How Many Door and Window Sensors Do You Need?
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Sensor count is where security-system pricing actually gets decided - and where both sales directions mislead. Provider package builders err generous (a sensor on every window sells more sensors), while starter kits err stingy (a low sticker price wins the comparison table, then you discover "8-piece kit" means four sensors for a house with nine openings). The honest answer comes from a simple principle:
Contact-sensor every door a person can walk through. Then cover ground-level windows by zone - with contacts on the reachable, openable ones, and motion sensors doing the bulk of the interior work.
Widely cited industry figures (treat them as approximate - primary sourcing is thin) put front doors at roughly a third of burglary entries, with first-floor windows and back doors each around a fifth. The pattern that matters for planning: entries overwhelmingly happen at ground level, through ordinary openings, and doors beat windows. Doors first, ground floor second, everything else a distant third.
Room-by-room planning table
| Opening | Recommendation | Why / notes |
|---|---|---|
| Exterior doors (front, back, side) | Contact sensor - always, no exceptions | The most-used entry points, and door sensors are the cheapest, least false-alarm-prone devices in the whole system |
| Door from attached garage into house | Contact sensor | Garage vehicle doors are often left open or defeated; treat the house door as an exterior door. A tilt/contact sensor on the overhead door itself is a nice-to-have, not a must |
| Sliding glass doors | Contact sensor; glass-break optional | A person-sized opening on the private side of the house. Add a rod/track lock too - a $10 mechanical upgrade worth more than a second sensor |
| Ground-floor windows that open, hidden from the street (rear/side) | Contact sensor | Concealment is what burglars want; these are the highest-value window sensors |
| Ground-floor windows facing the street | Contact sensor if budget allows; otherwise motion coverage inside | Visible openings are less attractive; a motion sensor in the room behind them catches entry anyway |
| Basement windows and basement door | Contact sensors, or one motion sensor covering the basement | Classic concealed entry; a single motion sensor often covers several small windows more cheaply |
| Upper-floor windows, no easy access | Skip | Money better spent elsewhere |
| Upper windows reachable from a porch roof, deck, fence, or ladder left outside | Contact sensor (and store the ladder) | "Second floor" only protects you if it's actually hard to reach |
| Interior hallways / stair landings / main living area | 1–2 motion sensors | The backstop: whatever opening you didn't sensor, an intruder still has to cross the house. Pet-immune models handle most cats/dogs under placement guidelines - read your brand's specs |
| Rooms with large fixed panes or many windows | Glass-break sensor (optional) | One acoustic unit covers multiple windows in the same room; catches smash-through-without-opening, which contacts miss. Costs more per unit and can false on loud crashes - a tradeoff, not a default |
Two placement notes that prevent the most common DIY mistakes: contact sensor halves must sit within the manufacturer's gap tolerance (roughly half an inch to an inch - check your brand) and adhesive struggles on textured or metal surfaces, so plan screws or extra adhesive there. And motion sensors shouldn't face big windows or heat sources, which drive false alarms - relevant because repeat false dispatches cost real money in many cities (see fines and permit rules).
Worked examples
One-bedroom apartment (upper floor)
- 1 contact sensor - entry door
- 1 contact sensor - balcony slider if the balcony is reachable; otherwise skip
- 1 motion sensor - main living area
Total: 2–3 devices. Most "8-piece" starter kits actually overshoot an apartment; a small kit plus renter-friendly adhesive mounting is plenty. (Renter specifics are covered in our choosing guide.)
Three-bedroom, two-story house (attached garage, slider to patio)
- 3 contact sensors - front door, back/patio slider, garage-to-house door
- 3–4 contact sensors - rear and side ground-floor windows that open and aren't visible from the street
- 2 motion sensors - main floor living space, stair landing or hallway
- Optional: 1 glass-break for a rear room with large panes
Total: 8–10 devices. Notice the mismatch with typical packages in both directions: an "8-piece kit" (which includes non-sensor pieces like the base station, keypad, and range extender) is short, while a salesperson's window-by-window quote of 15+ contacts is padded.
Large house (4+ bedrooms, walk-out basement, many ground-floor windows)
- 4–5 contact sensors - all exterior doors including basement walk-out and garage-to-house
- 5–7 contact sensors - concealed/reachable ground-floor and basement windows that open
- 3 motion sensors - main living area, basement, upstairs hallway
- 1–2 glass-breaks - rooms with large panes or French doors, if desired
Total: 13–17 devices. Past this point, each extra contact sensor buys less marginal protection than the motion sensors already provide - a defensible stopping rule when a quote keeps climbing.
The cost math per added sensor
As of July 2026 (verify current pricing - this category discounts constantly): entry/contact sensors from the major DIY brands generally list around $15–$30 each, with frequent multi-pack and promotional discounts well below that (Cove listed door sensors at $7.50 during one of its recurring sales); motion sensors run roughly $30–$50; glass-break sensors cost more than either, which is why they're an "add where justified" device rather than a default.
Put that against the decision it drives: the difference between under-covering and properly covering the example 3-bed house is three or four extra contact sensors - call it $50–$100 once. Compare that to upgrading a monitoring plan tier, which costs $120+ every year (monitoring costs here). Coverage is the better first dollar. The reverse also holds: a quote with six sensors you don't need is $100–$200 of padding, plus the drip of extra batteries to replace. Battery reality check: contact sensors typically run years on a coin or AA cell, so the maintenance cost of a right-sized system is a few dollars of batteries annually - not a reason to under-buy.
One brand-selection note: per-sensor price differences across brands compound with house size. For an apartment, ignore them; for the 15-sensor house, a $10-per-sensor difference is $150, worth checking in our no-contract system comparison before you commit to an ecosystem - expansion sensors only work with their own brand's system, so you're choosing future sensor prices too. Total-budget context lives in our costs and contracts guide.
Make it concrete: walk your home's exterior once, counting doors and reachable openable windows, then apply the table above. Our home security checklist turns the walk into a printable list - including the non-sensor items (locks, lighting, ladder storage) that cover several "should I sensor this?" edge cases for free.
FAQ
Do I need a sensor on every window?
No. Doors always; reachable, openable, concealed ground-floor windows next; motion sensors as the interior backstop for everything else. Upper windows without realistic access don't repay the hardware.
Are glass-break sensors better than window contact sensors?
Different jobs. Contacts detect opening; acoustic glass-breaks detect shattering and cover a whole room of windows at once, catching the smash-and-climb-through case contacts miss. They cost more and can false-alarm on loud sounds. Contacts plus motion sensors is the right default; glass-breaks are a targeted upgrade for big panes and sliders.
Can motion sensors replace window sensors entirely?
Largely, yes - with two caveats. A motion-only setup can't run in "home/stay" mode while you sleep (you'd trip it yourself), so perimeter contacts are what let the system protect you while you're inside. And motion detection means someone is already in the house; contacts on the likely openings give you the earlier alert. That's why the plan above uses both.
Sources
- MoneyGeek - US Home Burglary Statistics (entry-point figures; industry-cited) (accessed 2026-07-02)
- SimpliSafe - Entry Sensor (specifications, gap tolerance, battery life) (accessed 2026-07-02)
- Ring - Alarm Contact Sensor (2nd Gen) (accessed 2026-07-02)
- Ring - Glass Break Sensor (accessed 2026-07-02)
- Security.org - Cove Cost and Pricing (per-sensor sale pricing) (accessed 2026-07-02)
- SafeWise - SimpliSafe Cost (package sensor counts) (accessed 2026-07-02)
- DoItYourself.com forum - motion vs glass-break false-alarm discussion (representative owner experience) (accessed 2026-07-02)