CO2e calculator: convert methane and other gases to CO2 equivalent

Enter an amount of a greenhouse gas and the tool converts it to CO2 equivalent, using global warming potentials from the IPCC's Sixth or Fifth Assessment Report. It also turns tonnes of carbon into tonnes of CO2.

How it works

Greenhouse gases differ in how much heat they trap and how long they stay in the air. A global warming potential (GWP) folds both into one number: the warming caused by releasing 1 kg of a gas, added up over a set period, compared with the warming from 1 kg of carbon dioxide over the same period. CO2 scores 1 by definition.

The conversion is one multiplication:

CO2e = mass of gas × GWP

You can enter kilograms, metric tonnes, pounds or US short tons. The tool also works backward: give it an amount of CO2e and it tells you how much of a chosen gas has the same warming effect.

Three sets of values are built in:

Gas	AR6 GWP100	AR6 GWP20	AR5 GWP100
Carbon dioxide	1	1	1
Methane, fossil	29.8	82.5	28
Methane, biogenic (livestock, rice, landfills)	27.0	79.7	28
Nitrous oxide	273	273	265
HFC-134a (car air conditioning, older fridges)	1,530	4,140	1,300
HFC-32 (newer air conditioners)	771	2,690	677
Perfluoromethane, CF4 (aluminum smelting)	7,380	5,300	6,630
Nitrogen trifluoride (electronics)	17,400	13,400	16,100
Sulfur hexafluoride (switchgear)	25,200	18,300	23,500

The AR6 columns come from Table 7.15 of the IPCC's Sixth Assessment Report. The AR5 column comes from Table 8.A.1 of the Fifth, using the values without climate-carbon feedbacks; this is the set national inventories reported to the UNFCCC use, and it gives all methane the same value of 28. AR6 rates fossil methane a little higher than biogenic methane because, when fossil methane breaks down, it leaves CO2 that was not taken from the air recently. The carbon in biogenic methane was.

Short-lived gases look much stronger over 20 years than over 100, because most of their warming happens early. Methane lasts about a decade in the atmosphere. Very long-lived gases flip the other way: CF4, NF3 and SF6 score higher over 100 years because they are still there, still warming, long after year 20.

The tool also converts carbon to CO2. Each carbon atom (atomic mass 12) that ends up as CO2 picks up two oxygen atoms (16 each), for a molecular mass of 44:

t CO2 = t C × 44 / 12

Worked example

The tool opens on 1 tonne of fossil methane.

IPCC AR6, 100 years: 1 × 29.8 = 29.8 t CO2e.
IPCC AR6, 20 years: 1 × 82.5 = 82.5 t CO2e.
IPCC AR5, 100 years: 1 × 28 = 28 t CO2e, the figure a national inventory would show.

The same tonne counts for almost three times as much on the 20-year scale as on the 100-year one. Neither number is wrong. They answer different questions: how much warming over the next two decades, and how much over a century.

Switch to biogenic methane and the AR6 figures drop to 27.0 and 79.7 t, while the AR5 figure stays at 28.

For carbon, 12 t of carbon equals 44 t of CO2, so 1 t of carbon is about 3.67 t of CO2. Mixing up the two is an easy way to be off by a factor of 3.67, so check which one a project or offset claim reports. The soil carbon calculator uses the same ratio.

What it can't tell you

A GWP is a convention, not a physical constant. The IPCC tables give an uncertainty range around each value, a wide one for methane, and the values shift between reports as the science improves, as the AR5 and AR6 columns show. Never mix sets in one total, and always say which set you used.

The time horizon is a choice, and partly a political one. GWP100 is the standard in reporting and carbon trading. Critics argue it understates methane's near-term effect. Others prefer different metrics, such as the global temperature change potential (GTP), which appears in the same IPCC table, or GWP*, which treats a steady methane source differently from a growing one. The tool gives you the three common sets and leaves that argument to you.

CO2e is an accounting unit. It does not tell you whether a tonne claimed as avoided or removed was real, additional or permanent, which is where most carbon credit disputes start. Two of our pieces show how: one on ocean iron fertilization from Planktos to today's marine carbon removal, and one on the green economy and rainforest credits.

The tool covers nine gases. It does not handle aerosols, black carbon, ozone-forming pollutants or the non-CO2 effects of aviation, whose warming depends on where and when they are emitted and does not fit a single GWP.

Sources

IPCC, Climate Change 2021: The Physical Science Basis, Working Group I contribution to the Sixth Assessment Report, Chapter 7, Table 7.15.
IPCC, Climate Change 2013: The Physical Science Basis, Working Group I contribution to the Fifth Assessment Report, Chapter 8, Table 8.A.1.
UNFCCC reporting rules under the Paris Agreement's transparency framework, which call for AR5 100-year values in national inventories.

Questions

What is the GWP of methane?

In the IPCC's Sixth Assessment Report, fossil methane has a 100-year GWP of 29.8 and a 20-year GWP of 82.5. Biogenic methane has 27.0 and 79.7. National inventories under the UNFCCC use the Fifth Assessment Report's 100-year value of 28.

How do you calculate CO2 equivalent?

Multiply the mass of the gas by its global warming potential. Two tonnes of nitrous oxide at an AR6 100-year GWP of 273 equals 546 tonnes of CO2e.

What is the difference between GWP20 and GWP100?

They add up a gas's warming over 20 or 100 years after it is released. Short-lived gases such as methane score far higher on GWP20, because most of their warming happens in the first two decades.

Why does fossil methane have a higher GWP than biogenic methane?

Methane breaks down into CO2. For fossil methane, that CO2 is new to the atmosphere. For biogenic methane, the carbon was recently taken from the air by plants, and AR6 counts the difference.

How do you convert carbon to CO2?

Multiply tonnes of carbon by 44/12, which is about 3.67. To go from CO2 back to carbon, multiply by 12/44.