# 🔢 CO2e, Scopes, and Reading a Carbon Footprint Number
By Better Wise
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Last lesson we covered the mechanism: certain gases trap outgoing heat, and adding more of them holds more warmth. What that lesson did not give you was a way to *count*. That is this lesson. A footprint figure shows up in ads, food labels, bank statements, and job postings, and most of them are quietly describing different things. By the end you should be able to convert your own everyday quantities into kilograms of CO2e and read any number you meet with its assumptions in plain view.
What you will be able to do:
- Explain why we convert every gas into **CO2e** instead of adding raw tonnes
- Place your own activities into **Scope 1, 2, and 3**
- Work out kilograms of CO2e from a distance, a kilowatt-hour, or a kilogram of food
- Spot the **boundary** of a footprint claim and see why two numbers disagree
## Why one unit beats five gases
CO2, methane, nitrous oxide, and the fluorinated gases do not warm equally. A kilogram of methane traps roughly 28 times the heat of a kilogram of CO2 over 100 years. Nitrous oxide is around 273 times. Those multipliers are called **global warming potentials**, and they let us collapse a messy mix into a single figure: **CO2e**, carbon dioxide equivalent.
$$ \
\text{kg CO2e} = \text{kg of gas} \times \text{its global warming potential}
$$
Two things follow from that formula, and both matter later:
- **The multiplier depends on the time window.** Methane's 100-year value is about 28, but over 20 years it is over 80. A claim that uses a 20-year window is not wrong, just different. Ask which window.
- **Small masses can become large CO2e numbers.** A 1 kg methane leak is 28 kg CO2e. This is why gas leaks and food waste (which rots into methane in landfill) punch above their weight.
Then there is arithmetic: emissions are a **rate times a factor**. Find the activity, find its factor, multiply.
| Activity | Typical factor | Note |
| --- | --- | --- |
| Grid electricity | 0.2 to 0.4 kg CO2e per kWh | Varies hugely by country and year |
| Natural gas burned | ~0.18 kg CO2e per kWh | Combustion only |
| Petrol car | ~0.17 kg CO2e per km | Average mid-size car |
| Beef | ~60 kg CO2e per kg | Spread is roughly 33 to 100 |
| Chicken | ~10 kg CO2e per kg | |
| Tofu | ~3 kg CO2e per kg | |
| Short-haul flight | ~0.15 kg CO2e per passenger-km | CO2 only, economy |
## Scopes: the sorting hat borrowed from business
The **GHG Protocol** was built for companies, but the three-scope split is the clearest mental model for households too.
```mermaid
flowchart LR
H["🏠
Household footprint"]
H --> S1["🔥
Scope 1: direct
Gas boiler, car fuel you burn"]
H --> S2["⚡
Scope 2: purchased energy
Electricity, district heat"]
H --> S3["🛒
Scope 3: everything else
Food, flights, clothes, services"]
```
- **Scope 1** is what goes up a chimney or out of a tailpipe that you own or control. Your gas boiler. Your car's petrol.
- **Scope 2** is energy you buy as a utility. Electricity is the main one. You did not burn the coal, but you paid for the result.
- **Scope 3** is the supply chain: food, clothes, electronics, flights, hotels, streaming, banking. For most households this is the biggest and least visible slice.
That ordering is useful because it maps onto control. Scopes 1 and 2 are the ones a household can usually switch (a heat pump, a tariff, a boiler setting). Scope 3 is where the big spreads live, which is why the next module spends its time there.
## Reading a footprint number: check the boundary first
A **boundary** is the list of what a number includes and excludes. Two footprints are only comparable if their boundaries match. When you see a figure, ask four questions:
1. **What is counted?** CO2 only, or all gases as CO2e?
2. **Where does it start and stop?** Cradle to grave, or factory gate only?
3. **Which year's emission factors?** A grid factor from 2012 is roughly double today's in many countries.
4. **Per what?** Per person, per household, per kilogram of product, per year?
A "carbon neutral" coffee pod that counts only the roasting, not the aluminium, has a boundary problem, not a maths problem.
## Worked examples: your week in kilograms
**Driving.** 12,000 km a year in a petrol car: 12,000 × 0.17 = **2,040 kg CO2e**, about 2.0 tonnes. This is Scope 1.
**Home energy.** 8,000 kWh of gas: 8,000 × 0.18 = **1,440 kg**. Add 2,500 kWh of electricity at 0.25: **625 kg**. Home total is about **2.1 tonnes**, split across Scope 1 and Scope 2.
