CO₂ Equivalent (GWP) Calculator

Understanding the CO₂ equivalent (GWP) calculator — concept and principle

Different greenhouse gases trap different amounts of heat. To add them into one total, inventories convert each gas to carbon dioxide equivalent (CO₂e) using its global warming potential (GWP). This calculator combines CO₂, methane and nitrous oxide with 100-year GWP values from the IPCC Fifth (AR5) or Sixth (AR6) Assessment Report.

Just 1.1 t of methane and N₂O outweigh the 10 t of CO₂ more than five times over.

How it is calculated

CO₂e = CO₂ + CH₄ × GWP_CH₄ + N₂O × GWP_N₂O, all in tonnes.
AR6 setting: GWP_CH₄ = 27.9, GWP_N₂O = 273.
AR5 setting: GWP_CH₄ = 28, GWP_N₂O = 265.
CO₂ has a GWP of 1 by definition.

The principle

GWP compares the total extra heat (integrated radiative forcing) that a one-off emission of 1 kg of a gas causes over a chosen time horizon with the heat caused by 1 kg of CO₂ over the same period. Methane is a much stronger absorber than CO₂ per kilogram but breaks down in about a decade, while CO₂ persists for centuries, so the result depends strongly on the horizon: methane's 20-year GWP is about three times its 100-year value. The 100-year horizon (GWP-100) is the convention used in most reporting. Values change between IPCC reports as the science of lifetimes and indirect effects improves, which is why the report edition must be stated. AR6 also gives separate values for fossil methane (29.8), which includes the CO₂ formed when it oxidises, and non-fossil methane.

Worked example

Inputs: 10 t CO₂, 1 t CH₄, 0.1 t N₂O, AR6 values.
CH₄: 1 × 27.9 = 27.9 t CO₂e.
N₂O: 0.1 × 273 = 27.3 t CO₂e.
Total = 10 + 27.9 + 27.3 = 65.2 t CO₂e.
With AR5: 10 + 28 + 26.5 = 64.5 t CO₂e.

Each point is the calculator re-run with only “CH₄ (t)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

CH₄ (t)	Total CO₂e (t)
0.25	44.275
0.5	51.25
0.75	58.225
1	65.2
1.25	72.175
1.5	79.15
2	93.1

How to read the result

The total lets you compare and add emissions from energy, agriculture and waste. In the example, only 1.1 t of non-CO₂ gases accounts for more than 80% of the total, showing why methane and N₂O matter. Always compare totals calculated with the same GWP set.

Common mistakes

Mixing AR4, AR5 and AR6 values in one inventory or across years.
Entering CO₂e amounts again instead of the mass of each gas.
Using GWP-20 values where a programme requires GWP-100.

Keep in mind: GWP is a simplifying metric; it does not capture the different timing of warming caused by short- and long-lived gases. For official inventories, use the GWP set your reporting programme prescribes. This is not financial or compliance advice.

Frequently asked questions

Which GWP set should I use?

The one your programme specifies. Under the Paris Agreement transparency framework, countries report with AR5 100-year values; some corporate schemes have moved to AR6.

Why is methane 27.9 here but 29.8 elsewhere?

29.8 is AR6's value for fossil methane, which adds the CO₂ produced when the methane breaks down; 27.9 is the general value without that.

Can I add refrigerants such as HFCs?

The method is the same (mass × GWP), but this calculator includes only CO₂, CH₄ and N₂O.

Sources: IPCC AR6 WG1, Chapter 7, Table 7.15 and Supplementary Material 7.SM, 2021 · IPCC AR5 WG1, Chapter 8, Table 8.7, 2013

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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