Building trust in carbon calculators

Tuesday, 13 February 2024

In brief:

  • Carbon calculators help farm businesses estimate their greenhouse gas (GHG) emissions, but different calculators give different results
  • New Defra-commissioned research shows the areas these differences come from, such as what is and isn’t included, choice of emission factors, standards followed, and approaches to sequestration.
  • The report also gives recommendations to make the results from different calculators more consistent – “harmonisation”


Carbon calculators play a crucial role in achieving net-zero targets. They help farm businesses estimate their greenhouse gas (GHG) emissions and identify ways to reduce them. However, choosing a calculator is not so straightforward.

There are a lot of carbon calculators to choose from, each with varying levels of complexity and assumptions, which means that different calculators can report different results on the same farm. This can be very confusing for farmers and growers and make it hard to trust a calculator. Defra identified that this lack of trust was a barrier to the uptake of carbon assessments on farm and commissioned a report[1] to explore why calculators differ in their emission outputs and identify ways to improve harmony between calculators.  

Six calculators[2] that were most relevant to agriculture in England and Wales were selected and GHG emissions were calculated on 20 model farms spanning nine farm systems, with an intensive and extensive scenario for comparison. The results were then evaluated to determine where differences exist, as well as the causes of those differences.

What does the report tell us?

The report highlights that no one calculator consistently gave the highest or lowest emissions. In some instances, when carbon sequestration was included, the difference between calculators increased.

For cereal crops on cultivated peat, there was an eleven-fold difference between the highest and lowest calculator emission outputs. This big difference is because two calculators did not account for the emissions from cultivated peat. There was a 2.8- and 2.4-fold difference between the highest and lowest calculator emissions for intensive and extensive lowland grazing, respectively. Some of the remaining farm types (organic cereals, regenerative cereals, dairy, indoor pigs and extensive upland grazing) had similar levels of overall emissions, but this was through chance rather than because a harmonised approach was adopted.

The differences between calculators are explained by the variation in user-entry data, boundaries on what is included and what is not, the precision to which they use emission factors, how they deal with soil carbon sequestration, the standards and protocols to which they align, their transparency, rigour and consistency.

Calculators with the highest or lowest emissions cannot be considered the best or worst as there are many features driving this difference. For example, a more comprehensive assessment is likely to have a higher emission but may also provide greater functionality to support the user in reducing emissions, while a lower value may represent a calculator that is using up-to-date emission factors that reflect decarbonisation. It is important to look at both the scale of emissions per farm and understand the reasons for the differences.


To increase farmers’ and supply chain confidence and increase relevance to the user’s farm, the report made the following recommendations, which are directed at industry, government and calculator providers:

  • Clearly define what a farm-level assessment is, how it should be used and what parts of a farm business should and should not be included
  • Align with the latest recommendation in guidance and standards, such as those provided by GHG Protocol, Science Based Targets initiative, IPCC and ISO
  • Regularly review scientific knowledge and maintain up-to-date carbon accounting methodologies and new emission factors
  • Use UK-specific (Tier II and Tier III) emission factors and methodologies, such as those created for the UK GHG Inventory
  • Standardise guidance for setting boundaries and assessing carbon emissions and removals
  • Use emission factors from an agreed set of robust databases for embedded emissions in fertilisers, feeds and fuels
  • Support the development of appropriate emission factors for embedded emissions in purchased livestock
  • Mandate certain functionality (e.g. emissions from peat soils)
  • Require greater data granularity in areas with high emissions, such as enteric methane emissions
  • Improve transparency of approaches used and implement third-party verification of the alignment of calculators to minimum standards

What next?

This review of the calculators focused on the divergence between them, rather than areas of consistency, to look at how harmonisation can be achieved. Although full harmonisation of carbon calculators is unlikely, due to commercial and intellectual property sensitivities, it is important that there is a set of minimum standards or guidelines that are specific to farm-level carbon assessments against which calculators can align.

This report offers clear guidance to industry, government and the calculator providers on what needs to be done to progress towards increased consistency and standardisation, which should ultimately improve farmer and grower trust in carbon calculators.

When it comes to choosing a calculator, AHDB’s advice is to pick one and stick with it, this way you will always be tracking your progress against baseline data captured by the same calculator. It is important to make sure that you pick the right tool for you. To help you, AHDB has produced this useful guide: Carbon footprint calculators – what to ask to help you choose

[1] The full ‘Harmonisation of Carbon Accounting Tools for Agriculture’ report can be accessed here: Science Search (

[2] The calculators used were: Agrecalc Ltd.’s Agrecalc, The Cool Farm Alliance’s Cool Farm Tool, Eggcase Ltd.’s carbon footprint tool, Farm Carbon Toolkit’s Farm Carbon Calculator, Trinity Agtech’s natural capital navigator Sandy and Solagro’s The Farm Carbon Calculator