HERA came to us with a sophisticated cost-benefit model their researchers (Mode Economics ) had developed in a large Excel workbook — covering heat-related mortality, lost worker productivity, and the effect of five adaptation strategies. It was rigorous, but it lived in a spreadsheet form that would be difficult for inexperienced users to navigate. The challenge was to make it public and intuitive without compromising the robustness of the model. In other words, a tool that answers, for any city, three questions a decision-maker actually asks — what are the costs versus the benefits, what’s the impact on the local economy, and how many lives are saved.

The heart of the project was faithfully reproducing that workbook as a fast, pure-JavaScript engine that runs entirely in the browser. We treated the Excel model as the spec and validated our version against it at three levels — thousands of cell-by-cell assertions across the intermediate math, hundreds of end-to-end dashboard scenarios, and a differential test that ran hundreds of randomly sampled cities through both our engine and live Excel until the numbers matched to the cent.

With the new calculation engine in place, we iterated through different visualization-driven views into the results and worked with HERA and Mode to group them into categories that would make the most sense to city planners and economists who would be using the tool.

The result is a tool that translates extreme-heat risk into the language of budgets, benefit-cost ratios, and lives saved — giving cities everywhere a credible, science-backed way to build the case for acting on heat.