Carbon Accounting: I Have My Data - What’s Next? How to Calculate a Credible Baseline

Collecting activity data is the hard part- utility bills, fuel receipts, travel exports, procurement spend. Now you need to turn that data into a baseline carbon footprint you can defend.

At its simplest, corporate carbon accounting is still:

Emissions = Activity Data × Emission Factor

…but credible results depend on three additional choices:

• Which emission factor sources you use
• How you convert gases into CO2e (your GWP set)
• How you handle uncertainty, gaps, and Scope 2 reporting methods

This guide walks you through a repeatable calculation workflow for Scope 1, 2, and 3, aligned to widely used GHG accounting conventions.

A Quick Recap: confirm your boundaries and reporting period

Before calculations, confirm:

• Your organisational boundary (what entities/sites are included)
• Your operational boundary (what sources are included in Scope 1, 2, 3)
• Your reporting period (usually 12 months)

Why this matters: calculation spreadsheets are easy to change; boundary decisions are harder to unwind later. GHG Protocol guidance emphasises that consistent, well-documented choices improve comparability and credibility. [1]

Clean your activity data: units, dates, and completeness

Most calculation errors come from basic data hygiene issues:

• Wrong units (therms vs kWh vs MWh)
• Missing months that silently undercount
• Double counting the same invoice export
• Mixing location data (grid factor) across countries

A practical “pre-calc” checklist:

• Standardise units per data type (electricity: kWh; fuels: litres/gallons; refrigerants: kg)
• Track coverage (% of months, % of sites)
• Flag estimates clearly (don’t hide them)

Choose emission factor sources you can cite

Emission factors should be credible, transparent, and regularly updated.

Examples of widely used official sets:

• UK Government conversion factors (explicitly intended for company GHG reporting and updated periodically) [2]
• US EPA Emission Factors Hub (designed for organisational GHG reporting) [3]

If you operate globally, you may use location-specific factors by geography and document what you used per site.

Non-negotiable documentation (put it in your methodology note):

• Factor source name
• Publication year/version
• Geography coverage (UK, US, EU grid mix, etc.)
• Whether the factor is combustion-only or includes upstream

Convert gases into CO2e using a declared GWP set

CO2e is a conversion convention. The key is consistency and transparency.

GHG Protocol publishes a practical reference table of 100-year GWPs aligned to the latest IPCC assessment (AR6) and explicitly notes that AR6 is recommended, while recognising that AR4/AR5 are still used in some schemes. [4]

Practical takeaway for sustainability managers: Don’t argue about “the one true methane number.” Choose a GWP set that fits your reporting context, state it clearly, and keep it consistent year over year unless you have a documented reason to change (and then disclose the impact). [5]

Calculate Scope 1: fuels and refrigerants

Common Scope 1 calculations

• Natural gas: therms/m³/kWh × combustion factor = kg CO2e
• Fleet fuels: gallons/litres × fuel factor = kg CO2e
• Refrigerants: kg leaked/top-up × refrigerant GWP = kg CO2e (high impact; don’t ignore)

If you have multiple sites, keep Scope 1 calculations at site level first; it’s easier to troubleshoot.

Calculate Scope 2: electricity needs method clarity

Scope 2 is where disclosure expectations can differ.

GHG Protocol includes Scope 2 guidance updates and has long recognised location and market-based approaches; integration guidance notes that IFRS S2 requires location-based reporting and expects separate disclosure of contractual instruments, while GHG Protocol has a dual-method framing. [6]

What to do in practice

• Calculate a location-based Scope 2 footprint for each grid region
• If you use contractual instruments (such as certain renewable energy claims), track them separately and disclose methodology clearly, aligned to your reporting framework requirements

Calculate Scope 3: pick the right method for your maturity

Scope 3 can be estimated using multiple methods, and the “right” method depends on available data.

The GHG Protocol Scope 3 framework includes 15 categories and provides calculation guidance describing different approaches and examples. [7]

Common method types:

• Spend-based (good for first screening; less accurate; depends on sector averages)
• Activity-based (freight tonne-km, passenger-km, kWh from suppliers; more specific)
• Supplier-specific (best when suppliers provide product footprints or verified data)

A practical maturity path

Year 1: spend-based screening + a few activity-based categories (travel, freight)

Year 2: improve key categories (purchased goods/services, logistics) using activity-based and supplier engagement

Year 3+: supplier-specific for top suppliers and high-impact categories

Data quality scoring: make uncertainty visible

A credible inventory doesn’t pretend to be perfect, it makes limitations visible.

GHG Protocol updates and progress materials show that data quality and boundary clarity remain active development topics in the wider standards ecosystem, reinforcing that companies should build systems that can improve and become more granular over time. [8]

A simple data quality scorecard (you can add to your appendix):

• A (measured): primary meter readings, supplier-specific data
• B (high): invoices or activity exports + credible factors
• C (medium): some gaps; partial estimation
• D (low): spend-based only, heavy assumptions

A worked example you can copy into your methodology

Electricity example

• Site electricity: 10,000 kWh
• Grid factor: 0.2 kg CO2e/kWh (example; use your official factor source)
• Emissions: 10,000 × 0.2 = 2,000 kg CO2e (2 tCO2e)

Business travel (air) example

• 50,000 passenger-km (from travel provider export)
• Factor: X kg CO2e/passenger-km (from factor set)
• Emissions: 50,000 × X = Y kg CO2e

Turn calculations into a baseline you can track

Once you have totals by scope:

• Save a “frozen” baseline workbook/report
• Define intensity metrics (per employee, per revenue) for context
• Document what you will improve next year (better Scope 3 categories; better data coverage)

If you want to reduce manual recalculation and keep an audit trail (source files, versions, assumptions), book a GreenFeet demo and see how it produces reporting outputs to form a single carbon ledger.