Deadweight is the extent to which the outcomes would have happened anyway, and is estimated by using benchmarks. An example of deadweight at MillRace IT is the extent to which computer recycling would take place if MillRace IT did not exist. It turns out that there are many places local customers could go to have their computers recycled, even at the same site as customers could drop off computers with Millrace IT. Therefore, people could have recycled their computers anyway, and the deadweight for computer recycling is 100 per cent. We calculate the deadweight because it can help an organisation express the value of its work, and also weights the final impact depending on how difficult it is to achieve outcomes, which can vary across beneficiaries. For example, an employment project working with young people who have been unemployed for six months would probably find that the participants would be more likely to get jobs anyway (greater deadweight) than the participants of an employment project working with young ex-offenders who have been unemployed for two years.
Source: Measuring value: a guide to social return on investment (SROI) http://www.sroi-uk.org/component/option,com_docman/task,cat_view/gid,25/Itemid,38/