Carbon Basics & Converters — GWP, CO₂e & CO₂-to-Trees

What global warming potential actually means

Greenhouse gases differ in two ways that matter: how strongly they trap heat, and how long they last in the atmosphere. Global warming potential compresses both into a single ratio against carbon dioxide.

The number answers a specific question: if you release one tonne of this gas today, how much warming does it cause over the next hundred years compared with one tonne of CO₂? That hundred-year window is the GWP-100 basis, and it is the convention corporate reporting uses. CO₂ is the reference point, so its GWP is 1 by definition.

The lifetime half of the equation explains most of the surprises. Methane traps heat far more powerfully than CO₂ but breaks down within about a decade, so over a century its advantage decays — its AR6 GWP-100 is 29.8. Nitrous oxide is both potent and long-lived, giving it a GWP-100 of 273. Sulphur hexafluoride persists for millennia and sits at 25200. That range — from 1 to tens of thousands — is why a few kilograms of the wrong gas can outweigh tonnes of CO₂.

Multiply mass by GWP and you have CO₂-equivalent: the unit that lets a refrigerant leak, a diesel bill and a landfill sit in one total.

What are you converting? Start here

Three jobs, three tools.

Pick your converter

Know what you need? Go straight to the calculator.

Unit converters by family

The three calculators above answer a question. The converters below each do one arithmetic job — one unit in, one unit out. There are 136 of them, in nine families.

Which calculator do I need?

Two of these produce numbers for an inventory; one produces an illustration. The table makes the distinction explicit, because it decides where the output can legitimately go.

What each calculator covers

Converting gases to CO₂-equivalent

The GWP calculator does the conversion every inventory depends on: mass of gas × GWP = CO₂e (methodology). It covers the gases corporate reporting actually meets — methane, nitrous oxide, SF₆, the common HFC refrigerants, PFCs and NF₃ — and lets you pick the AR5 or AR6 basis explicitly rather than guessing which is baked in. One subtlety it handles that catches people out: methane comes in two flavours. Fossil methane and biogenic methane carry different AR6 values, because oxidising biogenic methane returns carbon that was recently in the atmosphere. Biogenic methane’s AR6 GWP-100 is 27 against 29.8 for fossil — a distinction that matters for landfill, livestock and biomass figures.

Putting a tonne of CO₂ in context

The CO₂ equivalencies calculator answers a different kind of question: what does a tonne actually mean (methodology)? A tonne of CO₂ is invisible and weightless-feeling in a spreadsheet, and that abstraction is a real obstacle when you are trying to explain a footprint to people who do not read inventories. Translating it into familiar terms — distance driven, homes powered — makes the magnitude legible. The important discipline is that these are communication devices. An equivalency never enters an inventory, never offsets anything, and should not be presented as though it did.

Trees: an illustration, not accounting

The CO₂ to trees calculator is the most-requested and most-misused conversion on the site, so it is worth being plain about what it is. It applies an average sequestration rate to give a rough sense of scale — how much growing forest it would take to absorb a given quantity of CO₂. That is a useful mental image and a legitimate way to make a number tangible.

It is not carbon accounting. A real tree’s uptake depends on species, age, climate, soil and management; it accumulates over decades rather than instantly; and it reverses if the tree burns, is felled or dies. The calculator’s average rate flattens all of that. Treat the output as an illustration of magnitude, never as a removal you can claim, an offset you have earned, or a deduction from a footprint.

AR5 or AR6 — which basis to use

GWP values are not constants; they are scientific estimates that the IPCC revises as understanding improves. The Fifth Assessment Report (AR5) and the Sixth (AR6) give different numbers for the same gases, and the 2019 Refinement updated the inventory guidance built on them.

The rule is simple. Use AR6 GWP-100 for corporate GHG reporting — GHG Protocol, CSRD, CDP and SBTi all expect it, and it is the default these calculators apply. Use AR5 only where a source you are quoting is itself pre-baked at AR5, most commonly DEFRA-sourced factors, where the AR5 basis is part of the factor’s definition and converting it would be wrong.

The revision is modest for most gases but not negligible. As a historical comparison: AR5 gave methane a GWP-100 of 28.0, while AR6 revised it to 29.8; nitrous oxide moved from 265 to 273; SF₆ from 23,500 to 25,200. A methane-heavy footprint therefore reports a few percent higher on AR6 than the identical activity would have on AR5 — which is why the basis has to travel with the figure rather than be assumed.

How the numbers are built

The GWP values on this page and in these calculators are read live from the MasterBrain data layer rather than typed into the text, so they reflect the current basis rather than whatever was true on the day this was written. The full set, with sources, is published on the IPCC AR6 GWP values data page.

This is how GWP-100 changed between AR5 and AR6 for the common gases — the shift is small but real, which is exactly why a total must state its basis. Both columns are read live from the data layer:

That matters more than it sounds. GWP values are stable between assessment reports — the AR6 set will hold until AR7, expected around 2028 — but they do get corrected within a cycle as the underlying science is refined, and a value frozen into prose at publication silently goes stale when that happens. Reading live means a correction propagates rather than leaving a page quietly wrong. The historical AR5 figures in the section above are deliberately fixed, because a historical comparison is a record of what a superseded report said and must not move.

The equivalency converters translate a tonne of CO₂ into something relatable using published EPA reference values, read live from the data layer at MasterBrain v2026.110:

Where these numbers go next

Conversion is the first step, not the destination. A CO₂e figure is only meaningful once it sits inside a boundary — which emissions, whose, over what period — and that is what an inventory provides. The corporate GHG inventory and accounting cluster covers assembling source-level figures into an auditable total across all three scopes.

The gases here show up everywhere in that work: methane in landfill and livestock lines, nitrous oxide in fertilised soils, refrigerants and SF₆ in fugitive emissions. Each has its own source-level calculator that applies these same GWPs, and the carbon calculator directory covers the full set.