A sponsor asks what the event’s footprint was. A client asks for the figure in a format their assurance provider will accept. Neither question is difficult to answer for electricity or waste — those arrive as invoices with quantities on them. Attendee travel arrives as nothing at all. Nobody sends the organiser a bill for the flights their delegates booked, and once the event closes, the only people who hold that information have gone home.
This is what makes travel the decisive line item. Not because its share of the total is fixed — it varies with the format, the geography, and the delegate mix, and the only way to know it for a given event is to measure it — but because it is the one category that cannot be reconstructed after the fact. Every other input leaves a paper trail. Travel leaves a gap that no amount of later effort will close.
Why attendee travel sits outside the usual categories
The GHG Protocol’s Corporate Value Chain (Scope 3) Standard organises value chain emissions into fifteen categories. Category 6, Business travel, is the one most people reach for. It is defined as emissions from the transportation of employees for business-related activities in vehicles owned or operated by third parties.
Delegates attending your event are not your employees. They are not commuting to your premises either, which rules out Category 7. The fifteen categories are built around a reporting company’s own value chain, and third-party attendee travel does not drop cleanly into any of them.
The consequence is not that attendee travel is excluded. It is that including it becomes an explicit decision the organiser has to make and document, rather than a box the framework fills in automatically. An assurance provider will accept a declared boundary that is applied consistently. It will not accept a number whose provenance is unclear.
So the boundary has to be written down before the event: whose travel is in scope, which journey legs count, whether accommodation travels with it, and how the same rules will apply next year so the comparison holds. ISO 20121:2024, the event sustainability management systems standard, exists precisely because these decisions benefit from being managed as a system rather than settled ad hoc for each event.
Collect it at registration or not at all
Registration is the single moment when every attendee is in direct contact with the organiser and has a reason to respond. It is the only reliable collection point for travel activity data, and it is open for a fixed window that closes permanently.
The GHG Protocol’s Scope 3 Calculation Guidance describes activity data for travel in terms of distance travelled by mode. That is the shape of what you need, and it reduces to a small number of fields:
- Origin — city or postcode, not country. Country-level origin collapses the distance calculation to an average that will not survive scrutiny.
- Mode — air, rail, coach, car. Emissions factors differ by an order of magnitude between them, so mode is not an optional refinement.
- Journey class, for air — cabin class materially changes the factor applied per passenger-kilometre.
- Return or one-way — a surprisingly common source of double counting, or of halving the real figure.
Four fields. They add perhaps fifteen seconds to a registration form, and they are the difference between a measured figure and an unsupportable one.
What separates a measured figure from an assertion
Once the activity data exists, the calculation is mechanical: distance by mode, multiplied by the appropriate emissions factors, aggregated across delegates. The methodology is not the hard part, and it is not where disclosures fail.
Disclosures fail on provenance. A defensible attendee travel figure needs the declared boundary it was calculated under, the source and vintage of every emissions factor applied, the response rate for the registration fields, and the documented method used to treat non-responses. That last one matters more than it appears: no registration form achieves complete responses, and how you handle the gap — extrapolating from responders, applying a conservative default, or excluding and disclosing — is a methodological choice that belongs in the disclosure rather than buried in a spreadsheet.
Structured this way, the figure supports year-on-year benchmarking, because the boundary and the factor sources are recorded alongside it. Collected loosely, it supports nothing, and the following year there is nothing to compare against.
The practical order of operations
Set the boundary before registration opens. Add the four fields to the form. Decide the non-response treatment in advance rather than when the deadline is already past. Record the emissions factor sources as you apply them, not retrospectively.
None of this is technically demanding. It is a data-collection discipline, and the reason it so often does not happen is that the deadline for starting it falls long before anyone is thinking about the sustainability report. By the time the report is due, the collection window has closed.
EventZero is built around this order of operations — structuring the activity data as it is generated, so that the reporting stage becomes an aggregation rather than an archaeology exercise. See how the measurement workflow fits together, or review what the platform captures across event categories.
If you have an event with registration still to open, that is the point of maximum leverage on the quality of the eventual number. Book Your Carbon Strategy Session and we will work through the boundary and the collection fields before the window closes.
