Science, Tech, Math › Science Percent Yield Definition and Formula Share Flipboard Email Print Adrianna Williams / Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated July 07, 2019 Percent yield is the percent ratio of actual yield to the theoretical yield. It is calculated to be the experimental yield divided by theoretical yield multiplied by 100%. If the actual and theoretical yield are the same, the percent yield is 100%. Usually, percent yield is lower than 100% because the actual yield is often less than the theoretical value. Reasons for this can include incomplete or competing reactions and loss of sample during recovery. It's possible for percent yield to be over 100%, which means more sample was recovered from a reaction than predicted. This can happen when other reactions were occurring that also formed the product. It can also be a source of error if the excess is due to incomplete removal of water or other impurities from the sample. Percent yield is always a positive value. Also Known As: percentage yield Percent Yield Formula The equation for percent yield is: percent yield = (actual yield/theoretical yield) x 100% Where: actual yield is the amount of product obtained from a chemical reactiontheoretical yield is the amount of product obtained from the stoichiometric or balanced equation, using the limiting reactant to determine product Units for both actual and theoretical yield need to be the same (moles or grams). Example Percent Yield Calculation For example, the decomposition of magnesium carbonate forms 15 grams of magnesium oxide in an experiment. The theoretical yield is known to be 19 grams. What is the percent yield of magnesium oxide? MgCO3 → MgO + CO2 The calculation is simple if you know the actual and theoretical yields. All you need to do is plug the values into the formula: percent yield = actual yield / theoretical yield x 100% percent yield = 15 g / 19 g x 100% percent yield = 79% Usually, you have to calculate the theoretical yield based on the balanced equation. In this equation, the reactant and the product have a 1:1 mole ratio, so if you know the amount of reactant, you know the theoretical yield is the same value in moles (not grams!). You take the number of grams of reactant you have, convert it to moles, and then use this number of moles to find out how many grams of product to expect.