Computing within Limits invites considerations of limits and constraints in design practice. We compare two projects which integrate constraints, the reduction of academic air travel and a solar powered internet, to show a distinction between two approaches to arriving at constraints. In the case of reducing academic air travel, the problem which greenhouse gas emissions pose for business-as-usual academic travel is addressed by proposing constraints on future flying. Constraints in the Flight project can be understood as a process of commensuration, of comparing that which is to be constrained according to a common metric. This gives rise to a future of academic travel understood in relation to CO₂ emissions and reduction targets. In the second case, we have explored the solar internet as a specific way to introduce constraints in the context of the rising electricity use associated with internet infrastructure. In the Solar Internet project, constraints have been approached relationally and iteratively, in reconfigurations of internet use practices and design practices, including the solar internet imaginary and the scale of battery and power supply.

We compare these two approaches, drawing on vocabulary from Sociology of Quantification and Science and Technology Studies, to help articulate their respective implications, while also acknowledging what they have in common, e.g. the ability to expand the frame of what is made relevant for design practice.The case of the Flight project suggests that constraints as a process of commensuration can be fruitful when pursuing a unified future, intervening over time with a trajectory towards a quantifiable target. On the other hand, when trying to account for indirect effects and the future as multiple, the introduction of constraints can better be understood as con-figurations, with a future negotiated iteratively in design practice. Rather than thinking about constraints as essentially requiring one or the other approach, we suggest that problems and the introduction of constraints may be more or less amenable to either approach at aspecific time.