The most commonly used indicators are: CO2 emissions per participant (transportation + energy + catering), the waste reduction rate (what percentage was sorted or reused), and the proportion of sustainable suppliers. For CO2 calculations, there are online calculators designed specifically for events.
Also ask your event agency for a post-event impact report; more and more agencies are able to provide this. In addition, you can measure participant satisfaction with sustainability aspects through a short post-event survey. Measuring without communicating has little value: share the results both internally and externally to demonstrate sustainability as a concrete commitment.
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