Where modern public health developed techniques to calculate probability, potentiality, risk and uncertainty, contemporary finance introduces instruments that redeploy these. This article traces possibilities for interrogating the connection between health and financialisation as it is arising in one particular example – the health impact bond. It locates the development of this very recent financial innovation in an account of public health's role within governance strategies over the 20th century to the present. We examine how social impact bonds for chronic disease prevention programmes bring two previously distinct ways of thinking about and addressing risk into the same domain. Exploring the derivative‐type properties of health impact bonds elucidates the financial processes of exchange, hedging, bundling and leveraging. As tools for speculation, the functions of health impact bonds can be delinked from any particular outcome for participants in health interventions. How public health techniques for knowing and acting on risks to population health will contest, rework or be subsumed within finance's speculative response to risk, is to be seen.