How your footprint is calculated

I estimate five kinds of harm for each part of your life, add a margin that depends on how good the data is, and turn the result into donations to charities that work on each kind.

One example, start to finish

A typical month of home electricity in California.

Step 1

What you use

503 kWh

From your bill, or your state's average when nothing is linked.

Step 2

The harm it does

138 kg CO₂e
51 L smog
1,124 L water
3.4 m² land

Your state's grid rates, plus a safety margin.

Step 3

What covers it

Climate$0.69
Air pollution$1.46
Water$0.09
Natureunder 1¢
To charities$2.25 a month

The five kinds of harm

Climate

Counted in kg of greenhouse gases

Paid at $5.00 a tonne, to climate policy charities

The detail

Greenhouse gases in kg of CO₂e, which counts methane and nitrous oxide as well as CO₂. Your donation gives $5.00 a tonne to climate policy charities. They are estimated to avert a tonne for $0.10 to $10, and I use the middle, but those tonnes can't be measured. Certified removal is measured per tonne but costs far more, so I don't buy it.

Air pollution

Counted in liters of smog gases, or grams of soot

Paid at 13% of the health damage, to clean-air work

The detail

The harm to health from the NOx, SO₂ and fine particles released by power plants, cars, home furnaces, jet engines and farms (mostly ammonia from manure and fertilizer), using EPA's per-ton estimates and published death counts for flights and farms. I show how much is released: liters of the smog gases (NOx, SO₂ and farm ammonia, at room temperature), or grams of soot where there's next to no gas, as with making clothes and electronics. Jet engines are counted at about 14 g of NOx per kg of fuel, and farms at EPA's 3.4 million tons of ammonia a year. The price follows the harm, not the amount, since a gram of soot does far more harm than a gram of NOx. Each item's details also give it as healthy life lost, since almost all of the harm is early deaths: EPA values a statistical life at about $10 million, and each early death from fine particles costs about 8.5 years of life, so $1 of damage is about 0.45 minutes. Your donation gives 13% of that damage to clean-air work: the middle of what US pollution controls cost per ton of fine particles (EPA puts the next ton at about $15,000, against about $113,000 of health damage).

Water

Counted in liters used up, weighted by scarcity

Paid at $0.30 per 1,000 gallons, to put water back in dry rivers

The detail

Fresh water consumed, in liters, to generate your electricity, cool data centers, grow your food, refine your fuel and make your electronics, weighted by scarcity: a liter from a water-stressed area counts in full and a liter from a well-watered one counts 20%. I assume 60% of food water, 67% of data center water and 40% of power plant water come from stressed areas. You choose how: expected impact at $0.30 per 1,000 gallons, or 1:1 restoration at $4.00 per 1,000 gallons. Expected impact, the default, goes to Colorado Water Trust, which leases water back into dry rivers for about $0.29 per 1,000 gallons all-in. Restoration certificates from Bonneville Environmental Foundation return every gallon to a stressed river.

Nature

Counted in square meters of land, for a year

Paid at About $2 an acre, to protect threatened forest

The detail

Land used to grow your food, to mine and make your electronics, and for the power plants and data centers behind your electricity and online use, in square meters. Power and data centers are a sliver of it: about 0.005 m² a year per kWh for power plants and their fuel (Lovering et al. 2022, weighted by the US mix) and 0.0005 for data centers. Your donation protects the same area of threatened forest through Rainforest Trust. Its projects cost $1 to $1,178 an acre, mostly legal protection at the low end and land purchases at the high end; across its projects it averages about $2 an acre.

E-waste

Counted in kg of electronics thrown away

Paid at $8.00 a kg, to reuse and proper recycling

The detail

What your phones, laptops, tablets, computers, TVs and consoles weigh when you replace them, plus the electronics you buy. Only about a fifth of the world's e-waste is recycled properly (Global E-waste Monitor 2024); the rest is dumped or burned, leaking lead, mercury and flame retardants. Your donation pays $8.00 a kg to a charity that refurbishes devices and recycles the rest responsibly. Nonprofit refurbishers spend about $8 to $14 for each kg they handle, so I take the low end.

