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.
- 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 |
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 and converted with ppm to mg/m3 and percent gas converter.
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 and Dew point to ppm calculator for gases.
- 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
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.
