There is growing interest in developing generalisable, standardised measurements of biodiversity, in particular to help the business and financial sectors to quantify biodiversity impacts. In our recent paper, we created a framework to explain how such units are defined in the rapidly growing voluntary biodiversity credits market. In this talk, I will present this framework and use it to discuss the many measurement challenges that are faced in trying to reduce something as complex as biodiversity to a single unit, and what this might mean for biodiversity credit markets.