Percent Purity & Sample Assay Calculator

Free Online Percent Purity & Sample Assay Calculator

This percent purity and sample assay calculator works out how pure a chemical, metal, drug, or lab sample really is, in three different ways. If you already know the mass of the pure substance and the mass of the full impure sample, it gives you the percent purity instantly, along with the impurity mass and impurity percentage. If you are planning a reaction or an order and you only know the purity grade, it can flip the problem around and tell you exactly how much impure material you need to weigh out to get a target amount of pure substance. And for anyone working in a QC lab, it can run a complete sample assay against a certified reference standard, the same way pharmaceutical, food, and industrial testing labs report a batch's active content.

Every result comes with the full working shown step by step, a purity breakdown chart, a comparison against common everyday purity benchmarks like gold karats and pharmaceutical-grade limits, and a one-click CSV export so you can drop the numbers straight into a lab notebook or report. The calculator updates instantly as you type, so you can try a few different numbers and see how the purity or assay result changes in real time.

What Is Percent Purity?

Percent purity tells you what fraction of a sample is actually the substance you care about, and what fraction is made up of unwanted impurities, water, filler, or by-products left over from how the material was made or extracted. Almost nothing you buy or make in a lab is 100% pure — even 'pure' table salt, gold jewelry, and pharmaceutical tablets all carry a small, known percentage of something else mixed in.

Chemists, jewelers, pharmacists, and quality control technicians all use percent purity for the same basic reason: it lets you compare two batches of the same material fairly, price a raw material honestly, and calculate exactly how much of the real, active ingredient you're actually working with, rather than guessing from the total weight alone.

The Percent Purity Formula

The core formula behind this calculator is short and easy to remember. In every case, purity compares the mass of the substance you actually want against the total mass of everything you weighed:

• Percent Purity = (Mass of Pure Substance ÷ Mass of Impure Sample) × 100
• Impurity Mass = Mass of Impure Sample − Mass of Pure Substance
• Percent Impurity = 100 − Percent Purity
• Required Sample Mass = Target Pure Mass ÷ (Percent Purity ÷ 100)

Worked Example: Finding Percent Purity

Suppose a student is given 20.00 g of an impure sample of copper sulfate and, after purifying and drying it in the lab, recovers 18.50 g of pure copper sulfate crystals. Using the formula above: Percent Purity = (18.50 ÷ 20.00) × 100 = 92.5%.

That means 92.5% of the original sample's weight was genuine copper sulfate, and the remaining 1.50 g — or 7.5% — was made up of impurities that were left behind or washed away during purification. This is exactly the kind of calculation covered in school and college chemistry practicals on recrystallization and purification.

Worked Example: Sizing a Sample for a Target Purity

Now flip the problem around. Suppose a manufacturer needs exactly 5.00 g of pure active ingredient for a batch, and the raw material they're buying is only 92.5% pure. How much raw material should they actually weigh out? Required Sample Mass = 5.00 ÷ (92.5 ÷ 100) = 5.41 g.

So 5.41 g of the 92.5%-pure raw material must be weighed out to guarantee 5.00 g of pure active ingredient once the impurities are accounted for — a calculation that comes up constantly in industrial batching, compounding pharmacies, and procurement.

What Is a Sample Assay? The Assay-Against-Standard Formula

In analytical and pharmaceutical chemistry, purity is rarely measured by simple weighing alone — instead, a test sample's response on an instrument (like an HPLC peak area, a UV absorbance reading, or a titration endpoint) is compared against the same measurement for a certified reference standard of known purity. This comparison is called an assay, and it's the standard way drug potency, food-additive content, and industrial reagent strength are officially reported.

The assay-against-standard formula used by this calculator's advanced mode is: % Assay = (Test Response ÷ Standard Response) × (Standard Weight ÷ Test Weight) × (Test Dilution ÷ Standard Dilution) × Standard Purity (%). Each ratio corrects for one real-world variable — instrument signal, how much was actually weighed, and how much each solution was diluted — so the final number is fully traceable back to a certified reference material.

Worked Example: Sample Assay Calculation

A QC lab tests a batch of an active ingredient by HPLC. The test sample gives a peak area of 48,250, while a 99.80%-pure reference standard gives a peak area of 49,010 under identical conditions. The test sample weighed 25.60 mg, and the standard weighed 25.20 mg, with no additional dilution on either side.

