Chemical Reaction Yield Calculator
Calculate the theoretical yield of a reaction product and find percent yield once you have an actual yield. Ideal for chemistry students and lab chemists checking reaction efficiency.
About this calculator
Theoretical yield is the maximum mass of product obtainable if the limiting reactant converts completely, with no losses. The formula is: Theoretical Yield = (mass of limiting reactant / molar mass of reactant) × molar mass of product × stoichiometric ratio. Converting the limiting reactant mass to moles first—moles = mass / molar mass—then scaling by the stoichiometric ratio gives product moles, which are converted back to grams using the product's molar mass. Percent yield then measures real-world efficiency: % Yield = (actual yield / theoretical yield) × 100. A percent yield above 100% signals an error such as impurities in the product or an inaccurate measurement. Typical laboratory reactions achieve 70–90% yield due to side reactions, incomplete reactions, and transfer losses.
How to use
Suppose you react 10 g of hydrogen gas (H₂, molar mass 2.016 g/mol) with excess oxygen to form water (H₂O, molar mass 18.015 g/mol) in a 1:1 stoichiometric ratio. Theoretical yield = (10 / 2.016) × 18.015 × 1 = 4.960 × 18.015 = 89.35 g. Enter 10 g for limiting reactant mass, 2.016 g/mol for reactant molar weight, 18.015 g/mol for product molar weight, and 1 for the stoichiometric ratio. If you collected 80.4 g of water, percent yield = (80.4 / 89.35) × 100 = 89.97%.
Frequently asked questions
What is the difference between theoretical yield and actual yield in chemistry?
Theoretical yield is the calculated maximum amount of product that can form based on the limiting reactant and stoichiometry, assuming perfect conditions. Actual yield is the mass of product you physically collect after the experiment. The difference arises from side reactions, product losses during filtration or transfer, incomplete reactions, and measurement error. Comparing the two gives the percent yield, which tells you how efficient your reaction was.
How do I identify the limiting reactant before calculating theoretical yield?
Convert the mass of each reactant to moles by dividing by its molar mass. Then divide each mole value by its stoichiometric coefficient from the balanced equation. The reactant with the smallest result is the limiting reactant—it runs out first and caps how much product can form. All theoretical yield calculations must be based on the limiting reactant; using any other reactant will overestimate the yield.
Why is percent yield rarely 100% in a real laboratory experiment?
Several practical factors reduce actual yield below theoretical: product may be lost during transfer between containers, filtration, or evaporation; side reactions consume some starting material; the reaction may not reach full completion; and product may remain dissolved in solution rather than being fully recovered. Additionally, impure starting materials or weighing errors can affect both the numerator and denominator of the percent yield calculation, making careful technique essential to maximize recovery.