Limiting Reagent Calculator
Find which reactant runs out first, plus the theoretical yield and what is left over.
What is Limiting Reagent?
When you mix reactants in amounts that do not match the equation, one of them runs out first and stops the reaction — that is the limiting reagent, and it alone determines how much product you can make. The amount predicted from it is the theoretical yield, the ceiling against which a real yield is measured.
How it works
Divide each reactant's moles by its coefficient in the balanced equation. The smallest quotient is the limiting reagent; every other reactant is in excess by exactly the difference. Products are then computed from that smallest quotient multiplied by their own coefficients. Comparing raw mole amounts instead of quotients is the classic error this table is designed to prevent — the reactant with fewer grams is often not the limiting one.
- Enter what you know. Every field recalculates as you type — there is no submit button. Leave the quantity you want blank and the page solves for it.
- Read the answer and the working. The result appears above the fold; underneath it is the formula with your numbers substituted, so the arithmetic can be checked rather than trusted.
- Check the units and the size. Units are shown on every value and selectable per field, so a result in kJ, kPa or km/h is converted rather than assumed. Sanity-check the magnitude against a known landmark such as 1 g/cm³ for water.
Examples
A textbook problem with a known answer
Every tool here is checked against standard example problems — the molar mass of C6H12O6 is 180.16 g/mol, and free fall from 20 m takes about 2.02 s at g = 9.8 m/s².
A result you want to reason backwards from
Leave the quantity you already know blank instead: given the final answer, most tools recover the starting value — the mass needed for a target concentration, or the height a fall must have had.
Common mistakes
Forgetting that a formula is only as good as its subscripts
H2O and H2O2 are different compounds. The formula field shows its parse — element by element — so a mis-typed subscript is visible before it reaches the arithmetic.
Mixing units in the middle of a calculation
The most common source of a wrong answer is a value in grams next to a formula expecting kilograms, or Celsius next to a gas constant expecting kelvin. This category converts on the way in and labels what it used.
Frequently asked questions
Is the limiting reagent always the one with the smallest amount?
No. It is the one with the smallest moles divided by its coefficient. A reaction needing 3 moles of hydrogen for every mole of nitrogen can run out of hydrogen despite having more of it by mass.
What is theoretical yield?
The mass of product predicted if the reaction went perfectly to completion with the limiting reagent fully consumed. A percentage yield compares what you actually got against it.
Why is there excess reagent left over?
Because the other reactant was supplied in greater proportion than the equation calls for. The table shows exactly how much of each is left when the reaction stops.