One part per million is 1,000 parts per billion. The factor is simple; the unit is not, because billion has meant two different numbers in different languages and the ambiguity has caused real errors in translated documents.
About the units
A part per billion is one in a thousand million under the short scale used in English. Under the long scale historically used in much of continental Europe, including Sweden, a billion meant a million million — so a European reading of ppb could be a thousand times smaller than an English one. The scientific community resolved this by discouraging ppb and ppt entirely: ISO and IUPAC recommend writing nmol/mol, µg/kg or an explicit power of ten instead. In practice ppb survives in environmental and analytical work, where it is always the short-scale meaning, but a document translated from an older European source deserves a second look.
The exact factor
The factor is exactly 1,000 under the short-scale definition. Reference values: atmospheric methane was 1,922 ppb as a global mean in 2024 and is still rising, which is 1.9 ppm; nitrous oxide 338 ppb over the same year; and the EU drinking water limit for lead, which the 2020 directive lowers to 5 µg/l by January 2036, is 5 ppb for water. Trace analysis routinely works at parts per trillion, where the naming problem becomes worse still — and where the recommended notation of picomoles per mole removes it entirely.
Where you meet this conversion
Environmental monitoring and trace analysis. Atmospheric trace gases, drinking water contaminants, pesticide residues in food and impurities in pharmaceuticals are all reported in the parts-per notation. Mass spectrometry and chromatography detection limits are quoted in ppb or ppt. The safest practice in any technical document is to state the ratio explicitly — 1.9 µmol/mol rather than 1.9 ppm — since that notation cannot be misread regardless of the reader's language.