No real reaction converts every bit of its starting material into the desired product. Some of it reacts the wrong way, some never reacts and some is lost on the filter paper or in the glassware. Percent yield measures how close you came to the maximum. This calculator divides the actual yield by the theoretical yield, or rearranges the relationship to predict how much product to expect from a known percentage. If you do not know the theoretical yield yet, it can calculate it from the limiting reactant and the formulas in your equation.
How to use the percent yield calculator
- Choose to Solve for the percent yield, the actual yield or the theoretical yield.
- For the Theoretical yield, either enter it directly or choose Work it out from the limiting reactant. In that mode, give the reactant’s formula, its amount and coefficient, and the product’s formula and coefficient from the balanced equation.
- Enter the Actual yield, the mass or moles of product you collected, and/or the Percent yield, depending on what you are solving for.
- Pick the unit of the result: g, mg, kg, lb, oz, mol or mmol.
- Read the result, the amount lost and a short rating. The steps show every molar mass and mole conversion.
Percent yield formula
The theoretical yield from a limiting reactant R forming product P is
where M is molar mass and ν the coefficient in the balanced equation.
Worked example
Making aspirin
Salicylic acid (C7H6O3) reacts with excess acetic anhydride to form aspirin (C9H8O4) in a 1:1 ratio. A student starts with 2.00 g of salicylic acid and collects 2.10 g of dry aspirin.
Moles of salicylic acid: 2.00 ÷ 138.122 = 0.01448 mol
Theoretical aspirin: 0.01448 mol × 180.159 g/mol = 2.609 g
Percent yield = 2.10 ÷ 2.609 × 100% = 80.5%
Planning ahead. Suppose a procedure usually gives 85% and the theoretical yield is 2.609 g. Solve for actual yield: you should expect about 2.22 g. If you need 21.3 g of product from a reaction that runs at 85%, the theoretical yield you must plan for is 21.3 ÷ 0.85 = 25.06 g, which tells you how much starting material to weigh out.
Why yields fall short
| Cause | Example |
|---|---|
| Incomplete reaction | Equilibrium reactions such as esterification stop partway |
| Side reactions | Competing products form from the same reactants |
| Mechanical losses | Product left on filter paper, in flasks or on spatulas |
| Purification losses | Some product dissolves during recrystallization |
| Impure reagents | The “2.00 g” of reactant was not 100% pure |
Industrial chemists also track atom economy, the share of all reactant atoms that end up in the desired product; a reaction can have a high percent yield and still produce a lot of waste by-product.
Multistep syntheses
Yields multiply across steps. A five-step synthesis with 80% at every step delivers 0.85 = 33% overall, and ten such steps deliver only 11%. That is why pharmaceutical routes are designed with as few steps as possible.
To find which reactant limits the theoretical yield in the first place, use the limiting reactant calculator. The molar mass calculator gives molecular weights for any formula, and the percent error calculator handles measured-versus-accepted comparisons.
Frequently asked questions
What is the percent yield formula?
Percent yield = actual yield ÷ theoretical yield × 100%. If a reaction could make 25 g of product and you recover 21.3 g, the percent yield is 85.2%. Both yields must be in the same units, either both masses or both moles.
How do I find the theoretical yield?
Convert the limiting reactant to moles, multiply by the product-to-reactant ratio from the balanced equation, then multiply by the product's molar mass. Choose “Work it out from the limiting reactant” and the calculator does all three steps from the two formulas.
Can percent yield be over 100%?
Not for a pure, dry product. A result above 100% usually means the product still contains water or solvent, is contaminated with impurities or by-products, or the theoretical yield was calculated wrongly. The calculator flags such results.
What counts as a good percent yield?
It depends on the reaction. Above 90% is excellent, 70–90% is good for most laboratory preparations, and 40–70% is common in teaching labs and multistep syntheses. Over many steps the losses multiply: five steps at 80% each give only 33% overall.
Is percent yield the same as percent error?
No. Percent yield measures how much of the possible product you obtained. Percent error measures how far a measured value is from an accepted value, whatever the direction. An 85% yield corresponds to a 15% shortfall, not a 15% error.