Percent Purity Calculator

Percent purity answers one narrow question: of the material sitting on the balance right now, what fraction is genuinely the compound you think it is? The formula is simple: percent purity = (mass of pure compound / total mass of sample) x 100. The "total mass of sample" is everything you weighed, including any solvent, unreacted starting material, byproduct, or moisture trapped alongside the real compound. The "mass of pure compound" is the portion of that weight that independent evidence (not just visual judgment) confirms is the target substance.

This calculation says nothing about how the sample was made or how it was cleaned up. It is a snapshot of composition at one point in time, taken after the sample already exists. That is what separates it from the two other percentages people often confuse it with.

These three percentages get mixed up constantly because all three divide one mass by another, but each one answers a different question. Percent yield compares the mass of product you isolated to the theoretical mass a balanced equation predicts, and is explained in full on the percent yield homepage. Percent recovery, covered on the percent recovery page, compares the mass that survived a purification step (recrystallization, extraction, distillation) to the mass that went into that step, and it says nothing about identity or cleanliness, only about how much material was not lost in handling.

Percent purity is different from both. It does not compare your result to a theoretical target and it does not compare a before-and-after purification mass. It compares the pure compound to the rest of the same sample, weighed at the same moment. A sample can have excellent recovery (almost nothing was lost during recrystallization) and still have poor purity (a lot of what survived was solvent or a co-crystallized byproduct). The three numbers are independent measurements, and a full lab report often needs all three.

Weighing a sample never tells you its purity by itself, it only gives you the total mass. Purity requires a second, independent measurement that identifies how much of that mass is the real compound. Common approaches, described here conceptually rather than as instructions for a specific instrument, include:

• Melting point comparison. A pure solid melts sharply at a value matching the literature reference; a depressed or broadened melting range signals a mixture, and the size of that depression gives a rough sense of how much impurity is present.
• NMR integration. Peak areas in a proton or carbon NMR spectrum are compared, and the ratio of signals belonging to the target compound versus any impurity or residual solvent gives a purity estimate.
• Titration. For compounds with a reactive functional group (an acid, base, or oxidizer), a titration measures how many moles of that group are actually present, which converts to a mass of "active" compound within the sample.
• HPLC or GC area percentage. A chromatogram separates components, and the area under the target compound's peak, as a fraction of total peak area, is used as a purity estimate.

Each method has real limits and none of them is infallible on its own, but the shared idea is the same: purity is inferred from an independent measurement, not assumed from the weighing.

An impossible percent yield reading (anything over 100%) almost always traces back to purity. The homepage's percent yield explanation covers the full mechanics of why yields above 100% are chemically impossible and what to check first; the purity connection specifically works like this: the balance cannot distinguish the target compound from whatever else is sitting next to it in the weighing vessel. If the isolated solid still carries residual solvent, unreacted starting material, or a byproduct, the total weighed mass is inflated past what the pure compound alone would weigh. Divide that inflated mass by the theoretical mass from stoichiometry and the resulting percentage climbs past 100%, even though not one extra mole of real product was ever created.

This is also why an unusually high yield deserves the same scrutiny as an unusually low one. A number like 108% is not evidence of an efficient reaction, it is evidence that the isolated mass is not 100% pure compound. Checking purity by one of the methods above, or comparing against common sources of measurement error, is the standard way to resolve a yield that reads over 100%.

1. Straightforward sample. A sample weighs 5.00 g in total. Independent testing confirms 4.20 g of that mass is the pure target compound. Percent purity = (4.20 / 5.00) x 100 = 84.0%. The remaining 0.80 g is solvent, moisture, or byproduct mixed in with the real compound.
2. High-purity recrystallized product. After recrystallization, a product weighs 3.50 g, and 3.42 g of that is confirmed pure. Percent purity = (3.42 / 3.50) x 100 = 97.7%. A result in this range is typical of a solid that has been recrystallized at least once and shows a sharp melting point close to the literature value.
3. Crude product before purification. A crude reaction mixture, filtered but not yet purified, weighs 6.00 g, of which only 3.90 g is confirmed to be the actual target compound. Percent purity = (3.90 / 6.00) x 100 = 65.0%. A result this low is expected for crude material straight out of a reaction, and it is exactly why a percent yield calculated from the crude mass would overstate the reaction's real efficiency; purification has to happen first before the isolated mass reflects the true product.

These labels describe common usage, not a fixed numeric cutoff; the same sample might be called "technical" in one lab and "reagent grade" in another depending on what it will be used for next.

If a purity check comes back low, the fix is almost always another purification step, not a recalculation. Recrystallizing from a more selective solvent, running a second extraction to wash out a co-eluting impurity, or passing the sample through a chromatography column again are the standard next moves. After each step, recheck the melting point, NMR integration, or titration value rather than assuming the new mass is automatically cleaner. For a wider view of how purity fits alongside the rest of a synthesis write-up, including stoichiometry and the accompanying error checks, the chemistry guide walks through the full sequence from balanced equation to final reported yield.