CO₂ Emissions Calculator

This calculator estimates emissions for the activities you enter: car travel, motorcycle travel, bus or coach journeys, rail journeys, flights, household electricity, natural gas, and one optional custom activity. It reports the selected total in kilograms and metric tonnes of carbon dioxide equivalent (CO₂e), then separates that total by category.

The result covers only the entered activity amounts and the disclosed factors. It does not infer an annual lifestyle total or include food, purchases, waste, accommodation, vehicle manufacture, infrastructure, or other activities that are not represented by an input. A zero result means no emissions-producing activity has been entered; it does not mean a person or household has no emissions.

Use the estimate to examine a journey, an energy period, or a controlled comparison. The calculation is not a carbon offset, a complete personal footprint, or a certified greenhouse-gas inventory.

CO₂e is a common reporting unit that expresses the warming effect of greenhouse gases as an equivalent amount of carbon dioxide. The official conversion-factor rows used here provide values in kilograms of CO₂e, so the calculator can add activities with different underlying greenhouse-gas profiles into one consistent unit.

CO₂ and CO₂e are therefore related but not interchangeable. CO₂ refers to carbon dioxide itself; CO₂e may combine carbon dioxide with other greenhouse gases using the global-warming-potential conventions applied by the source dataset. The calculator uses the published CO₂e value for each selected row rather than attempting to reconstruct its gas-by-gas components.

A CO₂e estimate is a calculation based on activity data and a factor, not a direct measurement of atmospheric impact. Its usefulness depends on choosing a factor whose unit, geography, year, and boundary fit the activity being estimated.

Every category starts with activity data: distance travelled, kilowatt-hours used, or a custom quantity. The calculator converts miles to kilometres when necessary and then multiplies the activity amount by an emission factor expressed in kilograms of CO₂e per matching unit.

An emission factor is an average relationship, not a reading from the specific vehicle, train, aircraft, meter, or appliance involved. Results can change when the selected vehicle size, powertrain, route type, cabin class, occupancy, energy factor, or activity amount changes. Keeping the input unit aligned with the factor unit is essential.

The built-in factors come from the UK Government 2026 dataset and are intended for UK activities. An editable electricity factor and the custom-activity controls support documented comparisons outside those defaults, but user-supplied factors carry the boundary and quality of their own source and do not make the result a certified inventory.

Car factors are selected by vehicle size—small, medium, large, or average—and by petrol, diesel, hybrid, plug-in hybrid, or battery-electric powertrain. Each built-in car value is a whole-vehicle factor per vehicle-kilometre, so a larger distance or a different size and powertrain selection changes the vehicle total before any personal allocation.

The calculator divides those whole-vehicle emissions by the entered number of occupants, from one through twenty, to show one person's equal allocated share. For example, the source factor for an average petrol car is 0.16152 kg CO₂e per vehicle-km: over 100 km the vehicle estimate is 16.152 kg CO₂e, or 8.076 kg CO₂e per person when shared equally by two occupants.

Occupancy is an allocation convention; it does not change what the vehicle emits during the journey. The average factors also cannot represent every model, driving style, load, road, temperature, charging pattern, or fuel blend, and this calculation does not add manufacture or infrastructure outside the selected source row.

The local bus, coach, national rail, and international rail factors are already expressed per passenger-kilometre. Enter the distance travelled by one passenger; the calculator multiplies that distance directly by the selected passenger-distance factor and does not divide it by a separate occupancy value.

A passenger-kilometre represents one passenger transported for one kilometre. Thus, 100 passenger-km may describe one person travelling 100 km. Published passenger-km factors embed the source methodology's average loading and operating assumptions, so adding a vehicle occupancy adjustment would count allocation twice.

The motorcycle option is the exception in this group: its UK reference value is per vehicle-kilometre and the interface does not provide a rider-occupancy allocation. Bus, coach, and rail averages can still differ from a particular operator, route, service load, traction source, or local transport system.

Flight estimates use the one-way distance, the one-way or return control, route type, and an available cabin class. The selected UK factor is already per passenger-kilometre and includes the dataset's distance treatment, including its built-in 8% distance uplift, so the calculator does not add another distance uplift. A return selection multiplies the entered one-way distance by two.

