What is an emission factor?

An emission factor (also known as a conversion factor, emission intensity, or carbon intensity) is used to calculate the rate at which an activity releases greenhouse gases into the atmosphere.

Every activity has an effect on the environment. An emission factor tells us how significant that impact is by measuring the greenhouse gas emissions per unit of activity.

Imagine you want to calculate your business’s carbon footprint, so you need to find out how many kg of GHG are emitted by burning a particular amount of fuel. To calculate this, you’d need an emission factor.

But how exactly do we use emission factors to measure the greenhouse gas emissions of a given activity?

Let’s dive in and find out.

Why is using an emission factor important?

Using emission factors help us fight climate change. Understanding the emissions factors of activities helps us build emissions inventories, which guide emissions control strategies.

We’re currently facing 1.5C of global warming, which would change our climate as we know it today. From air quality to natural disasters, every aspect of our lives will be affected by global temperature changes.

Scientists and members of the intergovernmental panel on climate change (IPCC) and the U.S. EPA are working on multiple solutions to the climate crisis. But in order to find solutions, we first need to understand the problem.

This is where emission factors come in.

Using an emission factor to calculate the average emission of a given greenhouse gas allows experts to work out which changes to make for emission reduction.

Emission factors can guide decision-making on almost everything, from the boilers we create to the cars we build. With the help of an emission factor, we can find out whether electricity generation or fuel combustion has a worse impact on the planet, informing future decisions.

How do we measure global warming from greenhouse gas emissions?

We can measure global warming from greenhouse gas emissions by finding out the emission rate of each gas. Here’s how.

If you’ve heard of global warming, you’ve probably heard of CO2 (carbon dioxide), one of the best-known greenhouse gases (GHGs). But it’s not the only one.

Methane and nitrous oxide are two other significant warming gasses.

Measuring the impact of these two gases is difficult because they affect the atmosphere in different ways.

Nitrous oxide, for example, has a 100-year global warming effect which is over 250 times more powerful than that of CO2, according to an index called the Global Warming Potential (GWP).

We tend to think of the climate emergency as occurring over the next 100 years, so the GWP of gases is typically measured in 100-year stints. Here’s a simple visual to show you the difference in warming potential of the three gasses.

So how do we find out the warming effect of each greenhouse gas?

To accurately compare the emissions of one GHG to another, we must standardise the measurement of emissions. Enter: emission factors.

Greenhouse gas (GHG) emissions are measured in CO2 equivalents, given in kilograms (kg) or tonnes/metric tons (t).

We use the CO2 equivalent to discover how much a given amount of a particular GHG would warm the atmosphere over 100 years compared to the same amount of CO2 over 100 years – i.e its emission rate.

Here’s an example of the GHG emissions of burning different fuel types, calculated using an emission factor.

Using the emission factor (cited here as the conversion factor), we can find a CO2eq in tons to measure which fossil fuel impacts greenhouse gas production most significantly.

How to use an emission factor


Now it’s time for the methodology. Let’s start with CO2 as the base.

We’ll call CO2 ‘1’ because all other GHGs will be measured against carbon dioxide. The table below shows the CO2 equivalent GWP for some of the best-known greenhouse gasses.

Carbon Dioxide

Nitrous Oxide







To convert an activity into its CO2 equivalent, we need an example activity.

Let’s imagine we want to find out how many kg of GHG are emitted by burning 50kg of natural gas. First, let’s pick an emission factor that allows us to calculate natural gas emissions in CO2e for a given unit of weight.

The UK government’s 2021 factor gives a factor of 2538.48kg CO2 equivalent per metric ton of natural gas combusted. To calculate the emissions emitted from 50kg of natural gas, we’d use the following equation.

Weight of gas combusted in metric tons x 2538.48.

We’d first need to convert 50kg into its equivalent metric tons, giving us 0.05t. So the result is:

0.05t x 2538.48kg CO2e/t = 126.921CO2e of GHG emissions.

Problems with developing emission factors

In some countries where the data is less specific, organisations try not to apply international values in case it skews the results. This uncertainty leaves the calculation of carbon footprints open to error.

Here are just a few of the uncertainties associated with developing an emission factor.

Parameter Uncertainty

Parameter uncertainty relates to the uncertainty of the emission estimation of various activities, such as running a power plant, raising cattle, and driving a vehicle.

Scenario Uncertainty

Scenario uncertainty relates to a lack of knowledge about the technological and societal developments of the future, resulting in a change in how GHGs are emitted. We don’t know, for example, how fuel-efficient future cars may be, so this limits sustainability plans.

Model Uncertainty

Model uncertainty relates to issues of how accurately a model can reflect real-world scenarios.

The bottom line

Developing an emission factor can be tricky, but it’s one of the best quantification tools at our disposal for estimating emissions to be used in data sets or an assessment report.

CO2 emission factors allow us to accurately compare the emissions of one GHG to another, informing our strategies for tackling climate change.

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Reduce emissions with actionable insights on all your suppliers, embedded seamlessly into your procurement stack