Percent yield tells you how efficient a chemical reaction actually was compared to the ideal, stoichiometric prediction. It's one of the most common calculations in general and organic chemistry lab work, used to grade lab reports, compare synthesis routes, and troubleshoot reactions that underperform.
How percent yield works
Percent yield compares two numbers: the actual yield (what you weighed on the balance after the reaction) and the theoretical yield (what stoichiometry says you should get if every molecule of limiting reactant converted perfectly to product). Dividing actual by theoretical and multiplying by 100 gives a percentage that's almost always below 100% in real lab work, because side reactions, incomplete conversion, and physical losses during transfer and purification all reduce the amount of product you can recover.
Theoretical yield itself comes from a separate stoichiometry calculation: convert the mass of your limiting reactant to moles, use the balanced equation's mole ratio to find moles of product, then convert back to mass using the product's molar mass. Percent yield only makes sense once that theoretical figure is correct — an error in the theoretical yield calculation will throw off every percent-yield result downstream.
Inputs and what they mean
Actual yield is a measured quantity — the mass of purified, dried product you recovered, in grams. Theoretical yield is a calculated quantity from stoichiometry, also in grams. Both must use the same mass unit for the ratio to be valid; if your lab notebook records actual yield in milligrams and theoretical yield in grams, convert one before dividing.
On the Solve tab, you can work in the other direction: given a percent yield (from a known reaction's typical performance, or a target set by a lab manual) and one of the two yields, solve for the other. This is useful for planning — for example, estimating how much product a scaled-up reaction will realistically produce before running it.
Limits and edge cases
A percent yield over 100% is a red flag, not a triumph — it means the measured actual yield weighs more than the theoretical maximum allows. Common causes are residual solvent, water of crystallization, or unreacted starting material still trapped in the product, all adding mass that isn't the intended compound. A very low percent yield (well under 50%) usually points to incomplete reaction, a side reaction competing for the limiting reactant, or product lost during filtration, extraction, or recrystallization.
This calculator assumes the theoretical yield you enter is already correct. If you're unsure how to calculate it, use a stoichiometry or theoretical-yield calculator first — garbage in from a wrong theoretical yield produces a misleading percent-yield figure no matter how carefully you measure the actual yield.