How much carbon should be stored in temporary and risky offsets to compensate an emission of 1 ton of CO2? We show how the Social Value of an Offset (SVO) is a well-defined fraction of the Social Cost of Carbon which depends on the offset's expected lifetime, risk of non-additionality and risk of failure. The key insight is that the SVO can be positive because delaying emissions is socially valuable. Offset projects could therefore be part of an efficient net-zero portfolio if their SVO to cost ratio exceeds the benefit-cost ratio of alternative projects. Since many offset projects are not riskless or perpetual, offset suppliers should supply transparent information about the permanence, risk and additionality of their offerings, so that the SVO can be calculated and offsets easily compared. We provide a matrix of risk correction factors to calculate the SVO for this purpose.