Scientists use a variety of different units to describe the concentrations of chemicals in the air. One common unit for low-level concentrations is parts per million, or ppm. Parts per million by volume refers to the volume units, such as millilitres, of a gaseous chemical found in one million equivalent units of air. The concentration of a gas in per cent by volume is a similar concept, except that in this case, the concentration gives the units of gas per one hundred equivalent units of air. You can convert between these two concentration units based on the ratio of one million to one hundred.

- Skill level:
- Moderate

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## Instructions

- 1
Enter the value of concentration, in ppm by volume, into the calculator. For example, if the concentration is 500 ppm, you would enter the number 500.

- 2
Divide the number you just entered by 10,000. Because one million is ten thousand times greater than one hundred, this converts the value into parts per one hundred, which is the same as percentage. In the case of the example, you would divide 500 by 10,000 to get 0.05.

- 3
Report the result of the previous calculation as the per cent by volume of the mixture, using the notation % v/v. The example result would therefore be reported as 0.05 % v/v.

#### Tips and warnings

- Adding the abbreviation "v/v" to your units of per cent makes sure that the reader knows the percentage is by volume, not by weight.