Motion Sensor Coverage Overlap Calculator

In test videos where a human walks directly from doorway to middle of room, most motion sensors do just fine. The real world, however, is not so tidy. There’s furniture blocking sensor sight lines, pets pacing near legs, and corners falling completely out of the detection cone. That difference between theory and practice is typically how security systems fall down. It is not because they turn on the lights or trigger an alarm. But simply because they don’t see you at all as you walk along side of living room instead of right in front of them.

You don’t need to visualize any overlap in your head. The calculator figures that for you. Most importantly: Know WHY you put the inputs in there.

How to Place Your Sensors Correctly

The first input is about mounting height. A wall sensor at eye-level (or at a 10 foot ceiling) will behave very differently than one eight feet off the ground. The distance from the lens to the target height reduce its horizontal reach. A “thirty foot range” doesn’t mean it sees all the way out to thirty feet. Actual range will be less if the lens is positioned higher. This is because angle tilts down toward floor instead of projecting out horizontally. That’s what folks misunderstand. They’ll spend big bucks on fancy gear and then cripple it with poor placement, starving it of effective coverage.

The equation also depends on sensor types. Standard PIR units sense heat differential relative to some kind of background, so they can absorbs sensitivity to amount of furnitures in a space, as well as the room’s overall temperature. Add dual-tech and you get radar/microwave sensing that can pick up motion even if thermal signatures are weak. Make it pet-immune and it’ll simply ignore anything near your base plane that looks like low-lying movement, avoiding false positive alerts caused by dogs chasing their tails. Every profile has its own reliability factor that changes how large an area it covers effectiveley.

An open bedroom will likely require fewer sensors than a cluttered office filled with shelves of books, since obstructions can block line-of-sight. To deal with this, the tool applies obstruction factors that limit usable coverage based off actual detection zone interruptions by sofas and walls.

That’s where the real world meets your plan: overlap. Ideally there will be some, so that if you walk from one area with a sensor into an adjoining one, it can pick up motion. But too much, and you waste money (and create confusing logs of which automations did what). Fifteen to twenty percent tends to be that sweet spot. Less than that means possible blind spots when moving from one covered area to another; more than that, and you’re paying for redundant coverage with no gain in security. If you chain sensors along a hall, or face them across a space, the math shows how far apart they need to be to achieve that sweet spot. In uneven rooms there will always be some blind spots, but ideally they shouldn’t exceed fifteen percent. Anything over 15 percent means that your system might not work reliably.

With the results you get a blind zone percentage along with a unique coverage number so that you know exactly what’s protected and what isn’t. Based off that, determine if a third sensor would help make sense or if you could just move one of the sensors near the troublesome corner to solve it. The page also has a series of reference tables illustrating typical range/angle combinations for various profiles, so that you can have some reasonable expectations going into wall measurements.

Wide angle sensors are generally good for corners (they tend to catch the entering motion). The opposite setup is best for cross-coverage, but does need to be aligned carefully enough so that there isn’t too much overlap. For hallways, narrow beams and/or close beam spacing help ensure no gaps between them. Where do people walk (not where you think they’ll be)? Put devices there so they capture movement over pathways. Don’t just monitor static locations. Mount at appropriate heights as this impacts range much more different than most people realize. You want moderate overlap to ensure reliable coverage while maintaining some efficiency.

When your blind zone stays small and the numbers align, congrats. You’ve got something that works, not just on paper, but out in the field. It’s really mostly knowing what to expect when you measure things, and how to place devices appropriatley to capture that. You should of known more about placement before starting.