Convert ppm CO₂ to Gt Carbon
| ppm CO₂ | GtC |
|---|---|
| 0.5 | 1.062 |
| 1 | 2.124 |
| 1.5 | 3.186 |
| 2 | 4.248 |
| 3 | 6.372 |
| 5 | 10.62 |
| 10 | 21.24 |
| 20 | 42.48 |
| 50 | 106.2 |
| 100 | 212.4 |
Each row uses the same factor as the converter above: 1 ppm CO₂ = 2.124 GtC.
Get one email when the source dataset revises this number: old → new, and what it means.
Atmospheric CO₂ is measured as a concentration — parts per million — but the carbon cycle is accounted in mass. Bridging the two is a single number: 1 ppm of CO₂ corresponds to 2.124 gigatonnes of carbon in the atmosphere. It is the constant that turns a Keeling Curve reading into a carbon-budget quantity, and it is the step where two silent mistakes are made: reading ppm as a mass fraction rather than a mole fraction, and reading the answer as CO₂ when it is carbon.
To convert ppm of atmospheric CO₂ to gigatonnes of carbon, multiply by 2.124. So 10 ppm = 21.24 GtC. To go the other way, divide GtC by 2.124 — one gigatonne of carbon is 0.4708 ppm.
GtC = ppm CO₂ × 2.124. Reverse: ppm CO₂ = GtC ÷ 2.124 (× 0.4708). The ppm is a dry-air mole fraction — the quantity NOAA and Scripps report — and the result is a mass of carbon, not of carbon dioxide. For gigatonnes of CO₂, multiply again by 3.664: 1 ppm = 7.782 Gt CO₂.
| ppm CO₂ | × 2.124 | GtC |
|---|---|---|
| 1 ppm | × 2.124 | 2.124 GtC |
| 10 ppm | × 2.124 | 21.24 GtC |
| 425 ppm | × 2.124 | 902.7 GtC |
| 0.4708 ppm | × 2.124 | 1 GtC |
How to convert ppm CO₂ to gigatonnes of carbon
The conversion is linear and exact through zero, so it works equally for a change in concentration and for the total standing in the atmosphere. Multiply the ppm figure by 2.124 and the answer is gigatonnes of carbon; divide by 2.124 to go from carbon mass back to a concentration.
Worked both directions. The atmosphere has risen roughly 145 ppm above its pre-industrial level of about 280 ppm. In mass terms that is 145 × 2.124 ≈ 308 GtC added to the air. Run it backwards to size a budget: 100 GtC of further emissions, if every tonne stayed airborne, would be 100 ÷ 2.124 ≈ 47.1 ppm — which is why the airborne fraction matters, since roughly half of emitted carbon is taken up by land and ocean rather than accumulating.
ppm CO₂ to Gt carbon conversion table
The table reads both ways — concentration to carbon mass on the left (× 2.124), carbon mass back to concentration on the right (÷ 2.124). The third column carries the same rows through the extra × 3.664 step for anyone working in CO₂ rather than carbon.
| ppm CO₂ | → GtC (× 2.124) | → Gt CO₂ (× 7.782) | GtC | → ppm (÷ 2.124) |
|---|---|---|---|---|
| 0.5 | 1.062 | 3.891 | 1 | 0.4708 |
| 1 | 2.124 | 7.782 | 10 | 4.708 |
| 2 | 4.248 | 15.56 | 50 | 23.54 |
| 5 | 10.62 | 38.91 | 100 | 47.08 |
| 10 | 21.24 | 77.82 | 250 | 117.7 |
| 50 | 106.2 | 389.1 | 500 | 235.4 |
| 100 | 212.4 | 778.2 | 1,000 | 470.8 |
| 425 | 902.7 | 3,308 | 2,000 | 941.6 |
What “ppm” actually measures
Parts per million is not a single quantity. Which one a source means changes the answer by a third, and almost no one states it — because within atmospheric science the convention is so settled it goes without saying. Outside that field it does not.
A dry-air mole fraction, not parts per million by mass
The 425 ppm you read from Mauna Loa is a mole fraction: 425 CO₂ molecules for every million molecules of dry air. It is reported as micromoles per mole (µmol/mol), and it is measured against dry air specifically because atmospheric water vapour varies from near zero to several percent, which would otherwise make the figure wander with the weather rather than with the carbon cycle.
