Weigh what you actually got, type in what the stoichiometry said you should have got, and it divides one by the other and prints a percentage to two decimal places. Two boxes, one button, one number.
It is for the lab report that ends with that line, for the supervisor who asks what you recovered, and for anyone who has stared at a watch glass wondering whether the pile in front of them is a decent result or a disaster. The arithmetic takes a second, so the interesting part is the two numbers you feed it.
The honest detail is that this tool does no chemistry. It never looks at your balanced equation and it cannot tell which reagent ran out first, so it will happily divide 4.5 by 4.0 and report that you beat the laws of nature. The theoretical yield you type in does all the work.
Calculate percent yield from actual and theoretical yield. Percent yield = (actual / theoretical) × 100.
How it works
Actual yield is what you have at the end, in grams if you are weighing a solid. Theoretical yield is the maximum your balanced equation allows from the limiting reagent, also in grams. Press Calculate and the panel prints Percent yield = 81.43% rounded to two decimals; Reset clears both fields.
The formula is the one you already know, (actual / theoretical) x 100. The tool refuses to divide by zero or a negative theoretical yield and asks for valid values instead, because an empty theoretical field is the easiest way to get a nonsense answer. An actual yield of zero is allowed, since sometimes the answer really is 0.00%.
The labels say grams, but nothing in the code cares about units: moles or millilitres work as well, as long as both boxes hold the same one. A ratio only means something when the two sides are measured the same way.
A worked example
The default pair in the placeholders: 3.2 g obtained against 4.0 g theoretical. That is 3.2 / 4.0 = 0.8, times 100, so Percent yield = 80.00%. Four fifths of what the equation promised is, in most courses, a reasonable afternoon.
A messier pair: 2.85 g out of a theoretical 3.50 g gives 0.814285…, so the panel rounds to 81.43%. That second decimal place is the calculator being polite. Your balance reads to 0.01 g and your theoretical figure is three significant figures, which supports 81.4% rather than 81.43%.
Now the two that start arguments. 4.5 g against 4.0 g gives 112.50% and 0.5 g against 4.0 g gives 12.50%. The first is not a triumph, the second is not necessarily a ruined experiment, and the tool will not comment on either.
Where the number misleads
Over 100% almost always means the material is not what you think it is. Wet product, residual solvent that never drove off, starting material still stuck to it, or grease from a spatula all add mass without adding product. Sometimes the theoretical yield was calculated from the wrong reagent, and the excess one is not what sets the ceiling.
Under 100% has a longer list: product left in the mother liquor, losses on the filter paper, a side reaction, the smear that stayed on the glassware during transfer. On a small scale one rinse that did not happen is worth several points. The number summarises everything that went on, so when it comes out low the useful question is where it went.
Purity and yield pull against each other. A smaller pile that passes the melting point test beats a bigger pile still containing solvent, and a recrystallisation recovering less solid for a clean product is usually the right trade.
And do not compare yields across different reactions as though they were grades. A reaction that is 60% and clean tells you more than one that is 90% and contaminated, and each method has a range this page cannot know. Treat the output as a guideline, not a promise, and let your procedure notes explain the gap.
Other tools
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Frequently asked questions
How do I find the theoretical yield?
From the balanced equation and the limiting reagent: take the moles of whichever runs out first, apply the mole ratio to get moles of product, then multiply by its molar mass. The tool does none of that, so the number you type in is only as good as that working.
Can percent yield be over 100%?
It can, and it usually points at wet or impure product, solvent that never evaporated, or a theoretical yield worked out from the wrong reagent. Dry the product and recheck the calculation before treating a figure above 100% as good news.
What is a good percent yield?
It depends on the reaction and the purification, so there is no universal pass mark. What your course expects is in the lab manual, and a yield that looks mediocre next to a literature value may be normal for a first attempt on your scale.
What if my actual yield is higher than the theoretical?
Recheck both inputs, especially whether the boxes are in the same unit and whether the theoretical value came from the limiting reagent. If the numbers are right, the product is probably still wet, so dry it and weigh again rather than reporting 112%.
Does it matter whether I use grams or moles?
Not for the division, as long as both sides match. Grams against grams works, moles against moles works, and grams against moles gives a meaningless percentage that the tool prints just as happily.