PPB to ng/g Converter
Common Conversions
| ppb | ng/g |
|---|---|
| 0.1 | 0.1 |
| 1 | 1 |
| 5 | 5 |
| 10 | 10 |
| 50 | 50 |
| 100 | 100 |
| 500 | 500 |
| 1000 | 1000 |
| 5000 | 5000 |
| 10000 | 10000 |
| 100000 | 100000 |
| 1000000 | 1000000 |
Why this conversion matters in chemistry
Fish-tissue contaminant analysis crosses this identity. A 20 ppb PCB advisory action level on a fish-consumption guidance document writes equivalently as 20 ng/g on the analytical laboratory output from EPA Method 1613B or 1668C. The numbers are the same because both notations describe parts of mass per parts of mass, with ng/g landing in the same scale as ppb. The identity is the standard type cast at the boundary between regulatory advisory documentation and analytical lab data — useful for any solid-matrix trace-contaminant report.
Formula
Where the factor comes from
Little needs deriving here beyond reading the prefixes off. A nanogram is 10⁻⁹ g, so ng/g is a mass ratio of 10⁻⁹ gram per gram — which is exactly what parts per billion means when ppb is used on a mass basis. The two numbers are identical, and identical exactly: both prefixes are stipulated, the gram cancels against the gram, and no density, molar mass or reference state enters at any point. The same reasoning makes µg/kg and mg per metric ton equal to both. What does need stating is the basis, because ppb on its own is ambiguous. In a liquid it is commonly meant as µg/L, which is a mass per volume and coincides with ng/g only when the density happens to be 1.000 g/mL.
Precision and significant figures
Multiplying by one changes nothing, so every significant figure survives and none is manufactured: a result reported as 20 ppb becomes 20 ng/g, not 20.0 ng/g. The precision worth arguing about all lives upstream of the conversion. Solid-matrix numbers depend on how the sample was dried, since wet-weight and dry-weight results for the same material can differ by a large factor when moisture content is high, and they are not comparable unless the basis is stated. Recovery through extraction and cleanup, tracked with surrogates, commonly leaves 10 to 30% relative uncertainty at these levels. Two significant figures is the practical ceiling; carrying more across an identity just relabels the same number with more confidence than it earned.
Worked Examples
The conversion anchor — same ratio in different prefix combinations.
About a typical regulatory action-level for fish-tissue contaminants.
0.5 ppm — about a moderate trace-level concentration.
1 µg/g = 1 ppm — the bridge step between trace and bulk regimes.
Common mistakes
Wet weight and dry weight reported alike
A tissue or sediment result on a dry-weight basis is numerically larger than the same result on a wet-weight basis, sometimes by several fold in a high-moisture matrix. Because the ppb-to-ng/g step changes nothing, an unstated basis passes straight through the conversion unnoticed. Two labs can report honest numbers for the same material that differ severalfold purely on this.
Liquid ppb converted as mass per mass
For aqueous samples ppb usually means µg/L, a mass per volume, and equating it to ng/g requires the density to be 1.000 g/mL. In a dense brine or an organic solvent that assumption costs several percent. The identity on this page holds unconditionally only when the source value was itself a mass ratio, which is the usual case for solids and not for liquids.
Gas-phase ppb is a mole ratio
Air measurements report ppb by volume, counting molecules rather than mass, and that quantity has no direct relation to ng/g. Converting it requires the molar mass of the species and the molar volume of the gas at a stated reference state. Treating an atmospheric ppb figure as though it were mass-based produces a number that looks reasonable and means nothing.