Convert GtC to Gt CO₂ (Carbon Budget)
| GtC | Gt CO₂ |
|---|---|
| 0.5 | 1.832 |
| 1 | 3.664 |
| 1.5 | 5.496 |
| 2 | 7.328 |
| 3 | 10.99 |
| 5 | 18.32 |
| 10 | 36.64 |
| 20 | 73.28 |
| 50 | 183.2 |
| 100 | 366.4 |
Each row uses the same factor as the converter above: 1 GtC = 3.664 Gt CO₂.
Climate science counts carbon two ways, and mixing them up throws every figure off by a factor of nearly four. A gigatonne of carbon (GtC) weighs only the carbon atoms; a gigatonne of carbon dioxide (Gt CO₂) weighs the whole molecule, carbon plus two oxygens. So 1 GtC = 3.664 Gt CO₂. This converter applies that ratio both ways — essential for reading carbon budgets, cumulative-emissions figures and TCRE numbers, which appear in both units.
To convert gigatonnes of carbon to gigatonnes of CO₂, multiply by 3.664 (the ratio of their molecular masses, 44.009 ÷ 12.011). So 100 GtC = 366.4 Gt CO₂. To go the other way, divide Gt CO₂ by 3.664.
Gt CO₂ = GtC × 3.664. Reverse: GtC = Gt CO₂ ÷ 3.664. The factor is the molecular mass of CO₂ (44.009 g/mol) divided by that of carbon (12.011 g/mol) — because oxidising one carbon atom to CO₂ adds two oxygens, tripling-plus its mass. The same scale (Gt vs Gt) applies on both sides.
| GtC | × 3.664 | Gt CO₂ |
|---|---|---|
| 1 GtC | × 3.664 | 3.664 Gt CO₂ |
| 10 GtC | × 3.664 | 36.64 Gt CO₂ |
| 100 GtC | × 3.664 | 366.4 Gt CO₂ |
| 0.273 GtC | × 3.664 | 1 Gt CO₂ |
GtC and Gt CO₂ are not the same thing
The distinction is physical, not a rounding choice. When carbon burns it combines with oxygen from the air, so a fixed mass of carbon becomes a larger mass of carbon dioxide. Carbon weighs 12.011 g/mol; the two oxygen atoms it picks up weigh 2 × 15.999 = 31.998, giving CO₂ a mass of 44.009. That is where the 3.664 comes from — and it means nearly three-quarters of the mass of CO₂ is oxygen taken from the air, not the original carbon.
You will see both. 3.664 uses the precise atomic weights — CO₂ 44.009 ÷ C 12.011 — and is the convention the IPCC uses for carbon budgets and the carbon cycle. 3.667 (44 ÷ 12) uses rounded whole-number masses and is the everyday stoichiometric ratio, used by the GHG Protocol and by our per-tonne carbon-to-CO₂ converter. The two differ by less than 0.1%, but for gigatonne-scale carbon-budget work this page uses the IPCC 3.664.
Key carbon-budget figures in both units
Carbon-cycle science and climate policy don’t agree on a unit: the carbon cycle and TCRE are usually in gigatonnes of carbon, while budgets and inventories are in gigatonnes of CO₂. This table gives the headline numbers in both, so you can read any source without tripping over the factor of 3.664.
| Figure | Gt CO₂ | GtC | Source |
|---|---|---|---|
| Remaining budget — 1.5 °C (50% chance) | 500 | 136 | IPCC AR6, from 2020 |
| Remaining budget — 1.5 °C (67% chance) | 400 | 109 | IPCC AR6, from 2020 |
| Remaining budget — 1.7 °C (67% chance) | 700 | 191 | IPCC AR6, from 2020 |
| Remaining budget — 2 °C (67% chance) | 1,150 | 314 | IPCC AR6, from 2020 |
| Cumulative CO₂ emissions, 1850–2019 | 2,390 | 652 | IPCC AR6 |
| Global CO₂ emissions, per year (recent) | ≈ 40 | ≈ 11 | Global Carbon Project |
The IPCC’s remaining budget for a 50% chance of staying under 1.5 °C is 500 Gt CO₂ (from the start of 2020). In carbon terms that is 500 ÷ 3.664 ≈ 136 GtC. At roughly 11 GtC (≈ 40 Gt CO₂) emitted a year, that budget is about 12 years of current emissions — which is why the year the world “passes” 1.5 °C keeps moving closer.
