Whether you’re trying to figure out how many students are attending your lectures or how many evil aliens have taken your Space Force brethren hostage, Wi-Fi can now be used to count them all.

The system, created by researchers at UC Santa Barbara, uses a single Wi-Fi router outside of the room to measure attenuation and signal drops. From the release:

The transmitter sends a wireless signal whose received signal strength (RSSI) is measured by the receiver. Using only such received signal power measurements, the receiver estimates how many people are inside the room — an estimate that closely matches the actual number. It is noteworthy that the researchers do not do any prior measurements or calibration in the area of interest; their approach has only a very short calibration phase that need not be done in the same area.

This means that you could simply walk up to a wall and press a button to count, with a high degree of accuracy, how many people are walking around. The system can measure up to 20 people in its current form.

The system uses a mathematical model to “see” people in the room based on signal strength and attenuation. The system uses off-the-shelf components and they’ve tested it in multiple locations and found that their total accuracy is two people or less with only one Wi-Fi device nearby.

Bodies and objects essentially absorb Wi-Fi as they move around in rooms, allowing the system to find discrete things in the space. Sadly it can’t yet map their position in the room, a feature that could be even more helpful in the future.