Photograph of a Drager hand pump used to measure carbon dioxide levels in the environment.How to Convert Between Percent & Parts Per Million (ppm) Concentrations in Liquids or Gases

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Convert ppm to percent or percent to ppm: this article describes the simple math used to convert between parts per million (PPM) and percentage, such as how we convert a measurement of 800 ppm of CO2 in air (indicated on our colorimetric gas measurement tube shown at page top) and a percentage concentration of that same gas in air.

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How to Convert Between Percent of a Gas in Air & Parts Per Million (ppm) of a Gas in Air

Percent concentration of substance X within another substance Y multiplied by 10,000 = measurement in ppm concentration of the substance X in Y.

Parts per million (ppm) of X divided by 10,000 = percentage concentration of X

We are calculating the percentage concentration of X in Y even if "Y" is not being stated. People often talk about concentrations without stating the "in what" substance (Y) because they think we know what the heck they're talking about. We don't. Or they don't.

If I make a Carbon monoxide (CO) measurement and my instrument tells me I've got 800 ppm of CO, and my hygienist asks me to write the report in percent concentrations what now?

Just watch the decimal point and we can avoid embarrassment.

Example of conversion from parts per million to the percentage concentration of a substance

To convert 800 ppm of gas X to percentage of gas X in air divide the ppm by 10,000

800 / 10,000 = 0.08%

Example of conversion from percentage concentration of a substance to parts per million or ppm

To convert a 0.08% concentration of substance X to parts per million of X (in whatever, air, water, gas)

0.08% x 10,000 = 800 ppm

Still nervous about converting between PPM and % concentration? Check your decimal point.

You can always check your math by using 100% - as Shelly Weinberg said, "Are you scared of the math, pick a number, any number. Still scared, OK let's try 1. -- or 100". [3]

Let's try converting 100 percent of something to parts per million.

If we have 100% or "all" of something expressed in parts per million, we should end up with exactly a million parts per million, right?

100 % x 10,000 = 1,000,000. which is a million (when speaking English).

Are ppm and concentration numbers accurate?

Well yeah, in some cases such as measuring gas concentrations in air in an enclosed, stable condition space. Otherwise, well probably not very.

Watch out: in the field when measuring levels of gas contaminants or airborne mold spore contaminants, people are lying to you. Except under controlled environments, the true level of airborne contaminants varies constantly as building conditions, weather, equipment operations, etc. vary.

If you are given an airborne toxic mold spore concentration of 81,926.939 spores per cubic meter of air as a result of a simple "air test for toxic mold", other than a general conclusion that "the number looked pretty big" the rest, the precision is nonsense. It's a very precise number (three decimal places!) but it's almost certainly not truly accurate.

Open a window, turn on a fan, walk through the room, move the sampler, change the time of day, and the number will almost always be enormously different. Not to mention that the most harmful mold in the building might not even have been collected in the air test.

OPINION: I'd prefer to report the number above as "80,000 +/- 50,000" or something like that, based on a combination of experience, building observations, case history, procedures and equipment used. The "right" answer to such studies is "it looks big" or "it doesn't look big". Why don't your labs and experts report like this? Competition. More digits = "credibility" in the minds of some customers. "If we don't report that way we'll lose business" is what lab directors have told me at conferences. So it's our fault for not paying attention in 6th grade math class. (Thank you Miss Morton, Tuckahoe Elementary School).

Details of this problem are at ACCURACY vs PRECISION of MEASUREMENTS.

Example: Concentrations of Carbon Monoxide

Bachrach indicates that for gas-fired equipment CO should not exceed 400 ppm in the flue gases. [1]

Bacharach explains that conversion burners and gas-designed equipment, if over-fired, can have flame impingement on cold surfaces and may produce CO even if they are being run with excess air and even though CO2 and O2 are at acceptable limits.[1][2]

Excess air is introduced in gas combustion to assure that no CO remains in the flue products. See Matzen's FURNACES & IAQ or for an engineer's view see COMPLETE COMBUSTION, STOICHIOMETRIC for details about "complete combustion".

RW Beckett Corporation also produces combustion and flue-gas measurement equipment. However a review of literature they provided to us did not find equipment specifically for CO measurements. Beckett's tests measure oxygen percentage.[1][2]

Other companies such as MSA, ISC, Monitox, and MDA also produce equipment which may be useable for these tests; these companies did not provide literature for this article.




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