Several public health groups have called for the creation of a global fund for 'social protection'-a fund that produces the international equivalent of domestic tax collection and safety net systems to finance care for the ill and disabled and related health costs. All participating countries would pay into a global fund based on a metric of their ability to pay and withdraw from the common pool based on a metric of their need for funds. We assessed how alternative strategies and metrics by which to operate such a fund would affect its size and impact on health system financing. Using a mathematical model, we found that common targets for health funding in low-income countries require higher levels of aid expenditures than presently distributed. Some mechanisms exist that may incentivize reduction of domestic health inequalities, and direct most funds towards the poorest populations. Payments from high-income countries are also likely to decrease over time as middle-income countries' economies grow.