Transparency
Methodology & Data Sources
Every number on this site is traceable to a primary source. This page explains exactly how we calculate carbon footprints and where our data comes from.
1. Our Data Sources
| Source | What We Use It For | Last Updated |
|---|---|---|
| EPA eGRID 2022 | State-level electricity emission factors (lbs CO₂e/kWh) for all 50 states + DC | 2024 (2022 data) |
| EPA GHG Emission Factors Hub | Combustion factors for natural gas, heating oil, propane, gasoline, diesel | 2024 |
| EPA WARM Model v15 | Waste reduction & recycling emission factors; landfill methane avoidance | 2023 |
| EPA EEIO Model | Consumer spending → lifecycle emissions (goods & services categories) | 2023 |
| EPA GHG Equivalencies Calculator | Result context: miles driven, smartphones charged, tree-years, gallons gas | 2024 |
| GHG Protocol | Scope 1/2/3 methodology framework; corporate and personal accounting standards | 2015 (current) |
| IPCC AR6 (2021) | Global Warming Potentials: CH₄ × 27.9, N₂O × 273 (100-year GWP) | 2021 |
| ICAO Carbon Emissions Calculator | Flight emission factors by haul length and cabin class; includes RFI ~2.0 | 2023 |
| Poore & Nemecek 2018 (Science) | Food lifecycle emission factors (kg CO₂e per kg food); diet archetypes | 2018 |
| EPA National GHG Inventory | US total emissions by sector; trends 2005–2022; US average benchmarks | 2024 (2022 data) |
2. How Each Category Is Calculated
Electricity
Formula: lbs_CO₂e = monthly_kWh × 12 × EGRID_FACTOR[state]
We use the EPA eGRID 2022 state-level output emission factor for each state. If a user enters a dollar bill amount instead of kWh, we estimate kWh using the US average electricity price of $0.13/kWh. Vermont (0.057 lbs/kWh) to West Virginia (1.817 lbs/kWh) represent a 32× range.
Natural Gas
Formula: lbs_CO₂e = monthly_therms × 12 × 11.7
Factor: 11.7 lbs CO₂e per therm. Source: EPA GHG Emission Factors Hub, Table 1 (Stationary Combustion). Includes CH₄ and N₂O in addition to CO₂.
Personal Vehicle (Gasoline/Diesel)
Formula: lbs_CO₂e = (annual_miles / MPG) × 19.60
Gasoline factor: 19.60 lbs CO₂e/gallon. Diesel: 22.51 lbs/gallon. Source: EPA GHG Emission Factors Hub, Table 2 (Mobile Combustion). EV calculation uses 0.346 kWh/mile (EPA avg efficiency) × state eGRID factor.
Flights
Formula: kg_CO₂e = distance_km × factor[haul][cabin] × num_trips × 2
Factors (kg CO₂e per passenger-km): Short haul economy: 0.255 · Medium: 0.195 · Long: 0.147 · Business multiplier: ~2.7× economy. Source: ICAO Carbon Emissions Calculator methodology. RFI (radiative forcing index) of approximately 2.0 is already incorporated in these factors. Multiply by 2 for round trips.
Diet
Formula: lbs_CO₂e = diet_archetype_kg × 2.20462
Diet archetypes sourced from Poore & Nemecek 2018 (Science) and Oxford University food systems research. Individual food factors range from 0.90 kg CO₂e/kg (lentils) to 99.48 (beef). Lifecycle includes land use change, farm, processing, transport, retail, packaging, and food loss.
Consumer Goods & Spending
Formula: lbs_CO₂e = annual_spend_$ × spending_factor[category]
Factors from EPA Environmentally Extended Input-Output (EEIO) model: Clothing 0.78 lbs/$, Electronics 0.58, Furniture 0.43, Food service 0.52, Services 0.34. Source: EPA USEEIO v2.0.
Recycling & Waste
Regular recycler saves ~440 lbs CO₂e/year vs. no recycling. Composting saves ~220 lbs CO₂e/year.
Source: EPA WARM Model v15. Savings represent avoided methane from landfill decomposition, plus manufacturing benefits of recycled materials. Aluminum recycling has the highest per-ton savings (9.13 tons CO₂e saved per ton recycled).
3. Units & Conversions
- All primary results expressed in lbs CO₂e for US audience familiarity, with metric tonnes shown for context
- CO₂e (CO₂ equivalent): converts all greenhouse gases to a common unit using Global Warming Potential (GWP)
- GWP values (IPCC AR6, 100-year): CO₂ = 1 · CH₄ = 27.9 · N₂O = 273 · HFC-134a = 1,530
- 1 metric ton = 1,000 kg = 2,204.62 lbs
- 1 US short ton = 2,000 lbs = 907 kg
- We display tons as US short tons (÷ 2,000) throughout the site unless otherwise noted
4. Limitations & Assumptions
- Regional variation within states: eGRID state-level averages mask sub-state variation. Your utility may have a very different fuel mix than the state average — particularly in states with a mix of municipal and investor-owned utilities.
- Diet factors have high uncertainty: Poore & Nemecek (2018) found that production method (conventional vs. organic, land use history, farming practices) causes ±30% variation even within a single food category. Our factors use median lifecycle values.
- Spending-based emissions are approximate: The EPA EEIO model maps spending categories to emission intensities at an industry level. Individual products within a category vary substantially. A sustainably produced cotton shirt differs greatly from fast fashion.
- Flight radiative forcing: We use ICAO's RFI factor of ~2.0. Academic consensus suggests the true climate impact may be higher (Lee et al. 2021 estimates 3.8× CO₂-only), but this remains an area of active research. Our estimates are conservative.
- Results are estimates, not precise measurements: Individual carbon footprints cannot be measured exactly. All results should be treated as approximations useful for relative comparison and decision-making, not precise audit figures.
- Scope 3 completeness: Our annual calculator captures the major Scope 3 categories but does not include all indirect emissions (e.g., emissions from financial investments, government services consumed, full supply chain of all purchases).