--- title: ppm, Percent and ppb Explained for Gas Analysis url: https://www.sarveshanalytics.com/learn/ppm-percent-and-ppb-units-explained.html description: What ppm, ppb and percent by volume mean, how to convert between them, why ppmv and ppmw differ, and how dew point relates to ppm. With worked examples. updated: 2026-11-03 source: html --- Learn · Foundations # ppm, percent and ppb: the units of gas concentration A number without its unit is a guess. This lesson fixes the units once, so that the rest of gas analysis reads clearly. Beginner 3 min read Lesson 4 of 9[Foundations](https://www.sarveshanalytics.com/learn/track-foundations.html) Helpful first: [How gases behave: pressure, temperature and partial pressure](https://www.sarveshanalytics.com/learn/how-gases-behave-pressure-temperature-and-partial-pressure.html) Quick answer Percent, ppm and ppb are volume fractions: 1 % equals 10,000 ppm, and 1 ppm equals 1000 ppb. A reading of 100 ppm oxygen is 0.01 %. Gas analysers report parts per million by volume (ppmv), not by mass. Moisture is often quoted as a dew point instead of ppm. Updated 3 November 2026 · Figures from Sarvesh datasheets **After this lesson you can** - Convert between percent, ppm and ppb in your head - Tell ppm by volume from ppm by mass - Match the unit to the measurement range - Know where dew point fits in ## Three scales of the same idea A concentration says how many parts of a gas there are in a hundred, a million or a billion parts of the mixture. All three are ratios of molecules, so they do not depend on temperature or pressure. | Unit | Meaning | Equals | |---|---|---| | Percent (%) | 1 part in 100 | 10,000 ppm | | ppm | 1 part in a million | 0.0001 %; 1000 ppb | | ppb | 1 part in a billion | 0.001 ppm | **Rule of thumb** To go from percent to ppm, multiply by 10,000. To go from ppm to percent, divide by 10,000. So 0.5 % is 5000 ppm and 250 ppm is 0.025 %. ## Which unit for which range - **Percent** for main components and wide ranges: oxygen 0 to 25 %, methane 0 to 100 %, CO2 0 to 20 %. - **ppm** for impurities and trace levels: oxygen in nitrogen 0 to 1000 ppm, CO in a furnace atmosphere. - **ppb** for ultra-trace work such as semiconductor gases. ppb analysis needs different instruments than ppm; the ACCURRA-S ranges start at ppm. A common specification error is to ask for a percent range when the process limit is in ppm, or the other way round. Write both: for example 0 to 1000 ppm (0 to 0.1 %). ## ppmv is not ppmw Analysers report parts per million **by volume** (ppmv), which for gases equals mole fraction. Some standards state limits **by mass** (ppmw, or mg/kg). The two differ by the ratio of the target's molar mass to that of the carrier gas, so 10 ppmv of oxygen in hydrogen is a very different mass ratio from 10 ppmv of oxygen in nitrogen. When a limit is written as ppm with no qualifier on a gas, it almost always means by volume. If you are unsure, ask. Mass concentrations such as mg/m3 are covered in [ppm to mg/Nm3: volume vs mass concentration](https://www.sarveshanalytics.com/learn/volume-mass-and-mg-per-nm3-concentration-units.html) and converted with [ppm to mg/m3 and percent gas converter](https://www.sarveshanalytics.com/calculators/unit-converter-ppm-percent-mg-per-m3.html). ## Where dew point fits For very dry gas, moisture is usually quoted as a **dew point** in degrees C rather than ppm. They carry the same information; each 10 degrees colder is about three times drier. For example minus 40 C is about 127 ppm and minus 60 C is about 11 ppm at atmospheric pressure. See [What is dew point and how is it measured](https://www.sarveshanalytics.com/learn/what-is-dew-point-and-how-is-it-measured.html) and [Dew point to ppm calculator for gases](https://www.sarveshanalytics.com/calculators/dew-point-to-ppm-calculator.html). One logarithmic scale from 1 ppb to 100 %. The bars are the ranges of three ACCURRA-S oxygen models, from the datasheet. **In short** - 1 % = 10,000 ppm; 1 ppm = 1000 ppb. - Analysers report concentration by volume, which is a mole fraction. - Always state the unit and the basis; write both percent and ppm when a limit is near the boundary. - Dew point and ppm are two ways to state moisture. ## Check your understanding Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading. **Convert 0.025 % to ppm.** 250 ppm: multiply by 10,000. **A nitrogen supply must hold oxygen below 5 ppm. How many percent is that?** 0.0005 %. **Why can 10 ppmv and 10 ppmw describe different amounts?** Mass ratio depends on molar masses. 10 ppmv of a gas lighter than the carrier is a smaller mass ratio, and a heavier one is larger. ## Key terms - [ppm](https://www.sarveshanalytics.com/learn/glossary.html#t-ppm) - [ppb](https://www.sarveshanalytics.com/learn/glossary.html#t-ppb) - [Volume fraction](https://www.sarveshanalytics.com/learn/glossary.html#t-volume-fraction) - [Dew point](https://www.sarveshanalytics.com/learn/glossary.html#t-dew-point) - [Trace measurement](https://www.sarveshanalytics.com/learn/glossary.html#t-trace-measurement) ## Frequently asked questions **How many ppm is 1 %?** 10,000 ppm. **Is ppm a mass or a volume ratio in gas analysis?** Volume, unless the specification says otherwise. **What does 99.999 % purity mean in ppm of impurity?** Five nines means 10 ppm total impurity, since 100 minus 99.999 is 0.001 %. **Why do some limits use mg/m3?** Environmental rules usually state mass per volume at reference conditions. See the converter. ## Related products [ ### ACCURRA-S Online analyser for trace, percent and purity O2, CO, CO2, CH4 and dew point; can measure two gases from one sample. View ACCURRA-S ](https://www.sarveshanalytics.com/safe-gas-analyser.html) ## Where this applies - [Dual gas trace oxygen and moisture](https://www.sarveshanalytics.com/trace-oxygen-and-moisture-analyser.html) ## Where to go next - [Dry basis vs wet basis in gas measurement](https://www.sarveshanalytics.com/learn/dry-basis-vs-wet-basis-gas-measurement.html)