These are all dimensionless ratios expressing the same thing at different scales, so the conversion is a shift of decimal point and nothing more.

Volume or mass, and why it matters

A ratio unit says nothing about what is being counted. For gases the convention is by volume, which for an ideal gas is the same as the mole fraction, and it is usually written ppmv or vol %. A ratio by mass, ppmw, is a different quantity for the same gas mixture, and the two differ by the ratio of molecular weights. Water vapour in air at 1.2 vol % is about 0.75 mass %, for instance. This tool converts between scales, not between bases, so whichever basis goes in comes out unchanged. Convert the basis first if you need to, then scale it here.

The scale of these numbers

• 1 % is one part in a hundred. Argon makes up about 0.93 % of dry air.
• 1 ppm is one part in a million, which is one drop of water in a 50 litre drum.
• 1 ppb is one part in a thousand million, which is the same drop in a 50 000 litre road tanker.

Trace measurement below ppb is quoted in ppt, parts per trillion, which is one part in a million million. Ambient background levels of several atmospheric gases sit in that range.

Note on ppb and ppt

The short scale is assumed throughout: a billion is 10⁹ and a trillion is 10¹². Older European usage sometimes means 10¹² by billion, which is a factor of a thousand. Standards bodies discourage ppb and ppt for this reason and prefer explicit powers of ten in formal documents, though the shorthand remains universal in instrumentation.