Chemical Reaction Yield Calculator
Compute the theoretical yield (in grams) of a chemical reaction from the limiting reactant’s mass, the reactant and product molar masses, and the stoichiometric ratio. The first step in evaluating reaction efficiency — compare against your actual experimental yield to get percent yield.
Last updated: May 2026
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About this calculator
Theoretical yield is the maximum mass of product obtainable from a reaction if the limiting reactant converts completely with no losses. The formula is: theoretical yield = (limiting reactant mass / reactant molar mass) × product molar mass × stoichiometric ratio. The steps: first convert the limiting reactant from grams to moles (mass ÷ molar mass). Then multiply by the stoichiometric ratio (product moles per reactant mole, from the balanced equation) to get product moles. Finally multiply by the product’s molar mass to convert back to grams. This is the upper bound on what you could collect. Actual yield from a real experiment is typically lower because of side reactions, incomplete conversion, losses during filtration/transfer/purification, and impurities. Percent yield = (actual yield / theoretical yield) × 100 measures the reaction’s practical efficiency — most undergraduate organic reactions achieve 50–90%, optimised industrial processes 90–99%, and very clean reactions can approach 100%. A yield above 100% indicates an error: impurities or residual solvent inflating the measured mass, weighing errors, or wrong limiting-reactant identification. Edge cases: identifying the limiting reactant requires comparing moles-per-stoichiometric-coefficient across all reactants — the one with the smallest ratio is limiting. The stoichiometric ratio here is product:reactant; the calculator uses values like 1, 2, 0.5 (= 1:2), or 1.5 (= 3:2) to handle common patterns. Reactions with non-integer ratios or multiple products require manual scaling. The formula assumes a single, well-defined product; for mixtures or polymerisation, separate yield calculations are needed per product.
How to use
Example 1 — Combustion of hydrogen. You burn 10 g of H₂ (MW 2.016 g/mol) in excess O₂ to form water (MW 18.015 g/mol). The balanced equation 2 H₂ + O₂ → 2 H₂O has H₂:H₂O = 1:1 (stoichiometric ratio = 1). Enter limitingReactantMass = 10, reactantMolWeight = 2.016, productMolWeight = 18.015, stoichiometricRatio = 1. Theoretical yield = (10/2.016) × 18.015 × 1 = 4.960 × 18.015 = 89.35 g of water. ✓ Example 2 — Aspirin synthesis. You react 25 g of salicylic acid (MW 138.12 g/mol) with excess acetic anhydride to produce aspirin (acetylsalicylic acid, MW 180.16 g/mol). The reaction is 1:1, so stoichiometric ratio = 1. Theoretical yield = (25/138.12) × 180.16 × 1 = 0.1810 × 180.16 = 32.61 g. ✓ If your reaction combined two reactant molecules into one product (2:1), you would set stoichiometricRatio = 0.5, halving the predicted yield.
Frequently asked questions
How do I identify the limiting reactant?
The limiting reactant is the one that runs out first and caps how much product can form. To identify it: (1) convert the mass of each reactant to moles using its molar mass; (2) divide each mole value by its stoichiometric coefficient from the balanced equation; (3) the reactant with the smallest result is limiting. Example: for 2 H₂ + O₂ → 2 H₂O with 10 g H₂ and 80 g O₂, moles are H₂ = 10/2 = 5 mol, O₂ = 80/32 = 2.5 mol. Dividing by stoichiometric coefficients: H₂ = 5/2 = 2.5, O₂ = 2.5/1 = 2.5. They are exactly stoichiometric — neither is limiting. If we had 10 g H₂ and 50 g O₂: H₂ = 5/2 = 2.5, O₂ = 1.5625/1 = 1.5625. O₂ is limiting (smaller value). All theoretical yield calculations must be based on the limiting reactant; using any other reactant overestimates the yield.
Why is percent yield rarely 100% in real experiments?
Several practical factors reduce actual yield below theoretical. Side reactions consume some starting material to form unwanted byproducts. The reaction may not reach completion — kinetically slow steps or unfavourable equilibria can leave reactant unreacted. Product is lost during filtration (some sticks to the filter, some passes through), evaporation of solvent, recrystallisation (purification often discards 10–20% of crude product), or transfer between containers (loss on glass walls). Weighing errors and impurities further muddy the numbers. Typical undergraduate organic-chemistry reactions get 40–80% yield, well-optimised industrial processes achieve 90–95%, and very clean reactions like simple acid-base neutralisations or precipitations approach 99%. A yield reported as ‘above 100%’ is a red flag — it means the measured product is impure (still wet with solvent, contaminated with byproduct, or contains unreacted starting material), the molar masses or stoichiometric ratio are wrong, or the limiting reactant was misidentified.
How do I handle a reaction with a non-1:1 stoichiometric ratio?
The product-to-reactant stoichiometric ratio comes from the balanced chemical equation: how many molecules of the target product are formed per molecule of limiting reactant. For 2A → B (two reactant molecules combining into one product), the ratio is 0.5 — you get half as many product molecules as reactant molecules. For A → 3B (one reactant splitting into three products), the ratio is 3. The calculator offers a dropdown with common cases (1, 2, 0.5, 1.5); for unusual ratios pre-multiply the limiting-reactant mass yourself before entering it. Always start from a balanced chemical equation — unbalanced equations or guessed coefficients are the most common source of wrong yield predictions. Concrete example: for 2 H₂ + O₂ → 2 H₂O, the H₂:H₂O ratio is 2:2 = 1:1, so use 1. For 3 H₂ + N₂ → 2 NH₃, the H₂:NH₃ ratio is 3:2, so per mole of H₂ you get 2/3 of a mole of NH₃, i.e., ratio ≈ 0.667.
What are the most common mistakes people make computing yield?
The first is using the wrong reactant for the calculation — basing yield on a reactant in excess rather than the limiting one, which always overestimates yield. The second is using an unbalanced or incorrectly balanced equation, propagating wrong coefficients into the stoichiometric ratio. The third is mixing up molar masses (reactant vs product) — leads to numerically plausible but completely wrong yields. The fourth is forgetting to convert between mass and moles at the right point — the calculation goes mass → moles → moles → mass, with stoichiometry acting on the middle (moles) step. The fifth is reporting a percent yield above 100% without questioning it; this always indicates an error and should trigger a recheck of weights, purities, and stoichiometry rather than being reported as ‘a great yield’. The sixth is failing to dry the product fully before weighing — retained solvent inflates the measured mass and produces falsely high yields. The seventh is treating yield as a quality measure in isolation; a 99% yield of an impure product is worse than a 70% yield of pure, characterised product.
When should I not use this calculator?
Skip it for reactions with multiple competing products of interest — the calculator computes yield for a single product, and a chemistry problem asking about ‘overall yield’ or ‘mass balance’ for two products needs separate calculations. Avoid it for reactions where the limiting reactant is itself a mixture (impure starting material), unless you know the active fraction — otherwise the yield calculation is based on a wrong starting mass. It is the wrong tool for reactions with no clear stoichiometric ratio, like polymerisation (variable degree of polymerisation), crystallisation (no chemical change in moles, just phase change), or catalytic cycles (turnover number is the metric, not yield). Do not use it for biological reactions where ‘yield’ is replaced by metrics like specific activity (enzymes), expression level (proteins), or transformation efficiency (gene cloning). Skip it for green-chemistry assessments where atom economy is the meaningful efficiency metric. And for any commercial or scaled-up process where economic yield (mass of pure product per dollar of input) matters more than chemical yield, this is just the starting calculation — true process yield needs additional accounting for solvents, purifications, and losses.