Last updated: 2026-06-14

Understanding percent yield

Percent yield measures how efficient a chemical reaction was — how much product you actually got compared with the most you could have gotten in theory. It is one of the first numbers a chemist calculates after running a synthesis, because it reveals how much material was lost and whether the procedure needs improving.

The percent yield formula

Rearranged, the same relation gives the other two quantities:

• Actual yield = theoretical × percent ÷ 100
• Theoretical yield = actual × 100 ÷ percent

Worked example

A reaction is predicted to produce 10.0 g of aspirin, but you isolate 8.5 g after purification. The percent yield is:

That means 85% of the theoretical maximum was recovered, and 15% was lost to incomplete reaction and handling.

Finding the theoretical yield first

Percent yield needs the theoretical yield, which comes from stoichiometry:

1. Convert the mass of the limiting reactant to moles.
2. Use the balanced equation's mole ratio to find moles of product.
3. Convert moles of product back to grams using its molar mass.

The result is the theoretical yield — the denominator in the percent-yield formula. Once you have it, weigh your purified product to get the actual yield and divide.

Why yield is rarely 100%

Real reactions lose material at several stages:

• Incomplete reactions that reach equilibrium before finishing.
• Side reactions that turn reactant into unwanted by-products.
• Physical losses during filtering, transferring, and drying.
• Impure reactants that contain less of the active substance than assumed.

A percent yield of 70–90% is typical for a careful synthesis. Consistently low yields point to a step worth optimizing.

Yields above 100%

A genuine percent yield cannot be more than 100% — you cannot create more product than the reactants permit. A value above 100% almost always means the product is still wet with solvent or water, adding mass it should not have. Drying the sample to constant mass usually corrects it; if not, recheck the weighing and the theoretical-yield calculation.