Theoretical Yield Calculator
Theoretical yield is the maximum product a reaction can form when the limiting reactant converts completely. Enter the limiting reactant by mass or moles, supply the stoichiometric coefficients and product molar mass, and get the theoretical yield in grams, kilograms, or pounds. Optionally add the actual yield you recovered to calculate percent yield in the same step.
Formula
Worked example
Burning 36.03 g of H2 (molar mass 2.016 g/mol) in 2 H2 + O2 -> 2 H2O: moles of H2 = 36.03 / 2.016 = 17.88 mol. Mole ratio of H2O to H2 = 2/2 = 1, so moles of water = 17.88 mol. With a molar mass of 18.015 g/mol, theoretical yield = 17.88 x 18.015 = 322.1 g of water. If you recovered 290 g, percent yield = (290 / 322.1) x 100 = 90.0%.
What theoretical yield means and how to calculate it
Theoretical yield is the maximum mass of product a reaction could possibly form, assuming the limiting reactant converts completely and no material is lost along the way. It is a stoichiometric ceiling derived entirely from the balanced chemical equation and the amount of limiting reactant you start with. The calculation follows a three-step path. First, convert the limiting reactant mass to moles by dividing by its molar mass (skip this step if you already know the moles). Second, multiply by the mole ratio, which is the product coefficient divided by the limiting reactant coefficient taken straight from the balanced equation. Third, multiply by the product molar mass to convert moles back to grams. This calculator also lets you express the result in milligrams, kilograms, or pounds for whichever scale your work demands.
Finding the limiting reactant
In most reactions the reactants are not present in the exact stoichiometric ratio the equation requires, so one of them runs out before the others. That reactant is the limiting reactant, and once it is consumed the reaction stops regardless of how much of the other reactants remain. To find it, convert the available mass of each reactant to moles and divide each result by its coefficient in the balanced equation. The reactant giving the smallest quotient is the limiting reactant. Only that reactant enters the theoretical yield calculation; the others are in excess. This calculator accepts either the mass (with the molar mass needed to convert to moles) or the moles directly, whichever you have available.
From theoretical yield to percent yield
Theoretical yield almost never matches what you actually weigh out of the flask. Competing side reactions divert some material into unwanted products, reactions may not reach full completion, and product is inevitably lost during filtration, transfer, and drying. The actual yield is what you genuinely recover and weigh, and it is always less than or equal to the theoretical yield. Dividing the actual yield by the theoretical yield and multiplying by 100 gives the percent yield, the most practical single number for judging a reaction. A percent yield close to 100 means the process is clean and well controlled. A low value signals losses, side reactions, or an incomplete reaction worth investigating and optimizing. Enable the percent yield toggle in this calculator to compute it immediately once you know what you recovered.
Unit conversions and laboratory scale
Bench-scale reactions often deal in grams, but preparative or industrial work may involve kilograms, and analytical or microscale work milligrams or micrograms. Theoretical yield always has the same units as the mass inputs you use, as long as you apply the correct conversion factors. This calculator handles the conversions automatically: enter the limiting reactant mass in any of four common mass units (grams, milligrams, kilograms, or pounds), choose your preferred output unit, and the calculator converts both internally and reports the yield in every unit alongside the primary display. The underlying moles calculation is always exact regardless of the unit you pick.
Theoretical yield, actual yield, and percent yield compared
| Quantity | Definition | How to calculate | Typical value |
|---|---|---|---|
| Theoretical yield | Maximum product possible if limiting reactant converts 100% | n(limiting) x mole ratio x M(product) | The ceiling |
| Actual yield | Mass of product you genuinely recover and weigh | Measured on a balance after isolation | Always <= theoretical yield |
| Percent yield | Efficiency of the reaction as a percentage | (actual / theoretical) x 100% | 50-95% for most lab reactions |
Three related quantities every chemistry student needs to know.
Frequently asked questions
What is the difference between theoretical yield and actual yield?
Theoretical yield is the maximum amount of product calculated from stoichiometry, assuming perfect and complete conversion of the limiting reactant. Actual yield is the mass of product you truly collect after the reaction is done and the product is isolated. Actual yield is always equal to or less than theoretical yield because real reactions have side products, handling losses, and may not reach full completion.
How do I find the mole ratio from a balanced equation?
The mole ratio is the coefficient in front of the product divided by the coefficient in front of the limiting reactant in the balanced equation. For N2 + 3 H2 -> 2 NH3 with H2 as the limiting reactant, the ratio of NH3 to H2 is 2 / 3, which is approximately 0.667. Enter the two coefficients separately in this calculator and it computes the ratio for you.
How do I identify which reactant is limiting?
Convert the available mass of every reactant to moles by dividing by its molar mass. Then divide each mole amount by the corresponding coefficient in the balanced equation. The reactant giving the smallest result is the limiting reactant. It caps the product formation, and only its moles go into the theoretical yield formula.
Can I enter mass directly instead of moles?
Yes. Switch the input mode to "Mass" and enter the mass of the limiting reactant in grams, milligrams, kilograms, or pounds, along with its molar mass. The calculator converts to moles internally before computing the yield, saving you a manual step.
How is percent yield calculated?
Percent yield = (actual yield / theoretical yield) x 100%. Enable the "Calculate percent yield" toggle, enter the mass you actually recovered (in any supported unit), and the calculator divides it by the theoretical yield in grams and multiplies by 100. A result above 100% indicates a measurement error or impure product.
Can the percent yield ever exceed 100%?
In theory, no: you cannot get more product than the stoichiometry allows. In practice, a reported percent yield above 100% means the recovered solid is impure (still contains solvent, by-products, or unreacted starting material) or there is a measurement error. A truly pure product can never exceed 100% yield.