Percent Yield Calculator & Formula

This percent yield calculator works out the percentage efficiency of a chemical reaction by comparing the actual amount of product you obtained to the theoretical maximum possible. You enter the actual yield and the theoretical yield (in any mass unit, as long as both are the same), and it returns the percent yield, plus the ratio and the amount of product lost. The result tells you how close the reaction came to the ideal, with 100% meaning you recovered everything expected.

Formula

The percent yield is the ratio of the actual yield to the theoretical yield, multiplied by 100 to express it as a percentage:

Percent yield (%) = (actual yield ÷ theoretical yield) × 100

• Actual yield – the mass of product you actually collected after the reaction, drying, and purification (grams, kilograms, or any mass unit).
• Theoretical yield – the maximum mass of product predicted by stoichiometry, assuming every molecule of limiting reagent converts to product with no losses (same unit as actual yield).

Because both yields are masses, the units cancel. You can use grams, milligrams, or kilograms – just make sure both inputs share the same unit.

Worked Example

Using the default values from the calculator above: actual yield = 8.5 g, theoretical yield = 10.0 g.

1. Divide the actual yield by the theoretical yield: 8.5 ÷ 10.0 = 0.85
2. Multiply by 100 to get the percentage: 0.85 × 100 = 85%
3. The ratio of actual to theoretical is 0.85 : 1, or 85 : 100.
4. The mass lost is 10.0 − 8.5 = 1.5 g.

The calculator displays 85.0% as the headline answer, matching the step-by-step result.

Why Percent Yield Matters

Percent yield is a standard metric in chemistry labs and industry because it quantifies reaction efficiency. A yield of 100% is almost never achieved in practice – some product sticks to glassware, some is lost during filtration or transfer, and side reactions may consume reactants. Typical values are:

• 90–99% – excellent, common for simple precipitations or distillations.
• 70–89% – good, typical for many organic syntheses.
• 50–69% – moderate, acceptable for multi-step reactions.
• Below 50% – poor; often indicates a flawed procedure or an incorrect theoretical calculation.

In teaching labs, a percent yield above 100% usually signals an error – the product is wet, contains impurities, or the theoretical yield was miscalculated. In industry, even a few percentage points of improvement can save significant material and cost.

How to Find Theoretical Yield

The theoretical yield comes from the balanced chemical equation and the limiting reactant. The general steps are:

1. Write the balanced equation for the reaction.
2. Convert the mass of each reactant to moles using its molar mass.
3. Identify the limiting reactant – the one that produces the fewest moles of product.
4. Use the mole ratio from the equation to calculate moles of product expected.
5. Convert moles of product to mass using its molar mass – that is the theoretical yield.

For example, if 10.0 g of reactant A (molar mass 50 g/mol) reacts with excess B to give product C (molar mass 80 g/mol) in a 1:1 ratio, then moles of A = 10.0 ÷ 50 = 0.20 mol, so theoretical yield = 0.20 × 80 = 16.0 g. If you actually collected 12.8 g, the percent yield is (12.8 ÷ 16.0) × 100 = 80%.

Common Mistakes and How to Avoid Them

• Mixing units – entering actual yield in grams and theoretical in milligrams gives a wildly wrong percentage. Always convert to the same unit first.
• Using the wrong limiting reactant – if you pick a reactant that is in excess, the theoretical yield will be too high, and your percent yield will look artificially low.
• Rounding too early – rounding molar masses to whole numbers can shift results by a few percent. Keep at least three significant figures until the final step.
• Ignoring side reactions – if the reaction produces multiple products, the theoretical yield applies only to the desired product.
• Reporting >100% without checking – a yield above 100% means an error in measurement or purity, not a miracle. Check for residual solvent or unreacted starting material.

Limits of the Percent Yield Formula

The formula itself is simple, but its usefulness depends on accurate inputs. It does not account for the purity of the product – a wet or contaminated sample will inflate the actual yield and give a misleadingly high percent yield. It also does not tell you why the yield is low; that requires examining the procedure, side reactions, and transfer losses. For reactions with multiple steps, the overall percent yield is the product of each step’s yield. For example, three steps each with 80% yield give 0.80 × 0.80 × 0.80 = 0.512, or 51.2% overall.

Frequently Asked Questions

Can percent yield be more than 100%?

Yes, but it indicates an error. If the product is wet, contains unreacted starting material, or the theoretical yield was calculated incorrectly, the actual mass can exceed the theoretical maximum. In a well-run experiment, percent yield should be 100% or less.

What is the difference between percent yield and percent error?

Percent yield compares actual to theoretical yield (how much product you got vs. how much you should have gotten). Percent error compares a measured value to an accepted value (how close your measurement is to the true value). They are different concepts – percent yield is about reaction efficiency, percent error is about measurement accuracy.

Do I need to use moles or grams for the calculator?

You can use any mass unit – grams, kilograms, milligrams – as long as both the actual and theoretical yields are in the same unit. The calculator simply divides the two numbers, so the units cancel. If you have yields in moles, you can also use those, as long as both are in moles.

Why is my percent yield so low?

Common reasons include incomplete reaction, product lost during transfer or purification, side reactions producing other compounds, or evaporation of volatile products. Also double-check that you identified the correct limiting reactant and that your theoretical yield calculation is correct.

What is a good percent yield for a lab experiment?

It depends on the reaction type. Simple precipitations often give 90% or higher, while multi-step organic syntheses may give 50–70% per step. A yield above 70% is generally considered good for most undergraduate lab work, but the expected range varies widely by reaction.

How do I calculate theoretical yield if I have multiple reactants?

Calculate the moles of product each reactant would produce if it were completely consumed. The reactant that gives the fewest moles of product is the limiting reactant, and that number of moles is the theoretical yield in moles. Then convert to mass using the product’s molar mass.

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in actual | Actual yield | g | 8.5 | 0
in theo | Theoretical yield | g | 10.0 | 0
out py | Percent yield | % | (actual / theo) * 100 | 1
out ratio | Actual : theoretical | - | actual / theo | 3
out lost | Mass lost | g | theo - actual | 1