% Assay = (48,250 ÷ 49,010) × (25.20 ÷ 25.60) × 1 × 99.80 = 96.66%. This tells the lab that the batch contains 96.66% of its labeled active ingredient — a result that would typically be checked against a specification range, such as 95.0%–105.0%, before the batch is released.

How to Use This Purity & Assay Calculator

Pick 'Percent Purity from Masses' if you already weighed both the impure sample and the recovered pure substance — just type both numbers in grams and read off the purity, impurity mass, and impurity percentage instantly, along with a donut chart of the split.

Pick 'Required Sample for a Target Pure Mass' if you're planning ahead and need to know how much raw, impure material to weigh out to guarantee a certain amount of pure substance at a known purity grade — useful for ordering raw materials, scaling up a recipe, or compounding a formulation.

Pick 'Sample Assay vs. Reference Standard' for the full analytical-lab workflow: enter the test and standard instrument responses, the weights of each, the standard's certified purity, and (optionally) any dilution factors, to get a complete, traceable percent assay result with every intermediate ratio shown.

Common Purity Grades and Benchmarks

Purity requirements vary hugely by industry. A few widely used real-world benchmarks are built into the comparison chart in this calculator:

• 24K gold — 99.9% pure gold, the fine-gold bullion standard
• Pharmaceutical (USP) active ingredients — typically must assay between about 98% and 102% of label claim
• Analytical Reagent (AR/ACS) grade chemicals — around 99.0% purity or higher, used for precise lab work
• 18K gold — 75.0% pure gold (18 out of 24 parts)
• Sterling silver — 92.5% silver, alloyed with 7.5% other metal for strength
• 14K gold — 58.3% pure gold (14 out of 24 parts)

Where Percent Purity and Assay Calculations Are Used

Percent purity and sample assay are two of the most widely used quality checks across science and industry, because they turn a raw weight or instrument reading into a number everyone can trust and compare.

• Pharmaceutical manufacturing and quality control — confirming a drug batch's active ingredient content against its label claim
• Precious metal trading and jewelry — reporting gold, silver, and platinum purity in karats or fineness
• Food and beverage testing — checking the concentration of additives, preservatives, or active compounds
• Chemical manufacturing and procurement — pricing and specifying raw materials by their purity grade
• Academic and school chemistry labs — recrystallization, titration, and gravimetric purification experiments
• Environmental and water testing — reporting the concentration of a target compound recovered from a sample

Common Mistakes When Calculating Purity or Assay

The most common mistake is dividing the wrong way round — percent purity always puts the pure substance mass on top and the total impure sample mass on the bottom, never the other way around. Mixing this up gives a number that looks plausible but is actually meaningless.

Another common mistake in assay calculations is forgetting to correct for the weight difference between the test sample and the standard, or forgetting the standard's own certified purity entirely — a reference standard that is itself only 99.8% pure will quietly shift every result if that factor is left out. Always double-check that response readings, weights, and dilution factors are all recorded under the same units and the same experimental conditions before comparing them.

Frequently Asked Questions

What is the formula for percent purity? Percent Purity = (Mass of pure substance ÷ Mass of impure sample) × 100.

How do you calculate percent purity from a titration or assay? Compare the test sample's instrument response against a reference standard's response, correct for the weights and dilutions used, then multiply by the standard's declared purity.

What is the difference between purity and assay? Purity usually refers to a simple mass-based measurement of how much of a sample is the genuine substance, while assay usually refers to an instrument-based comparison against a certified reference standard, common in pharmaceutical and analytical labs.

How much impure sample do I need for a target pure mass? Divide the target pure mass by the purity written as a decimal fraction — for example, needing 5 g of pure material at 92.5% purity requires 5 ÷ 0.925 = 5.41 g of the impure sample.

Can percent purity be more than 100%? No — under normal conditions, percent purity is capped at 100%. An assay result can occasionally read slightly above 100% due to measurement variation, but this normally signals the result should be re-checked.

What units does this calculator use? Purity from masses works in grams by default, and the assay mode works in milligrams, but the ratios involved are unit-independent — you can use any consistent mass unit for both sides of the same calculation.