Convert ppm CO₂ to ppb
Parts per million and parts per billion differ by a factor of a thousand, and nothing else: 1 ppm = 1,000 ppb. The conversion is definitional, so it holds for any trace gas — carbon dioxide, methane, nitrous oxide alike. What makes it worth a page is why you keep needing it: the major greenhouse gases are reported in different units because they sit at wildly different abundances, and putting them on one scale is the only way to compare them at all.
To convert parts per million to parts per billion, multiply by 1,000. So 425 ppm of CO₂ = 425,000 ppb. To go the other way, divide ppb by 1,000 — 1,866 ppb of methane is 1.866 ppm.
ppb = ppm × 1,000. Reverse: ppm = ppb ÷ 1,000. Both are dry-air mole fractions — ppm is micromoles per mole (10⁻⁶), ppb is nanomoles per mole (10⁻⁹). The factor is a definition, not a measurement, so it never changes and applies to any gas, not only CO₂.
| ppm | × 1,000 | ppb |
|---|---|---|
| 1 ppm | × 1,000 | 1,000 ppb |
| 425 ppm | × 1,000 | 425,000 ppb |
| 1.866 ppm | × 1,000 | 1,866 ppb |
| 0.001 ppm | × 1,000 | 1 ppb |
How to convert ppm to ppb
Multiply by a thousand to go from ppm to ppb; divide by a thousand to come back. Because both units express the same physical quantity — a mole fraction of dry air — nothing about the gas, the temperature or the pressure enters the calculation. It is a change of prefix, in the same way that converting kilometres to metres is.
Worked both directions. Atmospheric CO₂ at about 425 ppm is 425 × 1,000 = 425,000 ppb. Methane is conventionally reported in ppb: its 2019 level of 1,866.3 ppb is 1,866.3 ÷ 1,000 = 1.8663 ppm. Written on the same scale, the contrast is immediate — there is roughly 220 times more CO₂ in the air than methane by abundance.
ppm to ppb conversion table
The table reads both ways — ppm to ppb on the left (× 1,000), ppb back to ppm on the right (÷ 1,000).
| ppm | → ppb (× 1,000) | ppb | → ppm (÷ 1,000) |
|---|---|---|---|
| 0.001 | 1 | 1 | 0.001 |
| 0.01 | 10 | 10 | 0.01 |
| 0.3321 | 332.1 | 332.1 | 0.3321 |
| 1 | 1,000 | 1,000 | 1 |
| 1.8663 | 1,866.3 | 1,866.3 | 1.8663 |
| 10 | 10,000 | 10,000 | 10 |
| 409.9 | 409,900 | 100,000 | 100 |
| 425 | 425,000 | 1,000,000 | 1,000 |
The most common slip is inverting the direction, because the smaller-sounding unit carries the bigger number. A billionth is a smaller slice than a millionth, so it takes a thousand of them to make one — meaning the ppb figure is always 1,000× larger. If your answer got smaller going from ppm to ppb, you divided when you should have multiplied.
Why greenhouse gases are reported in different units
There is no standards body decreeing that CO₂ takes ppm and methane takes ppb. The convention is practical: each gas is quoted in whichever unit keeps the number readable. CO₂ sits in the hundreds of ppm, so ppm is natural. Methane and nitrous oxide are a thousand times rarer, so in ppm they would be awkward decimals — 1.87 and 0.33 — and ppb gives clean whole numbers instead.
The consequence is that you cannot read the headline figures side by side. This table puts the three main long-lived greenhouse gases on both scales, using the 2019 annual averages assessed by IPCC AR6.
| Gas | Usual unit | 2019 level | In ppm | In ppb |
|---|---|---|---|---|
| Carbon dioxide (CO₂) | ppm | 409.9 ppm | 409.9 | 409,900 |
| Methane (CH₄) | ppb | 1,866.3 ppb | 1.8663 | 1,866.3 |
| Nitrous oxide (N₂O) | ppb | 332.1 ppb | 0.3321 | 332.1 |
A third tier exists below these. The industrial fluorinated gases — SF₆, HFCs, PFCs, the CFCs they replaced — are present at parts per trillion (ppt), another factor of a thousand down from ppb. The same prefix logic runs the whole way: 1 ppm = 1,000 ppb = 1,000,000 ppt. Their vanishingly small abundance is exactly why they are so often misjudged, which the next section is about.
Same abundance is not the same warming
This converter rescales a prefix. It says nothing about climate impact, and reading it as though it did is the one way to go badly wrong with it.