Remaining budgets are measured from a fixed start date — the IPCC AR6 figures above are from the start of 2020 — and they shrink by roughly 40 Gt CO₂ (11 GtC) every year as emissions continue. Post-AR6 assessments put the 1.5 °C budget lower still. Always quote the budget’s baseline year alongside the number.
GtC to Gt CO₂ conversion table
The table reads both ways — carbon to CO₂ on the left (× 3.664), CO₂ back to carbon on the right (÷ 3.664).
| GtC | → Gt CO₂ (× 3.664) | Gt CO₂ | → GtC (÷ 3.664) |
|---|---|---|---|
| 1 | 3.664 | 1 | 0.273 |
| 10 | 36.64 | 100 | 27.3 |
| 50 | 183.2 | 400 | 109.2 |
| 100 | 366.4 | 1,000 | 273 |
| 500 | 1,832 | 2,500 | 682 |
Where these units are used
- Carbon budgets. The remaining budget for 1.5 °C is usually quoted in Gt CO₂, but the underlying carbon-cycle science is often in GtC — so you convert to compare them (IPCC AR6).
- Cumulative emissions. Total human CO₂ emitted since 1850 is on the order of 2,390 Gt CO₂ — about 652 GtC. Historical carbon-cycle datasets frequently report the carbon (GtC) figure.
- TCRE. The transient climate response to cumulative emissions — warming per unit of carbon released — is defined per 1,000 GtC (best estimate ≈ 1.65 °C, IPCC AR6). That equals about 0.45 °C per 1,000 Gt CO₂, so reading it against a CO₂-denominated budget needs this conversion.
- Related scaling. For a plain change of magnitude within one unit (megatonnes to gigatonnes, no carbon-to-CO₂ step), use the Mt to Gt converter; global annual emissions run around 40 Gt CO₂ (about 11 GtC).
The GtC/Gt CO₂ mix-up is one of the most common errors in climate reporting: a budget stated in GtC but read as Gt CO₂ (or vice-versa) is wrong by 3.664×. If a “carbon budget” of a few hundred looks like it might be carbon rather than CO₂, or an emissions total seems almost four times too big or too small, suspect a unit mismatch and convert before comparing.
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 gigatonnes of carbon by 3.664. For example, 100 GtC × 3.664 = 366.4 Gt CO₂. To reverse it, divide gigatonnes of CO₂ by 3.664. The factor is the ratio of the molecular masses of CO₂ and carbon.
About 3.664. When carbon is oxidised to CO₂ it gains two oxygen atoms, so one gigatonne of carbon becomes 3.664 gigatonnes of carbon dioxide — the added mass is the oxygen from the air.
3.664 uses precise atomic weights (CO₂ 44.009 ÷ C 12.011) and is the IPCC convention for carbon budgets. 3.667 uses rounded whole numbers (44 ÷ 12) and is the everyday stoichiometric ratio used by the GHG Protocol. They differ by under 0.1%; this carbon-budget converter uses 3.664, while the general per-tonne carbon-to-CO₂ converter uses 3.667.
GtC counts only the carbon; Gt CO₂ counts the whole carbon-dioxide molecule (carbon plus two oxygens), which weighs 3.664 times as much — nearly three-quarters of CO₂’s mass is oxygen from the air. A figure given without its unit is ambiguous by that factor, so always confirm whether a source means carbon or carbon dioxide.
The IPCC AR6 budget for a 50% chance of 1.5 °C is 500 Gt CO₂ from 2020, which is about 136 GtC (÷ 3.664); the 67% figure is 400 Gt CO₂ ≈ 109 GtC. Budgets are usually published in Gt CO₂, so divide by 3.664 for the carbon figure — and note they shrink by roughly 40 Gt CO₂ (11 GtC) a year.
Cumulative CO₂ emissions from 1850 to 2019 are about 2,390 Gt CO₂ (IPCC AR6), which is roughly 652 GtC. Global emissions currently add around 40 Gt CO₂ (about 11 GtC) each year.
Same chemistry, different framing. The carbon-to-CO₂ converter works per tonne and uses the 44/12 = 3.667 stoichiometric ratio for everyday accounting. This page is for gigatonne-scale carbon-budget work and uses the IPCC 3.664. Pick the one that matches your source’s unit and convention.