Percent

One part per hundred: Should be represented by the percent (%) symbol and denotes one part per 100 parts, one part in 10², and a value of 1 × 10–2. This is equivalent to one drop of water diluted into 5 milliliters (one spoon-full) or 14 minutes, 24 seconds out of one day.

Permille

One part per thousand: Should be spelled out in full and should avoid the use the “ppt” abbreviation, which is usually understood to represent “parts per trillion”. It may also be denoted by the permille (‰) symbol. Note however, that specific disciplines such as the analysis of ocean water salt concentration and educational exercises occasionally use the “ppt” abbreviation. “One part per thousand” denotes one part per 1000 parts, one part in 10³, and a value of 1 × 10–3. This is equivalent to one drop of water diluted into 50 milliliters (ten spoon-fulls), or one minute, 26.4 seconds out of one day

PPM

One part per million (ppm): Denotes one part per 1,000,000 parts, one part in 10 exp 6, and a value of 1 × 10–6. This is equivalent to one drop of water diluted into 50 liters (roughly the fuel tank capacity of a compact car), or one second of time in approximately 11½ days.

PPB

One part per billion (ppb): Denotes one part per 1,000,000,000 parts, one part in 10 exp 9, and a value of 1 × 10–9. This is equivalent to 1 drop of water diluted into 250 chemical drums (50 m3), or one second of time in approximately 31.7 years.

PPT

One part per trillion (ppt): Denotes one part per 1,000,000,000,000 parts, one part in 10 exp 12, and a value of 1 × 10–12. This is equivalent to 1 drop of water diluted into 20, two-meter-deep Olympic-size swimming pools (50,000 m3), or one second of time in approximately 31,700 years.