What is "livability" made of, exactly?

That is the elusive question that a new “livability index” from AARP wants to answer. The index allows you to punch in an address and find out how it scores, on a scale from 1 to 100, in seven different categories: housing, transportation, environment, health, engagement, opportunity, and the catchall “neighborhood” category, which encompasses proximity to services as well as personal safety. The site covers 200,000 communities around the country, and includes county- and state-level data as well.

It's a lot of fun to play with the tool. Click through to a given category and you'll find why a given address scores the way it does: My Brooklyn neighborhood, for instance, rates an impressive 83 on transportation (14 buses and trains per hour; estimated transportation costs of $5,324 per year compared to the U.S. median of $10,791), but only 37 on engagement (shamefully low voter turnout and few civic organizations). The apartment where I used to live in Portland, Maine, does well in the “housing” category (places to live are relatively affordable and abundant), but not so great in “environment” (there’s a heavily trafficked bridge close by). Sources for all the data are included, as well as explanations of the reasoning behind the rankings.

The index also allows you to customize your priorities. If, for instance, you value clean air and water over access to quality health care, you can weight “environment” more heavily and dial back the importance of “health.”

While the AARP index is designed to be of special use to people aged “50-plus,” as you might expect given the source, the researchers who put it together emphasize that it is useful for people of all ages who are trying to figure out where they want to live, either now or in the future. “When you plan for older adults, you plan for everyone,” says Jana Lynott, senior strategic policy adviser at the AARP Public Policy Institute.

The index also allows you to view policies that communities have in place that affect everything from housing affordability to access for people with disabilities.

Many of the categories measured by the index are of increasing concern to people who are aging—and the U.S. population as a whole is getting older, fast. By 2030, it is projected that 19 percent of Americans will be over 65, up from 12.4 percent in 2000.

The vast majority of those older Americans want to stay where they are after retirement, and AARP researchers hope the index will be a resource for cities and towns that want to examine how they can improve services for an aging population.

“The index will help communities become better,” says Dr. Rodney Harrell, director of livable communities at the AARP Public Policy Institute. “So people don’t have to move.”