The Six-Step Accounting Process
The Scope 3 Standard recommends a six-step process for building a Scope 3 inventory. These steps are not always strictly sequential — data collection and calculation are iterative — but the framework provides a logical structure for first-time practitioners.
Step 1: Review the accounting steps and principles Understand the five accounting principles (see below) and the overall framework before collecting any data. Setting up the inventory with correct boundaries and methods from the start avoids costly rework.
Step 2: Identify Scope 3 activities Map all business activities to the 15 Scope 3 categories. Not all categories will be relevant to every company. The standard provides decision trees and worked examples to help companies identify which categories apply.
Step 3: Set the Scope 3 boundary Determine which activities within each relevant category to include. The standard requires companies to include all significant Scope 3 emissions and disclose any excluded categories with justification.
Step 4: Collect data Gather activity data (quantities, spend, distances, etc.) and pair with appropriate emission factors. The standard defines a data quality hierarchy from primary supplier-specific data down to generic industry averages.
Step 5: Apply calculation methods Apply one of the accepted calculation approaches — supplier-specific, hybrid, average-data, spend-based, or direct measurement — to convert activity data into tCO₂e.
Step 6: Allocate emissions Where activities are shared across multiple products or customers, allocate emissions using physical or economic allocation. The standard provides guidance on preferred allocation approaches for each category.
The process is intentionally iterative. A company typically starts with a high-level spend-based screening to identify hotspots, then invests in better data quality for the most significant categories. The standard does not require perfect first-attempt accuracy — it requires improvement over time.
The Five Accounting Principles
The Scope 3 Standard adopts the same five accounting principles as the original Corporate Standard. These principles govern every decision made during inventory development.
1. Relevance
The inventory shall appropriately reflect the GHG emissions of the company and serve the decision-making needs of users — both internal managers and external stakeholders. A relevant inventory includes all significant Scope 3 categories and uses methods suited to the company's business model.
2. Completeness
The inventory shall account for and report on all GHG emission sources and activities within the chosen Scope 3 boundary. Omissions shall be disclosed and justified. The completeness principle drives the requirement to screen all 15 categories rather than accounting only for familiar or convenient ones.
3. Consistency
The same methodologies shall be used consistently to allow meaningful tracking of emissions over time. When methodologies change — for example, when a company upgrades from spend-based to supplier-specific data for Category 1 — the standard requires recalculation of the base year using the new methodology.
Consistency is particularly important for target-setting. If a company sets a 2030 target relative to a 2020 base year, both years must be calculated using the same methodology. Otherwise, apparent "reductions" may be artefacts of methodological change rather than actual emissions reductions.
4. Transparency
The company shall address all relevant issues in a factual and coherent manner, based on a clear audit trail. Transparency requires:
- Documenting all data sources and emission factors used
- Disclosing the calculation methods applied for each category
- Noting any assumptions or limitations
- Identifying any categories excluded from the boundary and explaining why
5. Accuracy
The inventory shall be sufficiently accurate to enable users to make decisions with reasonable confidence in the reported information. Accuracy does not mean perfection. The standard acknowledges that Scope 3 data will always be less precise than Scope 1 and 2, and that the best available estimate — disclosed with quantified uncertainty — is preferable to no estimate.
The five principles mirror the foundations of financial accounting. Relevance ensures the inventory is decision-useful. Completeness prevents cherry-picking favourable numbers. Consistency enables year-on-year comparison. Transparency supports external verification. Accuracy ensures the data is fit for purpose. A Scope 3 inventory built on these principles earns the same credibility as audited financial accounts.
Mandatory vs. Optional Requirements
The Scope 3 Standard uses the word "shall" to denote mandatory requirements and "should" or "may" to denote guidance and recommended practices. Understanding the distinction is important:
- Mandatory requirements (shall): Must be met for the inventory to claim conformance with the standard. Examples: disclose all relevant categories; explain any excluded categories; recalculate the base year when methodologies change significantly.
- Recommended practices (should/may): Best practice guidance that companies are strongly encouraged to follow but which is not required for conformance.
As Scope 3 disclosure becomes mandatory in jurisdictions including the EU (CSRD) and California, the accounting principles take on legal significance. Regulators and plaintiffs increasingly scrutinise whether disclosed Scope 3 inventories are truly complete (all material categories included), consistent (same method base year to target year), and accurate (not systematically understated). Companies that apply the five principles rigorously — and document their decisions — are far better positioned in regulatory reviews and legal challenges.
ISO 14064-1:2018 (Section 4) uses the same core principles as the GHG Protocol - relevance, completeness, consistency, accuracy, and transparency. The practical difference is in how ISO treats indirect emissions. Rather than grouping all non-energy indirect emissions into a single "Scope 3," ISO breaks them into four categories: transportation, products used by the organization, use of products from the organization, and other sources. This finer categorization can make it easier to prioritize data collection efforts, but the underlying accounting logic is the same. An inventory prepared under the GHG Protocol Scope 3 Standard will satisfy ISO 14064-1's indirect emission requirements with minimal additional work.
Key Takeaways
- 1The six-step accounting process provides a logical framework: review principles, identify activities, set the boundary, collect data, apply calculation methods, and allocate emissions
- 2The process is iterative - start with spend-based screening, then invest in better data quality for the most significant categories over time
- 3Five accounting principles govern every inventory decision: relevance, completeness, consistency, transparency, and accuracy
- 4Consistency is critical for target-setting - both the base year and target year must use the same methodology, or the base year must be recalculated
- 5The standard distinguishes mandatory requirements ('shall') from recommended practices ('should'/'may') - understanding this distinction is key for conformance