Calculating the carbon footprint of home-to-work travel (Scope 3, category 7)

The carbon footprint of home-to-work travel sits within Scope 3 of the corporate emissions inventory, specifically in category 7 of the GHG Protocol, known as employee commuting. Calculating it is compulsory for organisations entering the Carbon Footprint, Offsetting and Absorption Projects Registry held by MITECO (the Spanish ministry for the ecological transition) under the Reduzco or Compenso seals, and necessary for the corporate sustainability reporting of companies subject to Ley 7/2021 (the Spanish Climate Change and Energy Transition Act). This article sets out how the calculation is performed in technical terms.

Methodological framework

The operational reference is the MITECO technical note "Cómo calcular las emisiones de los desplazamientos in itinere" (How to calculate commuting emissions, January 2024). On that basis, the GHG Protocol applies in its Scope 3, category 7 for home-to-work travel and category 6 for travel on business. Where formal external verification is sought, ISO 14064-1:2018 applies.

Alternative sets of factors with international acceptance include those of the United Kingdom's DEFRA and those of the French ADEME. For the purposes of submitting data to MITECO, however, the national factors published by the ministry itself are mandatory.

Calculation expression

The basic formulation is conceptually simple:

Emisiones = Datos de actividad × Factor de emisión

Where:

• Activity data: kilometres per person travelled each year in each transport mode.
• Emission factor: kilograms of carbon dioxide equivalent per passenger-kilometre, specific to each transport mode (petrol car, diesel car, hybrid car, electric car, urban bus, metro, train, among others).

The technical difficulty lies in obtaining reliable activity data and in applying the factors correctly, far more than in the formula itself.

Operational steps

1. Questionnaire design

The questionnaire is the primary source of information. As a minimum, it must include the following fields:

• Workplace the employee travels to.
• Main transport mode and, where applicable, secondary modes used on the same journey.
• Distance travelled in each mode.
• Number of weekly journeys in each mode.
• Work shift.
• Home postcode, optional, useful for the mapping work that follows.

The methodological recommendations published by IDAE (the Spanish energy agency) in Guide 019 are as follows:

• A maximum of forty questions, mostly closed.
• An estimated response time of between fifteen and twenty minutes.
• Electronic format, anonymous and completed voluntarily.
• A covering letter signed by senior management.
• A pilot test with ten to twenty people before the general launch.
• Avoid Mondays and Fridays, holiday periods, strike days and one-off events.
• Suitable periods: March to April and October to November.

2. Calculating the response rate

Tasa de respuesta = número de respuestas válidas / plantilla total

3. Extrapolation to the whole workforce

For each transport mode:

kilómetros totales (modo) = kilómetros respondidos (modo) / (nº respuestas / nº empleados)

This operation assumes that those who did not reply follow the same pattern as those who did. Where the sample is stratified by sex, shift, workplace or job category, the extrapolation must be carried out stratum by stratum.

4. Adjustment for working days

Not every day of the year counts as a working day. The usual calculation considers:

• Full working day (January to June and September to December): approximately 152 working days.
• Shortened summer day (July and August): approximately 66 working days.
• Approximate annual total: 218 working days, varying according to the applicable collective agreement.

The corresponding frequency must be applied to each period whenever the questionnaire distinguished between the split working day and the shortened summer day.

5. Applying the emission factors

The official MITECO factors are published for each transport mode:

• Car: petrol, diesel, hybrid and electric.
• Motorcycle.
• Urban and interurban bus.
• Metro.
• Train: commuter, medium distance and long distance.
• Cycling and walking: emission factor equal to zero.

The unit normally used is kilograms of carbon dioxide equivalent per passenger-kilometre. Multiplying annual kilometres per person by the corresponding factor gives total emissions, expressed in tonnes of carbon dioxide equivalent per year.

Indicative case

The worked example published by MITECO, for a consultancy with sixty-five employees and a response rate of 41.5 %, shows the following orders of magnitude:

A calculation of this kind provides the baseline figure that is then fed into the company's Scope 3 inventory and carried across to the relevant modules of the corporate sustainability report.

Integration with the sustainable workplace travel plan

Calculating the footprint is not a standalone exercise. In a properly designed plan:

1. The baseline footprint is calculated with the data from the initial questionnaire, before any measures are applied.
2. Quantified reduction targets are set (for example, a 20 % reduction in emissions per person over five years).
3. The footprint is recalculated annually to measure how effective the measures introduced have been.
4. The result is carried across both to the plan's management dashboard and to the corporate sustainability inventory.

Common mistakes

• Assuming that the distance given by online mapping services reflects the real journey for every transport mode. By bicycle or on foot, real distances can differ appreciably from the straight-line distance or from the route designed for the private car.
• Ignoring the seasonality of homeworking or of the shortened summer day. Applying a single annual factor tends to overestimate private car use.
• Failing to distinguish the specific type of car (petrol, diesel, hybrid, electric) when the questionnaire did record it. The emission factor varies significantly between modes.
• Reporting only direct emissions, limited to vehicle combustion, instead of adopting the full life-cycle perspective (well-to-wheel). MITECO publishes both sets of factors.
• Failing to document the methodology used and the factors applied in detail. External verification under ISO 14064-1 demands this degree of precision.

Conclusion

Calculating the carbon footprint of home-to-work travel is not an opaque operation. A well-designed questionnaire, up-to-date MITECO factors and data processing supported by ordinary analysis tools are enough for any company subject to article 26 of Ley 9/2025 (the Spanish Sustainable Mobility Act) to build a traceable Scope 3 figure, usable both in the plan and in corporate sustainability reporting under the CSRD.

Fuentes oficiales

How to calculate commuting emissions

MITECO · Enero 2024

GHG Protocol - Scope 3 calculation guidance

GHG Protocol

IDAE Guide 019 - workplace travel plans

IDAE