The appearance of different LCA results when analyzing the same type of product is more common than it seems and can raise doubts when interpreting the findings. Understanding why a life cycle assessment varies requires reviewing aspects such as the defined functional unit, system boundaries, calculation hypotheses, or the inventory databases used.

When life cycle results do not match in detail, it is not advisable to compare only the final figures. Before comparing LCA studies, it is necessary to verify under which methodological criteria each model was built and which technical decisions may be influencing the result.

It is not always an error: it is simply that the comparison is not like-for-like

In technical environmental consultancy, it is common to think that a Life Cycle Assessment (LCA) should offer a single, invariable figure for a specific product. However, LCA is a modeling methodology standardized by ISO 14040 and ISO 14044 standards—as well as ISO 14025 program instructions and category rules when LCAs are presented as Environmental Product Declarations—which convert the reality of a production process into a mathematical and environmental model.

When different LCA results appear for equivalent products, it does not necessarily mean there is a methodological error. In many cases, the variation is explained because the studies pursued different objectives or were based on different hypotheses and calculation criteria.

The environmental performance of a product depends directly on the scope defined in the study. If the initial ground rules change, the resulting metrics will inevitably change, even if the raw material and the factory analyzed are exactly the same.

The three most common causes for variations in LCA results

There are multiple variables that influence the final calculation of a product’s environmental impacts, but when comparing LCA studies, the main discrepancies usually center on four specific methodological factors:

1. Definition of the functional unit and reference flow

The functional unit defines which function of the product is being evaluated and serves as a reference to relate it to its environmental performance. One of the most common errors when comparing two studies is assuming that both are measuring exactly the same thing.

• Difference between product and actual service: One study may evaluate “1 kg of paint,” while another evaluates “the amount of paint needed to protect 10 m² of surface for 10 years.” The first case measures material volume; the second measures the performance and durability of the product during its use stage.
• Difference in packaging or formats: The impact per liter will not necessarily be the same if a product is marketed in a 25-liter industrial container or in 250 ml individual units, as the materials and processes associated with packaging change. Often, differences may even appear because one functional unit includes its proportional share of packaging and another does not, and this difference appears in the description of the unit itself.

2. System boundaries and life cycle stages included

System boundaries determine which physical processes and stages are accounted for within the environmental assessment and which are left out.

• Cradle-to-Gate: This approach only accounts for impacts from the extraction of raw materials until the product leaves the manufacturing plant.
• Cradle-to-Grave: This approach expands the analysis to include transport to the customer, energy consumed during the use phase, and final waste management at the end-of-life phase, including potential recovery and reuse.

Therefore, if one study ends at the factory gate and another includes use, recycling, or final disposal, it is normal for the life cycle results not to match.

3. Data sources and inventory databases

A life cycle study combines primary data (provided directly by the manufacturer regarding actual consumption of energy, water, or inputs) with secondary data from international life cycle inventory databases.

• Geographic and temporal representativeness: If one model uses an updated Spanish electricity mix and another uses a European Union average from five years ago, the impact indicators will differ.
• Impact assessment methodologies: The choice of environmental characterization methods (such as EF 3.1, ReCiPe, or CML) introduces variations in how physical emissions are translated into final environmental impact indicators.

4. Availability of secondary data

• A Life Cycle Assessment requires a large amount of data, much of which is not always easily accessible to the LCA specialist or the manufacturer. For example, when a raw material is purchased in bulk from a wholesaler, it may not be possible to precisely know the origin of all or part of that raw material. In these cases, which occur frequently, it is necessary to fill those “gaps” with a specific methodological decision, which must be duly justified and documented.
• Product Category Rules: These rules define a common framework for inaccessible or generic data, allowing criteria to be unified to avoid differences in results, though they cannot cover every possible situation.
• Internal technical decision: The specialist or the company establishes a realistic scenario to fill these gaps, with a reasonable justification for third parties; however, these criteria may differ between experts while remaining valid in both cases.
• Specific studies and bibliographic references: When primary data is unavailable or insufficient, it is also possible to turn to scientific studies, technical publications, or bibliographic references that provide relevant information about the product. This data must be selected considering its representativeness, quality, and suitability for the context of the study.

How to know if two studies are indeed comparable

To determine if it is technically valid to compare the figures of two life cycle reports, it is necessary to verify that both have been developed under equivalent methodological criteria.

As seen previously, one of the main references to guarantee this comparability is Product Category Rules. These rules establish the specific requirements that studies of the same product category must follow so that their results can be communicated consistently through an Environmental Product Declaration (EPD).

To check if two studies are comparable, the following variables must be contrasted:

Methodological variable	What must be verified
Functional or declared unit	That they measure exactly the same function, performance, and service life.
System boundaries	That they cover exactly the same stages (cradle-to-gate or cradle-to-grave).
Cut-off rules	That they apply the same criteria for excluding minor mass or energy flows.
Impact allocation	That they use the same criteria for distributing environmental burdens among co-products.
Verification program	That they have been prepared under the same PCR within an ISO 14025 program.

If these variables do not match, the results cannot be considered directly comparable. Using them to make comparative environmental claims to the market or administration can lead to interpretations that lack rigor or are technically incorrect.

What if there are still differences despite everything?

Indeed, in every LCA study, there is a proportion of estimates, scenario creation, and other factors mentioned earlier that can cause the analysis of the same product to generate different results when the LCA is performed by two different teams, even when following the same criteria.

But we must not lose focus. Firstly, because the differences are usually very small, and secondly, because the objective of a Life Cycle Assessment should not be a competition for “who has the best result,” but rather to understand at which stages of the production process the greatest impacts are generated, in order to reduce them and achieve continuous environmental improvement.

How we guarantee comparability and rigor at Solid Forest

At Solid Forest, we have specialized in Life Cycle Assessment consultancy for over 15 years, helping companies and institutions measure, interpret, and communicate their environmental performance with technical rigor and reliability.

To ensure that each study is transparent, traceable, and comparable, we work with a structured process:

• Modeling in proprietary software: We develop and use Air.e LCA, our professional Life Cycle Assessment software. By having proprietary technology developed entirely by us, we maintain absolute control over data modeling, calculation traceability, and integration with the most demanding inventory databases on the market.
• Specialized technical profile: Our projects are executed by a multidisciplinary team that combines chemical engineers, environmental consultants, and computer engineers. This dual technical approach allows us to understand the actual industrial process and translate it into the mathematical model without distortions.
• Strict regulatory alignment: We develop each project in accordance with the most recognized international standards (ISO 14040, ISO 14044, EN 15804, or the European Environmental Footprint standard EF 3.1) and the instructions of the selected EPD program. This ensures that the client receives a study prepared to pass independent audits and verification processes.
• Support for organizations and consultancies: We provide software, training, and methodological support to companies and other consultancy teams so they can develop their own life cycle studies and inventories with a consistent methodology.

If you need to evaluate the environmental performance of your products, develop an Environmental Product Declaration, or resolve doubts about the comparability of your environmental metrics, we can help you.

Request technical advice from our team