Coupa Café—an extremely popular coffee shop in downtown Palo Alto, California, that’s typically jammed with Silicon Valley tech types—was experiencing a lull in traffic on a recent Friday morning.

I knew that not from visiting or calling, but by opening up a new app called Density on my iPhone that keeps track, in real time, of how busy businesses are. The app is made by a San Francisco startup of the same name, and it works by surfacing data collected by small, Internet-connected, infrared sensors  in the doorway of each business.

Density, which launched in July, is trying to solve a deceptively tricky problem: while there are tons of sensors in our phones and an increasing number of them in our homes that can tell us all kinds of things, it’s still hard to get a good idea of how busy a specific place is unless you’re there or get in touch with someone who is. Getting a better handle on this, says Density cofounder and CEO Andrew Farah, can make it simpler to figure out things ranging from when is a good (or bad) time to visit the gym to which conference rooms are free in your office.

So far, Density has installed prototypes of its sensors in over a dozen businesses, including several coffee shops in the Bay Area and in Portland, Oregon. Farah says the company will start installing sleeker sensors—small, square-shaped white boxes with a black plastic face, which connect to another gadget that houses the Wi-Fi radio—sometime in August. Density envisions the data collected by these sensors providing insights to businesses of all sorts, and potentially being added to apps that consumers can use, like the one I tried (though Farah says it’s meant to be more of a demo than a regular app, Density does plan to add more locations to it). The company charges $25 per month for the hardware and access to its service; it’s targeting businesses that sell things like cash register systems to retailers, which will roll them out to the retailers.

There are existing ways to count pedestrian traffic, such as by using surveillance cameras or so-called break-beam systems that keep a tally based on how often their infrared beam is broken by a passerby. But Farah says Density’s aim was to come up with another method that did away with privacy concerns people might have when it comes to cameras, while also collecting data in real-time.

Chris Harrison, an assistant professor of human-computer interaction at Carnegie Mellon University who recently worked on a project that used smartphone cameras as multifaceted monitors that can answer questions about the world around you, sees Density as more practical in the foreseeable future than a lot of other Internet of things gadgets, saying it’s dealing with a “very well-constrained problem.”

He is concerned about how accurate Density’s method of people counting using infrared sensors can be, though. Farah wouldn’t say how accurate the company’s sensors are at picking up humans entering and exiting a business, but says it’s “accurate enough to rely on,” especially when it comes to sensors placed by a standard-width, single door.