How to Calculate Your Carbon Footprint
The same five steps a professional assessment follows — written so you can do a credible first pass yourself, in an afternoon.
Use the free calculator insteadSet the boundary and the year
Decide what you are measuring — one person, a whole household, or a business — and pick a 12-month period. Mixing a year of energy bills with a month of car mileage is the most common source of error.
Gather your activity data
Home energy in kWh from your gas and electricity bills, car mileage and fuel type, public transport and flights taken, a rough weekly food pattern, and how much general waste versus recycling you produce.
Apply emissions factors
An emissions factor converts activity into emissions — for example kWh of gas into kg CO₂e, or miles driven into kg CO₂e. UK government conversion factors are published annually and are the standard reference for UK reporting.
Add it up in tonnes CO₂e
Sum every category and divide by 1,000 to get tonnes CO₂e. Keep the category breakdown — the split matters far more than the headline number when you decide what to change first.
Sanity-check against a benchmark
Compare your total against a typical UK household footprint. A result far below the benchmark usually means a category was missed, most often flights or heating.
A worked example
Suppose a household uses 11,000 kWh of gas and 2,700 kWh of electricity a year, drives 8,000 miles in a petrol car, and takes one return flight to southern Europe. You multiply each of those figures by its published conversion factor, add the results, then divide by 1,000. Heating and the car will almost always be the two largest lines, with the flight a surprisingly close third.
The point of the exercise is not decimal accuracy. It is knowing which two or three lines to attack, and roughly what each one is worth in tonnes.