Cabin class matters because the official passenger allocation differs by the space attributed to each class, and not every class exists for every route type. The interface limits the cabin choices accordingly rather than inventing a fallback factor. The result remains an average passenger-distance estimate, not a calculation for a specific aircraft, load factor, routing, or weather pattern.

The aviation non-CO₂ effects control selects the official factor that applies the dataset's radiative-forcing treatment, documented as a 1.7 multiplier on the CO₂ component. Turning it off selects the corresponding direct-GHG factor. Radiative forcing is scientifically uncertain and the selected factor is not a precise measurement of every aviation climate effect; the toggle is provided so both documented treatments can be viewed without applying the multiplier twice.

Electricity emissions depend on the generation mix and accounting boundary behind the factor. Grids with different shares of fossil, nuclear, renewable, imported, or other generation can have materially different kilograms of CO₂e per kWh, and the value can also change by reporting year.

The default is the UK 2026 electricity-generation reference factor of 0.13096 kg CO₂e per kWh. If the electricity was used elsewhere, replace it with a documented grid or supplier factor that matches the period and boundary you intend to compare. The calculator labels a changed value as user supplied rather than presenting it as part of the UK dataset.

Natural gas remains tied to the built-in UK 2026 gross-calorific-value factor of 0.18231 kg CO₂e per kWh. Editing the electricity factor does not alter gas, transport, or flight factors, and it does not add transmission, lifecycle, or supplier-specific elements unless those are already included in the replacement factor.

The total is the sum of all non-zero categories. Each category shows kilograms of CO₂e and its percentage share of that selected total, while the highest category identifies the largest entered contribution. Metric tonnes are the same total divided by 1,000 rather than a separate calculation boundary.

Shares answer a relative question: how the activities entered in this run divide the result. Adding or removing an activity changes the denominator and therefore every displayed share, even when another category's kilograms remain unchanged. Rounding can also make displayed percentages appear slightly different from values calculated at full precision.

When comparing alternatives, run each scenario separately or change one assumption at a time. Entering both a car journey and a bus journey together means both activities are included in the total; it does not tell the calculator that they are mutually exclusive options.

The main uncertainty comes from applying average reference factors to specific real activities. Vehicle efficiency, occupancy, routing, service loading, aircraft and cabin configuration, grid mix, weather, operational conditions, and data quality can all make an actual event differ from the estimate. Annual dataset revisions can also change a factor even when the activity amount is unchanged.

The built-in UK factors are intended for UK activities. They should not be treated as universal defaults for another country, and the calculator does not determine whether a user-supplied factor has the right geographic, temporal, or organizational boundary. Custom and editable factors improve scenario flexibility, not assurance.

This is an activity estimator, not a complete or certified carbon footprint, audited inventory, regulatory filing, or organizational Scope 1, 2, and 3 reporting system. It does not document evidence, consolidate entities, prevent double counting across inventories, or include categories that have no input in the interface.

A useful comparison holds the service constant—for example, the same journey distance—and changes one relevant input such as car occupancy, mode, vehicle selection, cabin class, or electricity factor. Separate runs make the difference attributable to that choice easier to interpret than a total containing both alternatives.

For car travel, occupancy changes only the displayed personal allocation; it does not reduce the calculated whole-vehicle amount. For bus, coach, rail, and flights, the official factor already represents one passenger-kilometre. For regional electricity comparisons, use factors with comparable years, units, and boundaries.

A lower calculated value means the selected factor-and-activity combination is lower within this calculator's boundary. It does not guarantee a lower complete lifecycle impact or a particular environmental outcome, because excluded manufacture, infrastructure, indirect effects, and local conditions may matter to a broader assessment.

Built-in values are taken from the UK Government GHG Conversion Factors for Company Reporting 2026, flat-file version 1.1, published by the Department for Energy Security and Net Zero. The interface discloses the active factor ID, label, value, unit, and boundary so the selected row can be traced to the versioned source.

The method applies each factor only to its matching activity basis, as summarized in the table below. Distances entered in miles are first converted using 1 mile = 1.609344 kilometres; return flights double the one-way distance; whole-vehicle car emissions are divided by occupants; category results are then summed and converted to percentage shares.

Government factors are updated over time as source data and methods change, so a result should retain its dataset year and assumptions when used in a comparison. All arithmetic runs locally in the browser, and neither the official defaults nor user-supplied values turn this selected-activity estimate into an audited or certified inventory.