Parts per million by mass is a different quantity. Because a CO₂ molecule is heavier than average air — 44.009 g/mol against roughly 28.97 — one ppm by mole is about 1.52 ppm by mass. Treat an atmospheric ppm as a mass fraction and 1 ppm resolves to roughly 1.40 GtC instead of 2.124: about 34% too low, in the same direction every time, with nothing in the output to signal it.
The answer is carbon, not CO₂
This converter returns gigatonnes of carbon — the carbon atoms alone. Carbon is only 27.3% of the mass of a CO₂ molecule (12.011 of 44.009 g/mol); the rest is oxygen taken from the air. Read a GtC figure as though it were CO₂ and it is wrong by a factor of 3.664. Both units are in routine use, and they are rarely labelled clearly: carbon-cycle science and the Global Carbon Budget work in GtC, while climate policy, national inventories and remaining-budget headlines are almost always in Gt CO₂.
Two substitutions account for nearly every bad ppm-to-mass figure. One: a source treats atmospheric ppm as parts per million by mass, understating the carbon by about a third. Two: a correct GtC answer is reported as though it were CO₂, understating by 3.664×. Both fail quietly — the number stays plausible. The sanity check is the standing stock: at today’s ≈ 425 ppm the atmosphere holds roughly 900 GtC, equivalently about 3,300 Gt CO₂. If your working lands far outside that pair, one of the two substitutions has happened.
Why 2.124 and not 2.13
You will find three values in circulation, and unlike most conversion factors on this site they do not differ because someone rounded. This one is measured, not defined. It follows from the estimated mass of the dry atmosphere, so it moves as that estimate is refined — which is exactly why it belongs in a cited, versioned data layer rather than being written into a web page.
| Value | Source | Where you’ll meet it |
|---|---|---|
| 2.12 | IPCC AR5 (2013), WG1 Chapter 6 | Older carbon-cycle literature and anything citing AR5 |
| 2.124 | Global Carbon Budget (after Ballantyne et al., 2012); IPCC AR6 WG1 Chapter 5 | Current practice — the value this converter uses |
| 2.13 | Legacy CDIAC / NOAA conversion tables | Still the most widely copied figure on the open web |
Where does the number come from? The dry atmosphere weighs about 5.1352 × 1018 kg. Divide by the mean molar mass of dry air (≈ 28.97 g/mol) to get its total moles, take a millionth of that, and multiply by the molar mass of carbon (12.011 g/mol). The arithmetic lands at roughly 2.13 GtC per ppm — which is where the legacy figure comes from, and it is not wrong so much as unrefined. The Global Carbon Budget’s 2.124 sits about 0.3% below it, reflecting both a tighter atmospheric-mass estimate and the assumption the budget makes explicit: the surface measurement network samples the well-mixed troposphere, and 2.124 is the factor that holds if the rest of the atmosphere tracks it within a year.
The spread is small in percentage terms but not invisible. At 425 ppm, the three published values give 901.0, 902.7 and 905.3 GtC — a range of about 4 GtC, or roughly 16 Gt CO₂ once converted. That is the sort of gap that shows up as an unexplained discrepancy when two sources are compared. This page reads 2.124 live from the GreenCalculus MasterBrain, so the figure it applies carries its citation and re-vintages when the Global Carbon Budget does — see the provenance line under the converter for the exact data version in use.
Converting to gigatonnes of CO₂ instead
If your source is denominated in carbon dioxide rather than carbon, take the carbon answer and multiply by 3.664 — the ratio of the molecular masses, 44.009 ÷ 12.011. One ppm of atmospheric CO₂ is therefore 7.782 Gt CO₂. The two steps are independent, and it is worth keeping them apart: 2.124 is an atmospheric constant that can be revised, while 3.664 is fixed chemistry that cannot.
That second step has a page of its own. The GtC to Gt CO₂ converter covers it in full, including why some sources use 3.667 (44 ÷ 12) instead, and carries the headline carbon-budget figures in both units. For the same chemistry at everyday tonne scale rather than gigatonne scale, use the carbon to CO₂ converter; for a plain change of magnitude with no carbon-to-CO₂ step, the Mt to Gt converter.
Where ppm-to-mass conversion is used
- The airborne fraction. The Global Carbon Budget’s central accounting identity compares emissions against the observed atmospheric growth rate. That growth is measured in ppm per year and must be converted to GtC per year before it can be set against emissions and sinks — 2.124 is the factor that does it.
