Parts-per notations are powers of ten
Each notation is a fraction times a power of ten. Percent is parts per 10², per mille (‰) per 10³, per myriad (‱, a basis point) per 10⁴, ppm per 10⁶, ppb per 10⁹, and ppt per 10¹². To convert, move the decimal point by the difference in the exponents.
Example: 250 ppm = 250 ÷ 10⁶ = 0.00025 = 0.025% = 0.25‰ = 250,000 ppb, or 1 part in 4,000.
| Notation | Parts per | 1 unit as a fraction |
|---|---|---|
| percent (%) | 100 | 0.01 |
| per mille (‰) | 1,000 | 0.001 |
| per myriad (‱) | 10,000 | 0.0001 |
| ppm | 1,000,000 | 10⁻⁶ |
| ppb | 10⁹ | 10⁻⁹ |
| ppt | 10¹² | 10⁻¹² |
The ppt ambiguity, and what ppm refers to
“ppt” usually means parts per trillion, but in oceanography and some chemistry it means parts per thousand, which is the same as ‰. A 35 ppt salinity is 35‰, not 35 × 10⁻¹². Choose the parts-per-thousand option if that is what your source means.
NIST recommends against ppm, ppb and ppt in formal work because they depend on language (in some countries a billion is 10¹², not 10⁹) and don't say what is being compared. Always state whether the ratio is by mass, volume or amount of substance. For dilute water solutions, 1 ppm by mass is close to 1 mg/L.