A life cycle assessment (LCA) is a structured method for measuring the environmental impact of a product, process, or service across its entire lifespan, from raw material extraction through disposal. The process follows four phases defined by ISO 14040 and ISO 14044: goal and scope definition, inventory analysis, impact assessment, and interpretation. A thorough LCA can take anywhere from a few weeks for a streamlined study to several months for a full, third-party-reviewed report. Here’s how each phase works in practice.
Phase 1: Define Your Goal and Scope
Every LCA starts by answering two questions: why are you doing this study, and what exactly are you measuring? The goal statement should spell out the intended application (comparing two packaging options, supporting an environmental product declaration, informing internal design decisions), the target audience, and whether the results will be made public. These choices matter because they determine how rigorous your data collection and review process need to be.
Scope definition is where the real planning happens. You need to pin down three things: the functional unit, the system boundary, and your data quality requirements.
The functional unit is the reference point that makes comparisons fair. It defines exactly what your product does and in what quantity. For example, “delivering 1,000 liters of drinking water to a household over one year” is a functional unit. Without it, you’d be comparing apples to oranges, literally comparing a glass bottle to a plastic pouch without accounting for the fact that they hold different volumes and last different lengths of time.
The system boundary determines which life stages and processes you include. Common boundary choices include:
- Cradle-to-grave: covers everything from raw material extraction through manufacturing, use, and end-of-life disposal or recycling.
- Cradle-to-gate: stops at the factory gate, excluding the use and disposal phases. This is common for business-to-business products where you don’t control what happens downstream.
- Gate-to-gate: covers only the manufacturing process itself, useful for comparing production methods within a single facility.
It’s normal to revisit and adjust the system boundary as you move into data collection. Just document every modification so the study remains transparent.
Phase 2: Build the Life Cycle Inventory
The inventory phase is the most labor-intensive part of an LCA. You’re creating a detailed ledger of every input (energy, water, raw materials) and every output (emissions to air, water, soil, and waste) for each process within your system boundary.
Data comes from two broad sources. Primary data is collected directly from the operations you’re studying: electricity bills, fuel consumption records, material purchase orders, waste manifests. Secondary data fills the gaps using published databases that provide average values for common processes like electricity generation, transportation, or chemical production. Most LCA software ships with one or more of these background databases.
One of the trickiest parts of the inventory phase is handling co-products. When a single process produces more than one useful output, you need a way to split the environmental burden between them. ISO 14044 prefers that you avoid allocation entirely by expanding the system boundary to include the additional function. When that isn’t practical, the two most common fallback methods are mass allocation and economic allocation.
Mass allocation divides the burden based on the physical weight of each co-product. It’s stable and straightforward, but it doesn’t work when one of the outputs isn’t measured by mass (electricity, for example). Economic allocation divides the burden based on the market value of each co-product. It captures the economic motivation behind production, but because prices fluctuate, it can shorten the shelf life of your results. Whichever method you choose, document it clearly and test how it affects your final numbers.
Phase 3: Assess Environmental Impacts
Once you have a complete inventory of emissions and resource use, the impact assessment phase translates those raw numbers into meaningful environmental indicators. Each emission gets multiplied by a characterization factor that reflects how potent it is for a given type of environmental damage. For climate change, for instance, one kilogram of methane is multiplied by a factor of roughly 28 to express its impact in kilograms of CO₂ equivalent.
The standard impact categories used in most LCAs include:
- Climate change: greenhouse gas emissions measured in CO₂ equivalents.
- Ozone depletion: damage to the stratospheric ozone layer.
- Acidification: emissions (like sulfur dioxide) that contribute to acid rain.
- Eutrophication: nutrient runoff that causes algal blooms in waterways.
- Human toxicity: cancer and non-cancer health effects from chemical exposure.
- Ecotoxicity: harm to ecosystems from toxic substances.
- Photochemical ozone formation: ground-level smog creation.
- Resource depletion: consumption of minerals, fossil fuels, and water.
- Land use: occupation and transformation of land areas.
You don’t always need to report on every category. The goal and scope phase should have identified which impacts are most relevant to your product and audience. A packaging study might focus on climate change, resource depletion, and eutrophication, while a building materials study might add land use and human toxicity.
Phase 4: Interpret the Results
Interpretation isn’t a step you save for the end. It runs continuously throughout the study as you refine boundaries, fill data gaps, and spot inconsistencies. But in its formal form, interpretation involves three activities: identifying the significant issues, evaluating data quality, and drawing conclusions.
Start by identifying which life cycle stages, processes, or materials dominate your results. These “hotspots” are where design changes or sourcing decisions will have the most leverage. If 70% of your product’s carbon footprint comes from one raw material, that’s the place to focus improvement efforts.
Next, test how robust your conclusions are. Sensitivity analysis is the primary tool here. You systematically vary key inputs, one at a time or in combination, to see how much the final results shift. If switching your electricity data source from a national average to a regional grid mix flips which product option looks better, that’s a finding your audience needs to know about. Both local sensitivity analysis (changing one parameter at a time) and global sensitivity analysis (varying many parameters simultaneously) help reveal which inputs matter most to the outcome.
Completeness checks confirm that you haven’t left critical data gaps that could skew the results. Consistency checks verify that you’ve applied the same methods and data quality standards across all the alternatives you’re comparing.
Choosing Your Software
You can technically build an LCA in a spreadsheet, but dedicated software makes the process dramatically faster and less error-prone. The most widely used tools fall into two camps.
On the commercial side, SimaPro is known for its flexibility and large user base in both academic and consulting settings. GaBi (now part of Sphera) is popular in industrial applications and has strong support for generating Environmental Product Declarations. Umberto specializes in energy and process modeling.
On the free and open-source side, openLCA is the most established option. It supports all major impact assessment methods and connects to several commercial and free databases. Brightway2, a Python-based framework, appeals to researchers who want full control over their modeling and want to run advanced sensitivity or uncertainty analyses programmatically. For more specialized needs, IDEMAT from TU Delft focuses on material selection, and Athena integrates with building information modeling for construction projects.
Your choice depends on budget, the complexity of your product system, and which background databases you need. Most commercial tools offer academic licenses at a steep discount.
When You Need a Critical Review
ISO 14044 requires a formal critical review by an independent expert or panel only when your LCA supports a comparative assertion that will be disclosed to the public. A comparative assertion is a claim that one product is environmentally preferable to another. If you’re publishing a report that says “Product A has a lower carbon footprint than Product B,” it needs a critical review. Every LCA report must include a statement declaring whether it’s intended to support such a comparison.
Even when a critical review isn’t mandatory, having an external expert check your methodology, data sources, and interpretation adds credibility. Many companies commission voluntary reviews for internal studies that will inform marketing claims or procurement decisions, knowing the results may eventually become public.
Practical Tips for Your First LCA
Define a narrow, realistic scope before you start collecting data. One of the most common pitfalls is setting a cradle-to-grave boundary when you only have reliable data for your own manufacturing process. A well-executed cradle-to-gate study is more useful than a cradle-to-grave study riddled with placeholder data.
Expect the inventory phase to take the bulk of your time. Tracking down primary data from suppliers, verifying units, and reconciling conflicting numbers is slow, iterative work. Build in time for at least one round of boundary adjustments once you see what data is actually available.
Document everything as you go. Record your assumptions, data sources, allocation choices, and any exclusions. This documentation isn’t just for the final report. It’s what allows you (or a reviewer) to trace any result back to its underlying data and understand why the numbers look the way they do.