- Reading the Keeling Curve as a stock. Turning a concentration record into cumulative carbon in the air is a single multiplication. The rise from roughly 280 ppm pre-industrial to about 425 ppm today is some 308 GtC added to the atmosphere.
- Translating carbon budgets. Remaining budgets are published as mass, but the constraint people intuitively track is concentration. Converting between them shows how much of a budget a given ppm of further rise represents — see carbon budget and IPCC AR6.
- Climate-science literature generally. IPCC WG1, carbon-cycle modelling and atmospheric-observation papers move between ppm and GtC constantly, often without restating the factor — the reader is assumed to know it.
This conversion has no place in a GHG inventory. Corporate and national reporting under the GHG Protocol works from activity data to emissions in tonnes of CO₂e; nothing in that chain passes through an atmospheric concentration. If you have arrived here looking to turn an activity into a footprint, that is a calculator, not a converter — start with the CO₂ equivalencies calculator. This page belongs to the atmospheric-science side: it converts an observation, not an activity.
Reading the number in context
The constant is easiest to trust once it is anchored against a figure you can check. At a concentration of 425 ppm the atmosphere holds about 903 GtC — roughly 3,300 Gt CO₂. Pre-industrial, at about 280 ppm, the same arithmetic gives around 595 GtC. The difference, about 308 GtC, is the carbon that human activity has added and that has stayed airborne; it is substantially less than total emissions, because land and ocean sinks have absorbed roughly half of what was released.
One consequence worth internalising: because 1 ppm is 2.124 GtC and only part of any emission remains in the air, the concentration moves far more slowly than emissions totals suggest. That asymmetry — between what is emitted, what is absorbed and what accumulates — is the whole subject of the airborne fraction, and this conversion is the unit bridge that makes the comparison possible at all. Note too that CO₂ is the reference gas for global warming potential and therefore has a GWP of 1 by definition, so no CO₂e basis or assessment-report vintage enters this conversion — unlike the gas-to-CO₂e converters, this page has no AR4/AR5/AR6 choice to make.
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 the concentration in ppm by 2.124. For example, 10 ppm × 2.124 = 21.24 GtC. To reverse it, divide gigatonnes of carbon by 2.124, so 1 GtC is 0.4708 ppm. The relation is linear through zero, so it works for both a change in concentration and the total standing in the atmosphere.
2.124 gigatonnes of carbon. This is the Global Carbon Budget’s standing conversion, after Ballantyne et al. (2012), and it is also the value used in IPCC AR6 WG1 Chapter 5. It follows from the mass of the dry atmosphere and the molar mass of carbon.
Because this constant is measured rather than defined, and the estimate has been refined. 2.13 is the legacy CDIAC and NOAA figure, still the most-copied value on the open web; IPCC AR5 used 2.12; the Global Carbon Budget and IPCC AR6 use 2.124. They differ by under 0.3%, but at today’s concentration that is a spread of about 4 GtC — roughly 16 Gt CO₂ — which is enough to show up as an unexplained discrepancy between two sources.
About 7.782 Gt CO₂. Take the carbon answer, 2.124 GtC, and multiply by 3.664 — the ratio of the molecular masses of CO₂ and carbon (44.009 ÷ 12.011). Keep the two steps separate: 2.124 is an atmospheric constant that can be revised, while 3.664 is fixed chemistry.
Neither, strictly — it is a mole fraction of dry air, reported as micromoles per mole, which for an ideal gas is equivalent to a fraction by volume. It is measured against dry air because water vapour varies enough to otherwise make the figure move with the weather. Parts per million by mass is a different quantity: because CO₂ is heavier than average air, 1 ppm by mole is about 1.52 ppm by mass, and treating an atmospheric ppm as a mass fraction understates the carbon by roughly a third.
At about 425 ppm, roughly 903 GtC — equivalently around 3,300 Gt CO₂. At the pre-industrial level of about 280 ppm the figure was near 595 GtC, so some 308 GtC has been added and remained airborne. That is well below total human emissions, because land and ocean sinks have taken up roughly half of what was released.
No. The 2.124 factor is specific to CO₂ and to carbon mass — it embeds the molar mass of carbon and assumes a mole fraction expressed per million. Methane is reported in parts per billion, has a different molar mass, and is normally expressed as a methane mass or as CO₂e via its global warming potential rather than as carbon. Use the gas-specific route instead: the CO₂ to CO₂e converter explains why CO₂ itself needs no GWP step.