Methodology

Overview

Coffset’s calculators ask a fixed set of questions for each kind of footprint: a household, a trip, an event, a business or a digital nomad year. You pick an answer or type one in your own words. When you type, an AI only matches your words to the question’s options or reads the figure you gave; it never chooses an emission factor or calculates a result. The calculator’s code then applies published emission factors to your answers to estimate your CO₂ equivalent (CO₂e) emissions.

Emission Factors

We use emission factors from authoritative sources including:

• Travel: UK Government GHG conversion factors 2026 (DEFRA) for flights (domestic, short-haul and long-haul, by cabin class where the calculator asks for it), cars, trains, coaches, buses and ferries.
• Energy: Electricity uses Ember’s CO₂ intensity for each country’s grid (2025, or the latest year published), and natural gas uses DEFRA 2026. Typical home electricity use by home size and solar panel yields come from the IEA.
• Diet: Monthly figures for four diets (vegan, vegetarian, average and meat-heavy) from Our World in Data, based on Poore & Nemecek (2018, updated 2024). Event catering uses figures per plate from the same source.
• Goods & Services: Business purchases and cloud services use spend-based factors from the US EPA Supply Chain GHG Emission Factors (v1.3). Personal shopping and waste use Coffset’s own screening estimates.
• Accommodation: Hotel and hostel nights on trips use Cornell Hotel Sustainability Benchmarking 2024 averages; the digital nomad calculator uses DEFRA 2026 hotel figures by country.

Calculation Approach

Our calculation follows these steps:

1. Questions: Each calculator asks a fixed set of questions. You pick an option or type an answer. If you type, an AI matches your words to an option or picks out the number you gave, and tells you when it had to assume something. You can also use the form, which has no AI.
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 1.7× as recommended by DEFRA.
5. Aggregation: The results for each category are added up to give your total: per month for a household, a business or a digital nomad year, and a single total for a trip or an event.

How our calculators work →

CO₂ Equivalent (CO₂e)

All results are expressed in CO₂ equivalent (CO₂e), which accounts for the global warming potential (GWP) of different greenhouse gases. The published factors we use are already given in CO₂e, so the calculators apply no warming potentials of their own, with one exception: refrigerant leaks from business cooling equipment, converted with the GWP that the EU F-gas Regulation (517/2014) uses for HFC-404A:

• HFC-404A: 3,922

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. The calculators show a ±10% range with each result, because they use country-level grid factors, real great-circle flight distances and the kilometres you enter rather than global averages. Business purchases estimated from spending are less precise than that range suggests, so treat them as rough. A single number to the kilogram would imply a precision that a screening estimate 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 2026 radiative forcing multiplier of ~1.7×, 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 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: When you type an answer instead of picking one, the AI may occasionally misread it. It tells you when it assumed something, and you can switch to the form, which uses no AI.

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, Climate Action Reserve and Puro.earth.

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: October 2026

Questions?

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