The theoretical yield is the maximum mass of product that could form, calculated from the balanced equation. The actual yield is the mass of product you really collect. The percentage yield compares them: percentage yield = actual yield ÷ theoretical yield × 100.
Example: burning 2.4 g of magnesium in 2Mg + O2 → 2MgO can make at most 4.0 g of magnesium oxide, which is the theoretical yield. A student who collects 3.4 g has a percentage yield of 85%. Collecting 3.0 g would give 75%.
The percentage yield should not be above 100%, because you cannot collect more product than the equation allows. A value above 100% suggests the product is impure or still wet. You can also rearrange the formula: actual yield = percentage yield ÷ 100 × theoretical yield.