The Decarb carbon footprint methodology covers eight lifestyle categories — flights, home energy, food, waste, goods and services, financed emissions, ground transport, and other carbon-intensive activities. We source every emission factor from verified data and document every assumption we make. This post explains exactly how we arrive at your number, where our confidence is high, and where it is not.
What we measure and why
Most personal footprint tools cover three or four categories. Decarb covers eight because the categories typically left out — financed emissions and goods and services — account for a significant share of a typical US footprint. EPA data puts the average US personal footprint at 14–16 tons CO₂e per year. Leaving out finance and consumption undercounts by 20–30% for many users.
Together, the eight categories we measure account for the vast majority of what an individual can observe and influence. We note categories outside our scope — government allocation, infrastructure embodied carbon, and upstream supply chain emissions — as limitations in every report.
Our data sources
We use emission factors from seven primary sources, chosen for their credibility, transparency, and US applicability.
| Source | What it covers | Categories applied |
|---|---|---|
| US EPA Emissions Factors Hub (2024) | US-specific factors for transport, waste, and energy | Ground transport, waste, home energy |
| IPCC AR6 (2021) | Global warming potential values; top-level emission factors | All categories — GWP100 basis |
| IEA Electricity Emissions Factors (2023) | Grid emission intensity by country, updated annually | Home energy (electricity) |
| Exiobase 3 (EEIO) | Consumption-based emission factors for goods and services | Goods and services, financed emissions |
| ICAO Carbon Emissions Calculator | Flight-level CO₂ estimates by route type | Flights |
| Poore & Nemecek (2018), Science | Lifecycle food emission factors, farm to retail | Food and diet |
| PCAF Global Standard (2022) | Financed emissions methodology for personal finance | Finance |
All factors are documented in full on our methodology page, including version numbers and last-updated dates.
How each category is calculated
Each of the eight categories uses a different approach depending on what data a user can reasonably self-report.
Flights
We calculate flight emissions using ICAO-based fixed factors per flight type — short-haul, medium-haul, and long-haul — multiplied by the number of roundtrip flights per year. We estimate a short-haul roundtrip at 0.296 tons CO₂e, medium-haul at 0.608 tons CO₂e, and long-haul at 0.775 tons CO₂e. The current version excludes non-CO₂ aviation effects (contrails, NOx) to maintain comparability with EPA and ICAO baselines.
Home energy
We calculate home energy emissions from housing type combined with energy source. The US EIA average electricity consumption of 10,500 kWh per year serves as the baseline, adjusted for housing size. We apply the eGRID national average grid intensity of 0.42 kg CO₂e per kWh for electricity, and the EPA factor of 5.3 kg CO₂e per therm for gas.
Food
We estimate food emissions from dietary archetype — vegan, vegetarian, flexitarian, or omnivore — using lifecycle emission factors from Poore and Nemecek (2018) and USDA dietary data. A vegan diet sits at approximately 0.4 tons CO₂e per year, rising to 2.3 tons CO₂e for a daily meat and dairy diet. These figures represent averages; individual variation within each archetype is not captured.
Ground transport
We calculate vehicle emissions from vehicle type at 12,000 miles per year — the US average per FHWA data. A standard petrol car produces approximately 3.1 tons CO₂e per year at 25 MPG. An electric vehicle produces approximately 0.9 tons CO₂e per year using the national grid average.
Goods and services
We multiply monthly spending by an emission-weighted basket of consumer categories using Exiobase EEIO factors, calibrated to the US Consumer Expenditure Survey. This is our most uncertain category — spending patterns vary widely and we cannot capture the emission intensity of individual purchases.
Financed emissions
Financed emissions reflect how banks and investment funds deploy your money to finance economic activity — including fossil fuel projects. We apply PCAF-standard factors to assumed US typical balances: $20,000 in a bank account and $100,000 invested. Standard banking and investment produces approximately 6 tons CO₂e per year — often the single largest category in a report. We state these assumptions explicitly in every report so you can assess how they apply to your situation.
Confidence levels by category
Not all categories carry equal confidence. The table below documents our assessment of each category’s reliability.
| Category | Confidence | Main source of uncertainty |
|---|---|---|
| Flights | High | Non-CO₂ effects excluded; route variation not captured |
| Ground transport | High | Assumes US average mileage of 12,000 mi/year |
| Home energy | High | Grid intensity varies by state; average applied |
| Waste | High | Based on US average waste generation per EPA WARM model |
| Food | Medium | Diet archetypes — individual meal variation not captured |
| Goods and services | Medium | Spending-based estimate; product-level intensity not captured |
| Financed emissions | Medium | Fixed balance assumptions may not match actual holdings |
| Other activities | Medium | Applicable only to explicitly high-emission hobbies |
Why your result is an estimate — and why that is sufficient
Every personal carbon footprint calculator produces an estimate. Inputs are self-reported and emission factors represent population averages. Decarb draws on the best available verified data for US users, but individual results will differ from a full lifecycle audit.
However, this is not a weakness. Estimates are sufficient for the purpose they serve: identifying which categories drive your footprint and which actions have the highest impact. If your transport category is three times larger than your food category, that ordering holds even when you account for measurement uncertainty. We build the action plan on relative magnitude, not false precision.
Decarb does not make carbon neutral claims. We will not tell you that you have offset or eliminated your footprint. We will tell you where you stand — and what to do about it.
Frequently asked questions
What emission factors does Decarb use?
Decarb uses emission factors from the US EPA Emissions Factors Hub, IPCC AR6, IEA Electricity Emissions Factors, Exiobase EEIO, ICAO, Poore and Nemecek (2018), and the PCAF Global Standard. All factors are documented in full on our methodology page.
How accurate is a personal carbon footprint calculator?
Personal carbon calculators produce estimates, not audits. Accuracy varies by category — transport and energy carry high confidence; food and goods carry more uncertainty. The estimates are accurate enough to correctly identify which categories are driving your footprint and prioritise reduction actions accordingly.
Why is financed emissions included in my footprint?
Financed emissions reflect how banks and investment funds use your money — including to finance fossil fuel projects. Under the GHG Protocol Scope 3 Category 15 framework, these are attributed to the account holder. For a typical US balance, this can produce 4–6 tons CO₂e per year — often the largest single category.
What is CO₂e and why does Decarb use it?
CO₂e stands for carbon dioxide equivalent. It is the standard unit for expressing the combined warming effect of all greenhouse gases — CO₂, methane, and nitrous oxide — on a common scale using GWP100 values from IPCC AR6. All results are expressed in tons CO₂e per year, consistent with the GHG Protocol.
Does Decarb cover all my emissions?
Decarb covers eight categories accounting for the majority of a typical personal footprint. Categories outside scope include government allocation, infrastructure embodied carbon, and some upstream supply chain emissions. Your actual footprint may be somewhat higher than the estimate.
Want to see how this methodology applies to your footprint?
Calculate yours in 3 minutes. Based on verified emission factors from EPA, IPCC, and IEA. No account required.
Sources
US EPA Emissions Factors Hub (2024); IPCC Sixth Assessment Report (AR6, 2021); IEA Electricity Emissions Factors (2023); Exiobase 3; ICAO Carbon Emissions Calculator; Poore, J. & Nemecek, T. (2018), Science 360(6392); PCAF Global GHG Accounting and Reporting Standard (2022).
Full factor documentation, version numbers, and assumptions: decarb.co/methodology