**Food, and the size of the spread.** One kilogram of beef a week is 52 kg a year: 52 × 60 = **3,120 kg CO2e**. Swap half of that beef for chicken: 26 kg beef (1,560 kg) plus 26 kg chicken (260 kg) equals **1,820 kg**. The same meals, roughly **1.3 tonnes saved**, all Scope 3.
**One flight.** A return trip of 1,200 km each way is 2,400 passenger-km: 2,400 × 0.15 = **360 kg CO2e**. Some calculators apply a multiplier near 1.9 for high-altitude non-CO2 effects, which pushes it to about **680 kg**. Same flight, two honest numbers, different boundaries.
**Putting it together.** 2.0 + 2.1 + 3.1 + 0.4 ≈ **7.6 tonnes** for this household. The global average is around 4.7 tonnes per person; a US household is well above 15. Notice that about 3.5 tonnes, nearly half, sits in Scope 3 where nothing shows up on a bill.
The unit that puts all greenhouse gases on one scale is _____.
Methane has a 100-year global warming potential of about _____ times that of carbon dioxide.
Direct emissions from a gas boiler you own are _____, while purchased electricity is _____.
Emissions from food, flights, and bought goods fall into _____.
Before comparing two footprint numbers, check the _____ and the emission factor year.
## Rules of thumb that survive bad data
- **Never compare a CO2-only number to a CO2e number.** It is the most common error in green marketing, and it usually flatters the seller.
- **Rough beats precise.** An estimate within 20 percent tells you where the big blocks are. Chasing decimal places on a flight calculator while ignoring a beef habit is misplaced effort.
- **Scope 3 is where the surprise is.** If a footprint feels low, the boundary probably stopped at the meter.
- **Keep your factors dated.** Electricity factors fall fast as grids decarbonise, so recalculate rather than reusing an old spreadsheet.
- **Judge a label by what it omits.** "We offset 100 percent of our shipping" is a sentence about shipping, nothing more.
## What to carry forward
CO2e is the common currency, and global warming potential is the exchange rate. Scopes sort emissions by who controls them, and the boundary tells you whether a number can be trusted or compared. Multiply an activity by its factor and you have kilograms you can act on.
Next up is the Checkpoint: Climate Foundations, where you will convert a handful of quantities yourself and label them by scope.
---
## Contents
Part of [🌱 Sustainable Living and Climate Awareness](https://everwise.study/courses/sustainable-living-and-climate-awareness.md)
- **🌍 Climate Science in Plain Terms**
- [☀️ The Greenhouse Effect Without the Jargon](https://everwise.study/courses/sustainable-living-and-climate-awareness/greenhouse-effect-without-jargon.md)
- [🔢 CO2e, Scopes, and Reading a Carbon Footprint Number](https://everwise.study/courses/sustainable-living-and-climate-awareness/co2e-scopes-reading-carbon-footprint.md) (sign-in required) — this lesson
- [✅ Checkpoint: Climate Foundations](https://everwise.study/courses/sustainable-living-and-climate-awareness/checkpoint-climate-foundations.md) (sign-in required)
- **🛒 The Big Four: Food, Home, Travel, Shopping**
- [🥦 Food Footprints: Meat, Dairy, Waste, and Fair Swaps](https://everwise.study/courses/sustainable-living-and-climate-awareness/food-footprints-meat-dairy-waste-swaps.md)
- [🔌 Home Energy and Water Savings That Pay Back](https://everwise.study/courses/sustainable-living-and-climate-awareness/home-energy-water-savings-payback.md)
- [🚲 Transport and Shopping: Choices With the Widest Spread](https://everwise.study/courses/sustainable-living-and-climate-awareness/transport-shopping-choices-widest-spread.md)
- [✅ Checkpoint: Everyday Emissions](https://everwise.study/courses/sustainable-living-and-climate-awareness/checkpoint-everyday-emissions.md)
- **♻️ Waste, Greenwashing, and Habits That Stick**
- [🧰 Waste, Repair, and Reuse: Worked Examples](https://everwise.study/courses/sustainable-living-and-climate-awareness/waste-repair-reuse-worked-examples.md)
- [🔍 Sustainable Purchasing and Spotting Greenwashing](https://everwise.study/courses/sustainable-living-and-climate-awareness/sustainable-purchasing-spotting-greenwashing.md)
- [📋 Building a Household Plan That Cuts Emissions and Bills](https://everwise.study/courses/sustainable-living-and-climate-awareness/household-plan-cuts-emissions-and-bills.md) (premium content)
- [✅ Checkpoint: Green Choices and Habits](https://everwise.study/courses/sustainable-living-and-climate-awareness/checkpoint-green-choices-habits.md) (premium content)
- [Course cheatsheet](https://everwise.study/courses/sustainable-living-and-climate-awareness/cheatsheet.md) (premium content)