Many cities try to mitigate the extreme heat by planting trees. But the cooling achieved by such programs is debated, as previous analyzes focus on much smaller scales than the city-wide ones planners are working on. To fill this gap, we performed a scaling analysis of cooling efficiency (CE)—the temperature reduction associated with the 1% growing tree canopy (UTC)—to predict citywide CE. The results show that CE increased with increasing spatial scales in a convex power form. The power law was consistently found across multiple cities with different climate contexts and was also robust under different summer weather conditions within the city. Power-law scaling of CE can provide a tool for urban planners to set UTC targets for extreme heat mitigation. (Jia Wang, Weiqi Zhou, Steward TA Pickett , more at pnas.org)