What is theoretical yield?

Theoretical yield is the maximum product obtainable if the reaction goes to completion with no losses. It equals moles of limiting reagent × the product-to-reagent mole ratio × the product's molar mass, expressed in grams.

How is percent yield calculated?

Percent yield = actual yield ÷ theoretical yield × 100. If you isolate 8.2 g of a product whose theoretical yield is 10.0 g, the percent yield is 82 percent. It measures how efficiently the reaction and workup converted reagent to isolated product.

What is the limiting reagent?

The limiting reagent is the reactant that runs out first and therefore caps how much product can form. All yield calculations are based on it. If you are unsure which reagent is limiting, compute the product each reagent could give and pick the smallest.

Can percent yield exceed 100 percent?

Not legitimately. A value above 100 percent usually means the isolated product is impure or still wet with solvent, the molar masses are wrong, or the stoichiometry is mis-entered. Re-check purity and your inputs before reporting it.

What is the stoichiometric ratio I enter?

It is the moles of product formed per mole of limiting reagent, taken from the balanced equation. For A → B it is 1. For a reaction producing 2 moles of product per mole of limiting reagent it is 2; for one producing half a mole, it is 0.5.

Chemical Reaction Theoretical & Percent Yield Calculator

Get one useful tool a week

Like this tool? Enter your email and we'll send you one genuinely useful Gera tool a week — plus a link to come back to this one. No spam, one-click unsubscribe any time.

Every synthesis is judged on yield, and yield only means something when it is referenced to the theoretical maximum set by the limiting reagent. This calculator computes theoretical yield from stoichiometry and then your percent yield from what you actually isolated.

How it works

Theoretical yield flows from the moles of limiting reagent through the balanced equation to the product mass:

moles_limiting   = mass_limiting / molar_mass_limiting   (if entered by mass)
moles_product    = moles_limiting × ratio                (ratio from equation)
theoretical (g)  = moles_product × molar_mass_product
percent yield    = actual_yield / theoretical × 100

The ratio is the moles of product per mole of limiting reagent read straight off the balanced equation. If you already know moles of limiting reagent, you can enter them directly and skip the mass and molar-mass step.

Worked example: Fischer esterification

Consider reacting acetic acid with ethanol to form ethyl acetate:

CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O

The stoichiometric ratio is 1:1 (1 mol of ethyl acetate per mol of acetic acid). Suppose acetic acid is the limiting reagent:

Mass of acetic acid used: 12.0 g
Molar mass of acetic acid: 60.05 g/mol → 0.200 mol
Ratio (product per mol of limiting reagent): 1
Molar mass of ethyl acetate: 88.11 g/mol
Theoretical yield: 0.200 × 1 × 88.11 = 17.6 g

If after distillation and drying you isolate 12.3 g:

Percent yield: 12.3 / 17.6 × 100 = 69.9%

For an equilibrium reaction like this, a yield around 65–75% is typical without driving the reaction — it is not a sign of poor technique, just thermodynamics.

Identifying the limiting reagent

The most common mistake when using this tool is entering the wrong reagent. If you have 0.20 mol of A and 0.30 mol of B in a 1:1 reaction, A is limiting and governs the theoretical yield. If the stoichiometry is 1:2 (one A produces two products), enter 2 as the ratio. To double-check: divide the moles of each reactant by its stoichiometric coefficient; the smallest quotient identifies the limiting reagent.

Interpreting a yield above 100%

A percent yield above 100% is a diagnostic flag, not a real result. Common causes:

The isolated product contains solvent or moisture — dry more thoroughly and re-weigh.
The molar mass used is wrong — recheck the product formula.
Stoichiometric ratio was entered incorrectly — recheck the balanced equation.
The wrong reagent was identified as limiting — recompute which one runs out first.

Never report a yield over 100% in a formal setting; correct the inputs instead.

Yield in manufacturing and scale-up

In a teaching lab, 70% is acceptable. In pharmaceutical manufacturing, yields below 85% raise costs significantly because reagents are expensive and waste must be disposed of safely. Process chemists often report yield across multiple steps as the product of each step’s percent yield — five steps at 80% each give an overall yield of only about 33%, which is why minimising step count matters as much as optimising each step.