The formula
How to calculate limiting reactant
The limiting reactant is the one that runs out first and therefore caps how much product can form. Everything else is in excess and will be left over when the reaction stops.
Divide each reactant's moles by its stoichiometric coefficient. The smallest quotient identifies the limiter, and multiplying it by the product's coefficient gives the maximum product.
The inputs, one by one:
- Moles of reactant A (mol)
- Coefficient of A
- Moles of reactant B (mol)
- Coefficient of B
- Coefficient of the product
No submit button: type and the answer moves. Your inputs end up in the link, so the page can be shared already filled in.
Where more decimal places matter than the fields above display, the underlying calculation is not rounded until the final figure is shown — the precision used internally is higher than what is printed, which matters for anyone chaining this result into a further calculation of their own.
Why limiting reactant matters
A limiting reactant calculation gets used both to check work already done by hand and to explore how a formula behaves without redoing the algebra every time an input changes — this page exists for both, since the underlying arithmetic is the same either way.
This also functions as a reference implementation of the formula itself: where the exact form of an equation is in question, the one used on this page, stated in the formula section above, is the standard version found in the relevant textbooks and reference material.
It is worth remembering that a formula is only ever as good as the assumptions built into it, and most of the standard equations used across science and statistics carry at least one simplifying assumption — a linear approximation, an idealised gas, a normally distributed error term — that holds well in most ordinary cases and breaks down at the extremes. The result here reflects the standard formula exactly; whether that formula's assumptions are appropriate for your particular situation is a separate judgement worth making deliberately rather than assuming automatically.
In practice, a formula like this one is most often reached for at the exact moment a manual calculation needs checking against a deadline — a lab report due, a problem set to submit — which is precisely the situation where a small arithmetic slip is easiest to miss and most costly to leave uncorrected. Running the same inputs through an independent calculator catches that class of error reliably.
Worked example
Work through the defaults on this page:
- Moles of reactant A: 3 mol
- Coefficient of A: 2
- Moles of reactant B: 2 mol
- Coefficient of B: 1
- Coefficient of the product: 2
That gives:
- A: moles ÷ coefficient: 1.5
- B: moles ÷ coefficient: 2
- Product formed: 3 mol
The figures above are the calculator's own default values, shown purely so the working is visible rather than hidden — the same steps apply exactly to your own numbers, entered in the fields at the top of this page.
Reading the result
Compare the two ratios above: whichever is smaller is your limiting reactant. Equal ratios mean the reactants are in exactly stoichiometric proportion and both are consumed completely.
Where this goes wrong. Comparing masses or raw mole counts rather than the ratios. Three moles of A against two of B looks like B limits, but with the coefficients above it is A that runs out first.
A result that is wrong by an exact factor of ten, a hundred or a similar round number is almost always a units error rather than a mistake in the formula itself — checking each input against the unit stated beside it is the fastest way to track it down.
It sets the theoretical yield, and therefore the percent yield the reaction is judged against. It also tells you which reagent to add in excess if it is cheap and which to measure carefully if it is not.
Take the moles consumed — the limiting ratio times that reactant's coefficient — and subtract from what you started with.
The headline figure is a: moles ÷ coefficient. With 3 mol moles of reactant A, 2 coefficient of A and 2 mol moles of reactant B, that comes to 1.5. Change any field and the figure moves with it.
Generally, no more than the least precise input justifies — a result reported to six decimal places from inputs measured to two significant figures is implying a precision the calculation does not actually have. The calculator shows full precision so you can round appropriately for your own use.
Yes — the equation shown in the formula section above is the standard form used in textbooks and reference material for this calculation, not a simplified or approximate version.
Yes, in the sense that it applies the correct standard formula and returns an accurate result for the inputs given — but check your own course or publication's requirements for how results should be rounded, presented and referenced, since those conventions vary and are not something a calculator can know on your behalf.