Put CO₂ and methane on one scale and CO₂ outnumbers methane roughly 220 to 1. That does not make methane a rounding error. Molecule for molecule, methane is a far stronger absorber of infrared radiation, which is why it is responsible for a large share of warming to date despite its tiny share of the air. The fluorinated gases go further still: some are tens of thousands of times more potent than CO₂ per unit mass, which is how a gas present at a few parts per trillion earns a place in the Kyoto basket at all.
The instinct after putting two gases on one scale is to multiply by a global warming potential and call the result CO₂-equivalent. That step is wrong as stated. GWP is defined per unit mass, and a mole fraction is not a mass. Going from ppb of methane to a CO₂e figure needs the molar mass of the gas and of dry air as well as its GWP — three inputs, not one. For gas comparisons that are already mass-denominated, use the gas to CO₂e converter or the dedicated kg methane to CO₂e page, which apply GWP to a mass, correctly.
What ppm and ppb actually measure
Both are mole fractions of dry air: ppm counts molecules of the gas per million molecules of air, ppb per billion. You will also see them written ppmv and ppbv — the v for volume — which for an ideal gas is the same thing, since equal volumes of gas hold equal numbers of molecules. Atmospheric monitoring networks such as NOAA and Scripps report on this basis.
They are measured against dry air deliberately. Water vapour ranges from near zero to several percent of the atmosphere depending on where and when you sample, and including it would make a CO₂ record wander with the weather rather than with the carbon cycle.
Parts per million by mass is a different quantity that shares the name. It is rarely used in atmospheric work but common in water and soil chemistry, so it is worth confirming which one a source means before converting anything downstream. That distinction matters most when a concentration is turned into a mass — the subject of the ppm CO₂ to Gt carbon and ppm CO₂ to Gt CO₂ converters, where reading a mole fraction as a mass fraction throws the answer out by about a third. Within this page it makes no difference at all: a prefix rescale is a prefix rescale on either basis.
Atmospheric concentrations do not appear in a GHG inventory. Reporting under the GHG Protocol runs from activity data to emissions in tonnes of CO₂e, and never passes through ppm or ppb. If you need to turn an activity into a footprint, that is a calculator — the GWP calculator handles gas-by-gas CO₂e on a mass basis.
The method behind this converter. The constants here — and why they are empirical rather than stoichiometric, why carbon is not CO2, and why an atmospheric burden is never multiplied by a GWP — are documented in the atmospheric CO2 unit conversion methodology.
Frequently Asked Questions
Multiply by 1,000. For example, 425 ppm × 1,000 = 425,000 ppb. To reverse it, divide ppb by 1,000, so 1,866 ppb is 1.866 ppm. The factor is definitional rather than measured, so it applies to any gas and never changes.
1,000 ppb. A part per million is one in 10⁶ and a part per billion is one in 10⁹, so a million-scale slice contains a thousand billion-scale slices. Going further down, 1 ppm is also 1,000,000 parts per trillion.
A part per billion is a smaller slice than a part per million — but because it is smaller, it takes more of them, so the ppb number is always 1,000 times larger for the same concentration. 1.87 ppm and 1,866 ppb describe the same amount of methane. If a figure got smaller when you converted ppm to ppb, the direction was inverted.
Only to keep the numbers readable. CO₂ is present at hundreds of ppm, so ppm suits it. Methane and nitrous oxide are about a thousand times rarer — 1.87 and 0.33 ppm respectively — so ppb gives clean whole numbers instead of small decimals. The fluorinated gases are rarer still and are quoted in parts per trillion. No standard requires any of this; it is convention.
No. They are the same abundance, not the same warming. Methane absorbs infrared far more strongly than CO₂ molecule for molecule, which is why it drives a large share of warming to date despite being roughly 220 times rarer in the air. This converter rescales a prefix and carries no climate-impact meaning.
Not with GWP alone. Global warming potential is defined per unit mass, and a mole fraction is not a mass, so the step also needs the molar mass of the gas and of dry air. Applying GWP straight to a ppb figure gives a number with no defensible meaning. For mass-denominated comparisons, use the gas to CO₂e converter, which applies GWP to a mass correctly.
In atmospheric science they are mole fractions of dry air, sometimes written ppmv and ppbv — equivalent to fractions by volume for an ideal gas. Dry air is used because water vapour varies enough to otherwise make records move with the weather. Parts per million by mass is a different quantity, common in water and soil chemistry; it makes no difference to this conversion, but it changes the answer substantially if you go on to convert a concentration into a mass.