Methodology

Overview

Coffset's carbon footprint calculator uses a combination of AI-powered natural language understanding and established emission factor databases. When you describe your activities — flights, drives, energy use, diet, purchases — our system extracts the relevant parameters and applies appropriate emission factors to estimate your CO₂ equivalent (CO₂e) emissions.

Emission Factors

We use emission factors from authoritative sources including:

• Travel: DEFRA 2024 Greenhouse Gas Reporting: Conversion Factors for flights (short/medium/long-haul, class), cars (by fuel type and size), trains, buses, and ferries.
• Energy: Grid electricity factors by country (IEA 2024 data), natural gas, heating oil, and renewable sources.
• Diet: Emission factors per kg of food type based on lifecycle assessments, distinguishing between plant-based, dairy, and meat products.
• Goods & Services: Estimates based on spend-based emission factors and product category averages.

Calculation Approach

Our calculation follows these steps:

1. Input extraction: Our AI parses your natural language description to extract activity type, quantity, distance, duration, and other relevant parameters.
2. Factor selection: Based on the activity type and any regional context, we select the appropriate emission factor from our database.
3. Calculation: Emissions are calculated as: Activity × Emission Factor = kg CO₂e.
4. Radiative forcing (flights): For aviation, we apply a multiplier to account for non-CO₂ effects at altitude (contrails, NOₓ, etc.), typically a factor of 1.9× as recommended by DEFRA.
5. Aggregation: Individual emissions are summed to provide your total footprint for the conversation.

CO₂ Equivalent (CO₂e)

All emissions are expressed in CO₂ equivalent (CO₂e), which accounts for the global warming potential (GWP) of different greenhouse gases. We use 100-year GWP values from the IPCC AR6:

• CO₂: 1
• Methane (CH₄): 27.9
• Nitrous oxide (N₂O): 273

Limitations

Our methodology has inherent limitations:

• Generalization: We use average emission factors which may not perfectly match your specific situation.
• Uncertainty range: Every result we show is the centre of a range, not an exact figure. Our calculators carry roughly ±15% overall, because we resolve country-level grid factors, real great-circle flight distances and the kilometres you enter rather than global averages. Spend-based supply-chain estimates carry ±30% on their own. We publish the band with each result — a single number to the kilogram would imply a precision that activity-data screening cannot deliver.
• Aviation non-CO₂ effects: Flying warms the climate through more than CO₂ alone — contrails, nitrogen oxides and induced cirrus roughly double its impact. Our flight factors therefore include the DEFRA 2024 radiative forcing multiplier of ~1.9×, already embedded in the per-km figures rather than applied twice. Calculators that report only the CO₂ from jet fuel understate aviation by about half.
• Scope boundaries: Personal calculators cover direct energy and transport plus estimated lifecycle emissions for diet, goods and waste. Business calculations are GHG-Protocol-coded across Scope 1 (on-site fuel, owned fleet), Scope 2 (purchased electricity and heat) and the Scope 3 categories we can meaningfully screen — Cat. 1 purchased goods, Cat. 4 freight, Cat. 6 business travel and Cat. 7 commuting. Remaining Scope 3 categories are outside our boundary.
• Scope 2 method: Electricity is reported location-based, using the country grid average. If you buy contractual renewable electricity (Guarantees of Origin, RECs or a PPA), your market-based figure would be lower; under GHG Protocol Scope 2 Guidance that figure must be reported alongside the location-based one, not instead of it.
• Household attribution: Home energy is a whole-dwelling figure and is never duplicated per occupant, while diet, shopping and waste scale with the number of people. Where you tell us your household size we also show the per-person share, because published averages such as the EU 6.3 t figure are per-capita and comparing a whole household against them would overstate your position.
• Data currency: Emission factors are updated periodically but may lag behind real-world changes.
• AI interpretation: Natural language processing may occasionally misinterpret ambiguous inputs.

Carbon Offset Quality

We partner with CNaught, which curates a diversified portfolio of carbon credits:

• Avoidance projects: Renewable energy, methane capture, forest protection.
• Removal projects: Reforestation, afforestation, soil carbon, direct air capture.
• Verification: Credits from Verra (VCS), Gold Standard, American Carbon Registry, and Climate Action Reserve.

All credits are retired on your behalf, ensuring they cannot be resold or double-counted.

Data Sources

We draw on the following primary sources:

Updates

We regularly update our emission factors and methodology as new data becomes available.

Last methodology review: March 2026

Questions?

If you have questions about our methodology, contact us at hello@coffset.org.