Parts per Trillion to PPB Converter
Common Conversions
| ppt | ppb |
|---|---|
| 1 | 0.001 |
| 5 | 0.005 |
| 10 | 0.01 |
| 50 | 0.05 |
| 100 | 0.1 |
| 250 | 0.25 |
| 500 | 0.5 |
| 1000 | 1 |
| 5000 | 5 |
| 10000 | 10 |
| 100000 | 100 |
| 1000000 | 1000 |
Why this conversion matters in chemistry
Common case: source-assessment cross-era reconciliation. A 5000 ppt PFOA reading from a modern LC-MS/MS run is 5 ppb on the older trace-contaminant convention used in pre-2020 contamination surveys. Either way the result sits orders of magnitude above the single-digit ppt range current drinking-water limits for PFOA are set in. You use it when bridging modern PFAS-detection technology against historical contamination records.
Formula
Where the factor comes from
No SI prefix appears anywhere in this pair, which makes it unusual company for the rest of the concentration set — the factor of 1000 comes out of English number words rather than the SI brochure. On the short scale a billion is 10⁹ and a trillion is 10¹², so ppb names the fraction 10⁻⁹ and ppt names 10⁻¹². Divide the second by the first and you get 10⁻³, exactly. Nothing measured enters the calculation, so the factor contributes no uncertainty. The exactness is real but conditional: it holds because both labels were built on the same counting scale and, more importantly, on the same denominator. Neither ppb nor ppt is an SI unit, and neither declares what it was divided by — a mass, a volume, or a count of moles.
Precision and significant figures
Dividing by an exact 1000 preserves the digit count untouched: 4300 ppt is 4.3 ppb, two figures on either side. What those figures are worth is decided at the ppt end, and that end is unforgiving. At ultratrace levels the method blank frequently sits within a small multiple of the sample, so the reported value is a difference between two similar numbers and the relative uncertainty inflates accordingly. A relative standard deviation of ten to twenty percent near a quantitation limit is ordinary rather than alarming. One significant figure is honest for a result a few multiples above the blank; two is the practical ceiling until the value climbs well up the calibration range.
Worked Examples
The conversion anchor — the prefix step from trillion to billion.
A single ppt — about a typical modern PFAS detection floor.
About a sub-ppb environmental contamination level.
About a typical trace-contaminant level in a contaminated source.
Common mistakes
Salinity ppt divided by 1000 anyway
Oceanography and older process literature write ppt for parts per thousand, and the division runs on regardless: a 35 ppt salinity becomes 0.035, a number that reads like an unremarkable trace result. On a parts-per-billion basis that same seawater is 3.5 × 10⁷ ppb. Nine orders of magnitude ride on three identical letters, so settle which exponent the source means before dividing.
Long-scale billion and trillion
Several European conventions run the long scale, where a billion is 10¹² and a trillion 10¹⁸, with milliard filling the 10⁹ slot. A translated report or a locale-set instrument can hand you a ppb that means what you would call ppt. Reading the exponent off the document rather than trusting the abbreviation settles it in one step.
Non-detects converted as if measured
A result reported as <2 ppt is a ceiling, not a value. Dividing it to <0.002 ppb is fine; dropping the less-than on the way across is not, and a censored value that loses its qualifier gets averaged and summed as though somebody had measured it. Carry the qualifier with the number, and decide separately how censored results enter any summary statistic.