Purpose: This paper explains how proponents of biodiversity offsetting have sought to produce an ecologically defensible mechanism for reconciling economic development and biodiversity conservation.Methods: The paper analyses a case study biodiversity offsetting mechanism in New South Wales, Australia. Michel Callon's framing and overflowing metaphor is used to explain how accounting devices are brought into the mechanism, to (re)frame a space of calculability and address anxieties expressed by conservationists about calculations of net loss/gain of biodiversity.Findings: The analysis shows that the offsetting mechanism embeds a form of accounting for biodiversity that runs counter to the prevailing dominant anthropocentric approach. Rather than accounting for the biodiversity of a site in terms of the economic benefits it provides to humans, the mechanism accounts for biodiversity in terms of its ecological value. This analysis, therefore, reveals a form of accounting for biodiversity that uses numbers to provide valuations of biodiversity, but these numbers are ecological numbers, not economic numbers. So this is a calculative, and also ecocentric, approach to accounting for, and valuing, biodiversity. Originality/value: This paper contributes to extant literature on accounting for biodiversity by revealing a novel conceptualisation of the reconciliation of economic development and biodiversity conservation, producing an ecologically defensible form of sustainable development. The paper also makes a methodological contribution by showing how Callon's framing and overflowing metaphor can be used to enable the kind of interdisciplinary engagement needed for researchers to address sustainable development challenges.