, with the remaining 15% lost to incomplete conversion, side reactions, or purification losses rather than any single identifiable cause.
Questions
Can percent yield be over 100%?
It can be reported over 100%, but it can never genuinely be over 100% for a pure product — that would mean more product mass exists than the limiting reagent could stoichiometrically supply. A reading above 100% almost always signals that the isolated sample still contains residual solvent, unreacted starting material, or another impurity adding extra mass to the balance reading, not a reaction that outperformed chemistry.
Why is actual yield almost always less than theoretical yield?
Because real reactions rarely reach full completion, side reactions siphon off some of the limiting reagent into byproducts instead of the intended product, and purification steps like filtration, washing or recrystallization physically lose some material even when the underlying chemistry worked as intended. Theoretical yield assumes a perfect world with none of these losses, which real lab work never fully achieves.
What's the difference between this calculator and the theoretical yield calculator?
Theoretical yield computes the maximum possible product mass from moles of limiting reagent, the stoichiometric ratio, and the product's molar mass — it needs no lab measurement at all. This calculator takes that theoretical figure plus your actual measured mass and divides one by the other to report how efficiently the reaction performed. Run theoretical yield first, then bring its answer here as your Theoretical yield (g) input.
What counts as a 'good' percent yield?
It depends heavily on the reaction, but 70-90% is often considered solid for a single, well-optimized synthetic step, while multi-step routes compound losses at each stage and can end up much lower overall even when every individual step performs reasonably. Industrial processes optimized over years can push well above 90%; a first attempt at a new reaction in a teaching lab commonly lands lower.
Do actual and theoretical yield need to be in the same unit?
Yes — this instrument divides one mass by the other directly, so both must be expressed in the same unit, here grams, and must refer to the identical product. Entering actual yield in milligrams against a theoretical yield in grams, for instance, would silently distort the percentage by a factor of 1,000.
Does percent yield tell me anything about purity?
Not directly — percent yield measures quantity recovered relative to the stoichiometric maximum, not how chemically pure that recovered material is. A sample could have a high percent yield while still containing impurities that inflate its measured mass, or a low percent yield while being extremely pure; yield and purity are assessed with different measurements and often reported side by side.