How we calculate embodied carbon
16th May, 2023

People using our Carbon Calculator database often ask us how we produce the results. This brief article outlines our process.

The extent of embodied carbon's impact on climate change has increased in prominence in recent years. As a practice and as an industry, structural and civil engineers are working to limit that impact. But the adage goes that 'if you can't measure it, you can't manage it', so the accurate and consistent calculation of embodied carbon is crucial. How do we do it?

Emissions come from both the operational energy used to run buildings (heating, cooling, lighting, technology, and the like) and the embodied energy from the production of materials and their transportation, and the construction, maintenance, and demolition of buildings.

Embodied carbon is the name given to carbon dioxide and equivalent gases with high global warming potential, which are released into the atmosphere because of the production of goods and materials. While carbon dioxide is the primary greenhouse gas responsible for climate change, it’s not the only one. Others, like methane, can have a great impact in terms of their global warming potential - sometimes by significant orders of magnitude. To make direct comparisons, emissions are compiled into a single figure known as Carbon Dioxide Equivalent (CO2e). This is measured in kilograms kgCO2e and is standard across all industries.

To make comparisons between one building and another, the figure is usually adjusted to take account of its size and an embodied carbon rate is used, in kgCO2e /m2.

Price & Myers has developed two calculator tools to measure embodied carbon. The first, Parametric and Numerical Design Assessment tool (PANDA) developed in conjunction with University of Cambridge, is a carbon cost estimator and allows assessments of multiple different building framing, grid, and material choices at concept stage. It reports relative costs and embodied carbon for each option. This allows the design team to make informed choices on options at early scheme stages. University of Cambridge analysis has indicated that PANDA has reduced embodied carbon on non-refurbishment projects by at least 100 tonnes per Price & Myers project where it has been used.

The second is our in-house Carbon Calculator which calculates the embodied carbon within the structure of the building over different life cycles. It is based on the methods outlined in 'How to Calculate Embodied Carbon', first published by the IStructE (August 2020) and in line with BS EN 15978. It’s used throughout the later stages of the project - once embodied carbon targets have been set - and our database is generated directly from this tool. We have been measuring carbon and publishing our data over the past three years. The figures show that over that period we have managed to reduce the embodied carbon of our designs by approximately 13%.

The process is relatively straightforward. The quantity of each material contained within the primary structure (which amounts to the superstructure plus the substructure) is first calculated or taken from the BIM model. Each material quantity is then multiplied by a series of embodied carbon factors which relate to parts of the building lifecycle, such as the production of materials or transportation to site.

The data used in these calculations primarily covers the embodied carbon to practical completion of the building, otherwise known as modules A1-A5, within the overall building life cycle. This includes kgCO2e released during extraction, processing, manufacture, and transportation of materials/products, as well as any energy usage and material wastage on site during the construction phase.

Price & Myers has carried out extensive research into the embodied carbon figures, and the data used to generate these results is a combination of this research and standard industry published data. All Embodied Carbon Factors (ECFs) used in our calculations are displayed, to allow for direct comparison with calculations published by others. Our philosophy is to ensure transparency in the figures and assumptions made, and to use conservative-worst case figures at early-stage design to aid decision making, with the values refined as designs progress. Our calculations therefore fix variables, such as recycled steel content and cement replacement materials, to global average values for scheme design, with our engineers only allocating more realistic values once procurement for the materials is completed.

Beyond practical completion, the carbon calculations continue to cover the remainder of the building's lifecycle - over modules B, C and D. This is vital for understanding the impact on the building's Whole Life Carbon (WLC). While our tools are all capable of calculating these latter modules using standard practice methods, we do not present this data as part of our headline figures. This is because in most cases, it makes little practical difference to the decision-making process that we need to take as engineers, and the values are often based on a range of assumptions that mean the accuracy is questionable.

The only exception to this is in timber sequestration, which accounts for the carbon that is locked into timber as a tree grows. It remains locked in during the time that the timber is used as building materials, meaning the timber acts effectively as a carbon store. For early-stage lifecycles, A1-A3 or-A5, this value is given as a separate, negative figure and represents carbon that is removed from the atmosphere. The negative sequestration value is only considered if the timber is sourced from sustainable forests. Once the timber is deposed of, by either burning or rotting away, the stored carbon will be returned into the atmosphere, and so the sequestration figures are cancelled out by the C3-C4 modules. As guidance recommends, the calculation therefore reports the sequestration figures separately for lifecycles A1-A3, or A1-A5. Sequestration is included in A-C or A-D totals but gives a net contribution to the totals of zero.

Carbon calculations are subject to a wide range of assumptions and are based on data with a high degree of uncertainty. Our data provides an estimated range of this uncertainty. The range represents the degree to which the actual embodied carbon total may vary, based on the cumulative uncertainties of the different factors. While the headline figure should align with standard calculations, we recommend the use of the higher, bound value in decision making, as this represents a realistic worst-case - particularly in the early stages of a project.

We express the results of our calculations as both a total embodied carbon for the primary structure and an embodied carbon rate (total embodied carbon divided by the gross internal areas of the building).

Calculating embodied carbon consistently and as accurately as possible helps us to set meaningful targets for its reduction. Price & Myers makes the data from our Carbon Calculator available to everyone for free, to help increase the sample, and thereby the accuracy, of embodied carbon calculation in the built environment. You can download our latest data here.