Each part of your life

Home electricity
Your bill or your state's average use, times your state's grid rates for greenhouse gases, NOx, SO₂ and water. Those rates count only what leaves the power plant, so I add 12% to carbon for getting the fuel there (drilling, processing and methane leaking from gas lines) and 5% to everything for power lost on the wires. The same applies to all electricity on this page.
Home heating
Your fuel bill or a typical home's use. Natural gas is 5.3 kg CO₂ a therm, plus its NOx and particles.
Car travel
Miles divided by fuel economy, at 8.89 kg CO₂ a gallon, plus tailpipe NOx and particles, plus about 1.5 gallons of water consumed to make each gallon of gasoline. Electric cars use 0.3 kWh a mile on your state's grid.
Flights
Round trips a year, spread over 12 months, times 1.7 for contrails and other high-altitude warming. Air pollution is each kilogram of jet fuel's share of the roughly 16,000 early deaths a year that aviation causes worldwide. Water is the water used to refine the fuel.
Bus and train
Miles a week at about 0.1 kg CO₂e a passenger-mile. Air pollution and water use are assumed to match a car's per kilogram of CO₂, a rough figure.
Food
Yearly per-person figures for your type of diet, for carbon, water and land, split into meat and fish, dairy and eggs, and plant foods by typical shares. Air pollution is your share of the about 15,900 US deaths a year from farm pollution, mostly from meat, scaled by how your diet compares to the average.
Shopping
A yearly per-person carbon figure for your level of spending, split into clothing, electronics, home goods and services by typical shares. Electronics also counts the water and land to mine and make them. Clothing, home goods and services get water, land and air pollution from the EPA's USEEIO supply-chain model, with clothing water raised for imported cotton (Levi's 2015 jeans study).
AI
About 0.3 Wh a chatbot prompt, plus its share of training the model, plus data center cooling water. Because AI is new demand on the grid, its carbon is counted at the plants that ramp up to meet new demand, mostly gas, at about 1,100 lb CO₂ a MWh (EPA AVERT, rough) instead of the 770.9 average. If you add your ChatGPT, Claude or Gemini data export, I measure each prompt instead (longer conversations, reasoning models and images all use more), and your past use is your real history. With nothing linked, past use grows steadily from when you say you started (late 2022 at the earliest) to today's rate, which works out to half of today's rate over that time. Gemini's export lists prompts without their conversations, so long Gemini chats count as if each prompt started fresh.
Video streaming
About 36 g CO₂ and 0.077 kWh an hour. Your devices' own power is already in your electricity bill.

Margins

Linking an account replaces an estimate with measured data and shrinks its margin.

Estimated+30%
From your bill+15%
Measured+5%

Fees

I make nothing on fees. You pay me once, through Stripe, and I give each charity its part through Pledge. The payment carries Stripe's card fee (30¢ + 2.9%) and Pledge's 5% technology fee and 2.9% card fee, worked out on the whole payment and added on top, so each charity gets its part in full. The fees go to Stripe and Pledge. Each harm's share goes to its own charity to the cent, with no rounding up to a dollar. Paying yearly means one 30¢ fee a year instead of twelve.

Lifetime internet, AI and devices

The one-time lifetime offsets count from age 12 (or 1995, whichever is later), and from late 2022 for AI (or when you started), through age 85. Past years count at today's rates, except AI, which most people took up gradually: unless your chat history is linked, its past use grows steadily to today's rate. Future years count at today's amount of use, since networks get more efficient about as fast as use grows, but the power for it gets cleaner. I assume the US grid's carbon and power plant water per kWh fall 2% a year until 2050, then hold steady. That's a cautious pace; the EIA's outlooks range from about 2% to 6% a year depending on clean-power policy. Power plant air pollution falls faster, since coal is its main source and retires first (in 2023 sulfur dioxide fell 24% and NOx 15%, against 7% for carbon), so I take 4% a year for air. Data center cooling water and making devices, mostly overseas, aren't discounted. Future AI use counts 1.5 times over in case it keeps growing. Prices use expected-impact climate pricing, plus fees.

Everyday items (driving, flights, food, home and shopping) can include past use too, if you tick it: today's monthly amount times the months since the year you turned 18, or a year you pick. That probably undercounts, since cars and the grid were dirtier then. Past use can be paid all at once or spread over up to 10 years; by default it's spread just enough to add no more than $10.00 a month, with nothing added or taken off.

Internet is split into video, social media, cloud gaming, video calls, music and browsing, each with its own data center and network power per hour. Video uses the IEA figure (0.077 kWh an hour); the others are scaled from typical data use and are rough, so they carry an extra 50% margin.
AI adds each prompt's share of training the models: about 40% of machine learning energy goes to training. On top of that, making the chips and servers and building the data centers adds about 25% of AI's carbon. Some data centers run their own gas power, so their air pollution counts 1.2 times. AI built into other products, like search answers, counts as 4 typical prompts a day for everyone. A short AI video counts about 944 Wh, the one published measurement.
Devices counts the carbon to make each phone, laptop, tablet, computer, TV and console you'll go through (for example 70 kg for a phone kept 3 years), the water and land used to mine and make them, plus the power to run them. If you also cover home electricity month to month, the running power overlaps with it.

What “Fully Offset” means

A Fully Offset month is one where you approved donations covering the estimate plus its margin for every kind of harm I count. Covering an item or a whole area earns its badge; covering everything for all five kinds of harm earns the complete badge. Donations fund charity work, not carbon credits. If you lower an estimate by hand by more than its margin, the badge pauses until a linked account confirms the new number.

What I don't count yet

Some harms are real but have no reliable price per prompt or per purchase yet, so I name them rather than guess: toxic waste and working conditions in mining the metals for chips and devices; “forever chemicals” (PFAS) used in chip factories; noise and diesel generators near data centers, and higher power bills for their neighbors; and toxic harm from e-waste beyond paying to recycle it. Making the hardware is counted for carbon, water, land and e-waste. Tell me what's missing.

Sources

Shopping, heating, transit, food-group splits and car tailpipe figures are approximate and will be refined. The car pollution rates date from 2008 and overstate today's cars.